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General Category => Equipment and Software => Topic started by: tygo on May 24, 2010, 01:26:00 AM

Title: ATC Refractometers
Post by: tygo on May 24, 2010, 01:26:00 AM
Just picked one of these up and the directions say that the automatic temperature correction will handle samples up to 50F - 86F.  Obviously a sample pulled right from the kettle is going to be higher than that.  So how is that handled?  Should I let it cool a little before dropping it on the prism?

It wouldn't take long for a little bit of wort to cool down to below 86 but my impression when reading people describe using these things is that I could take the sample directly out and drop it on.
Title: Re: ATC Refractometers
Post by: a10t2 on May 24, 2010, 04:21:37 AM
The ATC range refers to the temperature of the refractometer, not the sample. Once the hot wort hits the room temperature lens it'll cool down very quickly.

It's probably a good idea to have some way to cool the sample down immediately anyway, though. Otherwise evaporation will skew the readings, IME.
Title: Re: ATC Refractometers
Post by: richardt on May 24, 2010, 01:26:25 PM
One or two drops of wort (regardless of temp) quickly cools down when it hits the much cooler glass plate of the refractometer.
Then you flip the clear plastic cover plate on top of it (which is also cooler). 
This takes all of 2 seconds.  There's no evaporation taking place once your cover plate is on.
Now you can take all the time in the world reading it
Just multply your brix reading by 4 to get a rough estimate of your SG.

There's no need to worry about evaporation nor the need to rapidly chill your drops, IMO.
Just use the plastic dropperettes to sample a few drops onto the refractometer, flip the cover plate, and read it.
Then flip the cover plate off and LICK the refractometer!  It's a great way to assess for astringency or process problems, esp during sparging.
Congrats on the refractometer--it's a useful tool.
Title: Re: ATC Refractometers
Post by: tygo on May 24, 2010, 01:55:33 PM
Thanks, that all makes sense.  I actually got one that has both a Brix and SG scale so at least I don't have to do any math.  I'll be taking the new toy for a spin this weekend.
Title: Re: ATC Refractometers
Post by: Kaiser on May 24, 2010, 03:18:48 PM
you should check the calibration with water before you use it. Though mine is an ATC meter its ATC seems to be broken since the calibration changes with temperature.

Kai
Title: Re: ATC Refractometers
Post by: hopfenundmalz on May 24, 2010, 03:22:39 PM
Be sure to calibrate with distilled or RO water.  If you have a really high mineral content in your tap water like me, your cal will be off.
Title: Re: ATC Refractometers
Post by: tygo on May 24, 2010, 05:18:51 PM
I calibrated it at room temperature (70 or so) with distilled water but I'm planning to recalibrate it outside in the garage immediately prior to using it.
Title: Re: ATC Refractometers
Post by: Kaiser on May 24, 2010, 06:34:17 PM
I calibrated it at room temperature (70 or so) with distilled water but I'm planning to recalibrate it outside in the garage immediately prior to using it.

Also, the ATC works based on the temperature of the unit itself and not necessarily the temperature of the prism. The latter determined the temperature of the sample. I can see how they can have different temperatures if the refractometer was stored in a cold room and the moved into the warm outside air for measurements.

Ideally you want the unit to acclimate to the temperature of the area where you will be measuring.

Kai
Title: Re: ATC Refractometers
Post by: nyakavt on May 24, 2010, 07:12:15 PM
There's no need to worry about evaporation

This has not been my experience.  I use the refractometer for lots of things on the hot side, including pre-boil OG measurements and mash gravity measurements.  The reading would be way too high (pre boil gravity > post boil gravity, or mash gravity much greater than predicted) with just a drop on the lens compared with a cooled hydrometer sample, and also when pouring a bit onto an open dish to cool before sampling.  Then a couple of times I took a hydrometer sample to chill in the freezer and checked with the refractometer right away and after chilling - the hot sample was 2 Brix higher.  I also could not get any consistency when taking two readings in a row.

For the last dozen or so batches I've switched to cooling the sample in an airtight container, either a small screw top test tube or a 10 mL syringe.  The results have been much more consistent to each other, as well as matching the hydrometer sample and the predicted mash gravity.  There could be other factors at work here (like stratification of sugar concentration in the MT or kettle) that affected my readings, but I've seen enough to convince myself that this evaporation is a real concern and must be addressed, even with small samples that are quickly covered.
Title: Re: ATC Refractometers
Post by: richardt on May 25, 2010, 01:38:58 AM
Short version:  I respect the fact that you’ve given this a lot of thought and are dedicated to improving the accuracy of your measurements.  I merely disagree with the premise that your refractometer readings are off because of evaporation.  Instead, I think it is due to sample stratification.  After all, don’t we wait 15 minutes or so for the hot and cold break to settle before transferring?  Why wouldn’t sugars and proteins of various shapes, sizes, and weights be any different?

Long version:   Three key parts to evaporation are heat, humidity and air movement (source:  http://en.wikipedia.org/wiki/Evaporation) (http://en.wikipedia.org/wiki/Evaporation)).  As I see it, plunging a dropperette beneath the surface of the liquid to draw a sample of wort (regardless of temp), promptly placing a very small sample mass of just one or two drops on the cooler refractometer surface (which has a much larger mass and serves as a heat sink), and placing the cover plate over it pretty much removes all three parts necessary for evaporation.  There’s even less kinetic energy to drive evaporation as the liquid cools, the humidity probably does decrease (but you’re reading within a few seconds, so it should be negligible), and the air movement is nil (as the cover plate prevents exposure to air movement and may limit humidity changes since there’s practically no exposed surface area to permit it).

The refractive index is dependent on
1.   The material/media in which it is traveling
2.   The frequency of light
3.   The temperature
(source:  http://en.wikipedia.org/wiki/Refractive_index (http://en.wikipedia.org/wiki/Refractive_index))

As you mentioned, I think it is quite possible that you may be noticing temperature and specific gravity stratification of your pulled samples.  This sample inhomogeneity may be yielding slightly lower readings if you’re pulling smaller samples from the upper and/or outer portions of the larger sample.

Assuming you’re doing the readings in the same place regardless of your sampling method, the frequency of light isn’t changing (whether you’re consistenly brewing outside or in the kitchen).  If you’re doing one reading outside with one method and another reading inside with a different method and using a different light source—give it some thought and eliminate that variable.

Temperature gradients can cause fluctuation in refractive index within a single medium to generate a refractive effect, e.g., mirages and looming artifacts (source:  http://micro.magnet.fsu.edu/primer/lightandcolor/refractionintro.html (http://micro.magnet.fsu.edu/primer/lightandcolor/refractionintro.html)).  I don’t know if there’s enough of a temperature gradient with a refractometer to create this refractive effect to a significant degree that we’d notice it.  My bias is that a temperature gradient is not playing a role with the refractometer readings.  IMO, even the chilled specimens are more likely to be causing specific gravity stratification in the time it takes to chill the sample, which I perceive as having a greater effect on one’s refractometer readings than temperature gradient effects would.

And, like Kai said earlier, it is best to acclimatize your refractometer to ideal conditions.  Thanks for letting me participate in such an interesting discussion.
Title: Re: ATC Refractometers
Post by: a10t2 on May 25, 2010, 06:53:10 AM
Instead, I think it is due to sample stratification.

I really can't believe that there's any stratification in a kettle full of boiling wort.
Title: Re: ATC Refractometers
Post by: jimbo44 on May 25, 2010, 07:24:32 AM
I simply pull of a pipette full of wort (about four drops) and run the bulb under cold water for about 5 seconds.  then I drop a couple on the refrac and look.  I have broke a refrac with 212 water.  also be gentle when you flip the lid down.  really hot wort and an impact can break the prism.  It doesn't happen every time, but if it happens once, thats one to many.  Side Note:  I use beer smith and there's a nice conversion tool for measuring fermenting wort.  I have ran some tests and that can be off by more than three points.  That can really start to add up, so I suggest always using a hydrometer once it is fermenting.
Title: Re: ATC Refractometers
Post by: tygo on May 25, 2010, 10:55:19 AM

Ideally you want the unit to acclimate to the temperature of the area where you will be measuring.

I can definitely see the logic of this.  If the ATC range is 50-86F though I can see situations where the ambient temp outside in the garage where I brew would be higher than that range.  In those cases I assume that will affect the accuracy of the reading?  I guess if it's higher than 86 outside I should take the reading inside.
Title: Re: ATC Refractometers
Post by: tygo on May 25, 2010, 10:56:37 AM
I simply pull of a pipette full of wort (about four drops) and run the bulb under cold water for about 5 seconds.  

Are you using the plastic pipette that came with the refractometer?  Will that hold up to boiling wort?

Using this method it seems like evaporation wouldn't really be significant since you're cooling it down quickly inside the pipette.
Title: Re: ATC Refractometers
Post by: richardt on May 25, 2010, 01:26:40 PM
Instead, I think it is due to sample stratification.

I really can't believe that there's any stratification in a kettle full of boiling wort.

I agree--there shouldn't be any stratification in a kettle when the wort is boiling.

However, when the wort has been chilled down to yeast pitching temps (60-80 F) during the 15-30 minutes of whirlpooling and using the immersion chiller, then stratification has probably occurred (as evidenced by the improved clarity of the wort and the sedimentation of the break material and trub.

Personally, I tend to perform my refractometer readings during lautering (T <170 F) and at the end of the boil/post chill (T 60-80 F).  I'm not putting hot wort (T =212F) on my refractometer.  I really don't see the need to check the SG during the boil--I trust my BeerSmith brewsheet and want to follow the hopping and boil schedule as accurately as possible--that's what is going to determine what the beer tastes like. 
Title: Re: ATC Refractometers
Post by: Kaiser on May 25, 2010, 02:00:12 PM
However, when the wort has been chilled down to yeast pitching temps (60-80 F) during the 15-30 minutes of whirlpooling and using the immersion chiller, then stratification has probably occurred (as evidenced by the improved clarity of the wort and the sedimentation of the break material and trub.

I don't think there is any stratification possible after the wort has been mixed. Yes there is settling of suspended particles but the sugars are dissolved and won't statify. As little as a sugar solution you put on the counter will stratify.

Kai
Title: Re: ATC Refractometers
Post by: a10t2 on May 25, 2010, 03:37:31 PM
I agree--there shouldn't be any stratification in a kettle when the wort is boiling.

The reason I said that is that I take a refractometer reading right as the wort comes to a boil (because prior to that point I *have* seen stratification). And unless I pull that sample with a sealed syringe it reads around 1°Bx higher than a hydrometer sample that's chilled, covered, in an ice bath. Stratification couldn't account for that - especially since we're talking about having higher-density liquid at the top.
Title: Re: ATC Refractometers
Post by: Kaiser on May 25, 2010, 04:55:57 PM
Because of the small sample size you have to be careful not to contaminate the sample with water. When I place a drop onto the refractometer I always discard a few drops first. This way I get the wort that is in the center of the sample.

But I haven't used my refractometer much so far and I still have to get comfortable with it as well.

Kai
Title: Re: ATC Refractometers
Post by: richardt on May 26, 2010, 10:21:35 PM
I agree--there shouldn't be any stratification in a kettle when the wort is boiling.

The reason I said that is that I take a refractometer reading right as the wort comes to a boil (because prior to that point I *have* seen stratification). And unless I pull that sample with a sealed syringe it reads around 1°Bx higher than a hydrometer sample that's chilled, covered, in an ice bath. Stratification couldn't account for that - especially since we're talking about having higher-density liquid at the top.

Actually, the opposite.
Higher density liquid goes to the bottom; lower density liquid goes to the top. 
That's why colder water sinks and warmer water rises (convection currents).

Without sufficient and continuous mixing, all solutions and mixtures have strata and gradients, be it thermal, salinity, dissolved solids, etc.
Title: Re: ATC Refractometers
Post by: richardt on May 26, 2010, 10:23:30 PM
A refractometer doesn’t directly measure the density of the liquid, but it does measure how light is bent through the liquid.  The fundamental scale of a refractometer is the Refractive Index (RI).  As discussed earlier, temperature of the medium and the wavelength of the light do affect the Refractive Index of a substance.  To be truly “standardized,” RI measurements with a refractometer should be done at 20 degrees Celsius using sodium light (589.3 nm).  On a homebrewing level, that’s not really practical, nor necessary.  The presence of dissolved substance(s) in water (i.e., its concentration) also affect(s) the refractive index.  The Brix scale is a sucrose concentration scale utilizing standardized settings (sodium light, weight % of sucrose in water, and 20 degrees C).  It is a two-component system.
 
Wort is not a two-component system, nor a true (i.e., homogeneous) solution.  Wort is a complex, heterogenous liquid mixture.  While most of the dissolved substances in wort are sugar, a significant portion (>5%) is not.  Furthermore, sucrose only comprises a small percentage (3-8%) of the sugars, like maltose, maltotriose, glucose, fructose, and other dextrins, which, incidentally, exist in greater percentages and have different relative densities than sucrose.  Liquid mixtures that are not homogeneous include colloids, suspensions, and emulsions; wort has features of all three.

Yet, the Brix scale has usefulness in brewing.  As home brewers, we should be using the term “apparent Brix” for non-sucrose based liquids like wort as it gives us a measure of the total dissolved solids in the wort which are predominately sugars.  It would not be unreasonable for the “total dissolved solids” measurement to decline over time if given a reasonable period of time for substances to stratify or sediment based on their relative density and weight.  Refractometer readings taken immediately after sampling permits no time for evaporation or stratification.

Brix RDS (Refractomeric Dissolved Solids) is a measurement of percent by weight of total dissolved solids in solution; and is determined using a refractometer.  It is NOT the same thing as Brix degrees, which is a measurement of the mass ratio of dissolved sucrose to water in a liquid and is determined through a specific gravity method, e.g., a hydrometer or a saccharimeter.

Hydrometers aren’t perfectly accurate, either.  Given that alcohol is lighter than water, how many home brewers really boil off the alcohol when taking FG readings (to determine real attenuation rather than measured attenuation)?
 
I just don’t think “evaporation” is causing your readings to be variable with the refractometer.

It would be interesting to see results of the following experiment when you do your next batch:  Go ahead and chill your covered samples before taking your refractometer and hydrometer readings to determine OG.  Take one reading without disturbing the sample, and then take another refractometer reading after agitating the entire sample.  If it is due to sedimentation, your second reading should be higher.  Do your hydrometer reading last.  As long as your sample remains covered during chilling,  evaporation should not be a factor.
Title: Re: ATC Refractometers
Post by: a10t2 on May 26, 2010, 11:29:18 PM
Higher density liquid goes to the bottom; lower density liquid goes to the top.

... which is why I said it wouldn't make sense for a refractometer sample pulled from the surface to read high.
Title: Re: ATC Refractometers
Post by: nyakavt on May 27, 2010, 01:08:40 PM
Richardt, have you noticed your refractometer readings to be consistent with post-boil volume corrected gravity when taken with a drop of boiling liquid placed directly on the lens?  Like I said, every time I've done this the gravity was higher than anticipated, sometimes to the point that the pre-boil gravity was greater than or equal to the post boil gravity (impossible!).  What can I do differently in sample taking to eliminate the issue, it's done with boiling wort in the center of the kettle? 

If you are saying that my chilled samples are inaccurate due to stratification, and the chilled sample is consistent with the post boil (and batch cooled) wort, then you are also saying that the post-boil sample is inaccurate by the same amount.  This is hard to believe, especially considering the height of the kettle vs. the height of a 5 mL sample (a sample taken at the top of a 24" kettle should be much lower than one taken at the top of a 1/2" column of wort).  Clearly more experimentation is needed since I don't have a good enough understanding of the fundamentals going on here.
Title: Re: ATC Refractometers
Post by: Kaiser on May 27, 2010, 02:26:36 PM
I concur that there is a good chance of evaporation between placing the drop and finally covering it with the lid. Once you place the drop(s) they quickly spread out and being as hot as they are they may also evaporate some water. And it does not take much evaporation to create a significant error.

This is also easy to test and one test that can be tried is to see if there is any correlation between the size of the error and the time that the drop is on the prism but not yet covered.

I also use 10 ml test tube to pull samples. I just tied a 14 gauge copper wire around the neck to lower it into the boil. Another thing these test tubes are good for is for evaluating the quality of the hot break.

Kai
Title: Re: ATC Refractometers
Post by: tygo on May 27, 2010, 02:48:36 PM
So you just dip out a sample with the test tube, cap it, and chill it down somewhat?
Title: Re: ATC Refractometers
Post by: richardt on May 27, 2010, 03:00:56 PM
This is a great discussion with no shortage of opinions!  

Check out this article from the November 2003 BYO for more info on how to use your refractometer.
http://www.byo.com/stories/projects-and-equipment/article/indices/29-equipment/1343-refractometers (http://www.byo.com/stories/projects-and-equipment/article/indices/29-equipment/1343-refractometers)

Brewing coffee isn't that different from brewing wort/beer.  Both use refractometers.  Check out this site and draw your own conclusions.  http://vstapps.com/blog/wp-content/uploads/2010/03/Coffee-and-Refractometers.pdf (http://vstapps.com/blog/wp-content/uploads/2010/03/Coffee-and-Refractometers.pdf)

And this site has a nice schematic of the internal workings of an ATC refractometer:  http://www.misco.com/traditional.php (http://www.misco.com/traditional.php)

What I've taken away from this discussion and my online investigation are the following:

Refractometers should be acclimated to ambient temps and calibrated with distilled water before use.
The sample should be thoroughly mixed before being placed on the refractometer.  This minimizes the impact of any stratification that may have occurred.
Wait 30 seconds after placing the sample and closing the cover plate to allow the sample to come to ambient temp (i.e., same temp as the refractometer).  This minimizes/eliminates the impact of sample temperature as the refractive index of a sample is a function of temperature.

Nyakavt, I am unable to explain your observations and results that you're obtaining with your equipment.
Kaiser, I remain doubtful that, despite its brief increase in exposed surface area, a significant amount of evaporation can occur during cooling (rather than heating) of a sample and the short time (delta t) that the sample is exposed to the atmosphere before the cover plate is placed over it when it is placed on the refractometer prism.

Title: Re: ATC Refractometers
Post by: a10t2 on May 27, 2010, 03:48:46 PM
I concur that there is a good chance of evaporation between placing the drop and finally covering it with the lid. Once you place the drop(s) they quickly spread out and being as hot as they are they may also evaporate some water. And it does not take much evaporation to create a significant error.

If you "hover" the cover plate over the sample before closing it, you'll see condensation form. So - at least to me - it seems clear that evaporation is occurring. The only question is how much it will skew the readings. Like nyakavt, I've found the discrepancy to be large enough that it has to be managed.

Wait 30 seconds after placing the sample and closing the cover plate to allow the sample to come to ambient temp (i.e., same temp as the refractometer).  This minimizes/eliminates the impact of sample temperature as the refractive index of a sample is a function of temperature.

I would disagree. You want to close the plate as quickly as possible. In addition to any benefits from minimizing evaporation, more mass in contact with the sample can only help in reducing its temperature.
Title: Re: ATC Refractometers
Post by: Kaiser on May 27, 2010, 04:07:18 PM
Brewing coffee isn't that different from brewing wort/beer.  Both use refractometers.  Check out this site and draw your own conclusions.  http://vstapps.com/blog/wp-content/uploads/2010/03/Coffee-and-Refractometers.pdf (http://vstapps.com/blog/wp-content/uploads/2010/03/Coffee-and-Refractometers.pdf)

The measurement technique outlined in that article also recommends to cool the sample before placing it onto the prism. I'd love to copy that section but the Acrobat Reader plug-in I have doesn't let me. WTF.

Kai

Title: Re: ATC Refractometers
Post by: bluesman on May 27, 2010, 04:43:59 PM
I use a refractometer to qualify the mash and the brewing process on a routine basis. I have used a refractometer for over a year now and would not brew without it. It is a very useful QA tool.

I have found through my home brewing experiences that measuring boiling wort can be tricky. The repeatability of the actual measurements is highly dependant on the technique employed. I use a clean and dry 10mL dropper to transfer the boiling wort from the kettle to the glass. I try to minimize the exposure (transfer time) of the liquid to air as much as possible in an effort to minimize evaporation and increase the accuracy of the actual readings.

I have concluded that the longer the transfer time (exposure of liquid to air), the higher the reading which leads me to believe that evaporation is taking place thereby concentrating the sugar concentration in the liquid.

Therefore by minimizing the trasfer time one will increase the accuracy of the actual readings.
Title: Re: ATC Refractometers
Post by: richardt on May 27, 2010, 05:40:17 PM
I concur that there is a good chance of evaporation between placing the drop and finally covering it with the lid. Once you place the drop(s) they quickly spread out and being as hot as they are they may also evaporate some water. And it does not take much evaporation to create a significant error.

If you "hover" the cover plate over the sample before closing it, you'll see condensation form. So - at least to me - it seems clear that evaporation is occurring. The only question is how much it will skew the readings. Like nyakavt, I've found the discrepancy to be large enough that it has to be managed.

Wait 30 seconds after placing the sample and closing the cover plate to allow the sample to come to ambient temp (i.e., same temp as the refractometer).  This minimizes/eliminates the impact of sample temperature as the refractive index of a sample is a function of temperature.

I would disagree. You want to close the plate as quickly as possible. In addition to any benefits from minimizing evaporation, more mass in contact with the sample can only help in reducing its temperature.

Interesting observation, Sean--I have never seen that; but, then again, I don't sample the boiling wort (at 212 F).  I only sample the run off from the lauter tun (T <170)  and the chilled wort before transferring to the fermenter (T around 80 F).  I can see how a mixture at 212 F (or slightly higher since it is a sugary solution) could have enough energy to continue to drive off water (phase change from water to steam) which you witness as condensation on the inside aspect of your "hovering" cover plate.  Why is it necessary to sample the SG at full boil?  How do the results change what you do during a brew day?  And if it doesn't, then why do it? 

With regards to the second comment, please re-read my original post.  This is another way of saying it:  After placing the sample and [promptly] closing the cover plate, wait 30 seconds to allow the sample to come to ambient temps.  I sample, spread, and cover in less than 2 seconds.  Is that enough time to realistically cause evaporation to any significant degree?  The hard part for me is waiting 30 seconds to read the refractometer.
Title: Re: ATC Refractometers
Post by: hopfenundmalz on May 27, 2010, 05:54:01 PM
Good discussion and the evaporation may be a thing that has caused variation in my readings.

I have also read somewhere that refractometers are more accurate in natural light.  Had not seen that mentioned, or I missed it.
Title: Re: ATC Refractometers
Post by: Kaiser on May 27, 2010, 06:14:46 PM

I have also read somewhere that refractometers are more accurate in natural light.  Had not seen that mentioned, or I missed it.

I wonder why that would be. Natural light covers a wide range of light colors. If the refraction changes with the color of the light, which it does to some extent, then any light source that has a more narrow color spectrum would lead to a more accurate reading. Given that calibration and measurement are done with the same light.

Could it be that incandescent light is biases toward the red end of the spectrum while natural light is actually “white” light?

One thing to consider is that in my refractometer, I don’t see any significant chromatic aberration which are the colored fringes around edges when using a cheap lens. If you see that it means that the color of the light caused a significant difference in the position of the transitional line that you use to read the refractometer.

Kai
Title: Re: ATC Refractometers
Post by: a10t2 on May 27, 2010, 07:10:48 PM
Why is it necessary to sample the SG at full boil?  How do the results change what you do during a brew day?  And if it doesn't, then why do it?

I check the pre-boil gravity at a rolling boil because I've found that if I don't wait, the reading will be low due to the lower-gravity sparge runoff stratifying. Based on that reading, I'll adjust the length of the boil so that I hit my target OG exactly.

Are you mixing your 170°F wort to avoid stratification, or is that somehow unnecessary in your setup?
Title: Re: ATC Refractometers
Post by: richardt on May 28, 2010, 12:43:59 AM
Brewing coffee isn't that different from brewing wort/beer.  Both use refractometers.  Check out this site and draw your own conclusions.  http://vstapps.com/blog/wp-content/uploads/2010/03/Coffee-and-Refractometers.pdf (http://vstapps.com/blog/wp-content/uploads/2010/03/Coffee-and-Refractometers.pdf)

The measurement technique outlined in that article also recommends to cool the sample before placing it onto the prism. I'd love to copy that section but the Acrobat Reader plug-in I have doesn't let me. WTF.
Kai

The document seems to be password protected--maybe that's why you couldn't copy it.  I tried too--unsuccessfully.
The coffee link also mentions that one should thoroughly mix the coffee before taking a sample.
Title: Re: ATC Refractometers
Post by: richardt on May 28, 2010, 12:46:01 AM
Good discussion and the evaporation may be a thing that has caused variation in my readings.
I have also read somewhere that refractometers are more accurate in natural light.  Had not seen that mentioned, or I missed it.
You are correct.  Refractometers are calibrated with sodium light (yellow-orange, 589.3 nm), and a known [sucrose] solution at 20 degrees Celsius.  From a practical standpoint, natural light and incandescent bulbs are preferable to fluorescent bulbs which possess different color spectra and “warmth” of light depending on the vapor (usually mercury) within the tube and the age of the tube.
Title: Re: ATC Refractometers
Post by: richardt on May 28, 2010, 12:51:46 AM

I have also read somewhere that refractometers are more accurate in natural light.  Had not seen that mentioned, or I missed it.
I wonder why that would be. Natural light covers a wide range of light colors. If the refraction changes with the color of the light, which it does to some extent, then any light source that has a more narrow color spectrum would lead to a more accurate reading. Given that calibration and measurement are done with the same light.
It is true that natural light is a blend of colors.  Given that the red and green cones of our retinas play a more prominent role than the blue cones, it makes sense to “straddle the color fence” and calibrate with yellow-orange sodium light (which lies between the two colors) and minimize any deviation due to chromatic dispersion.  For those of you who wear glasses, you may have had your eye doctor perform a “duochrome test” with Red and Green colors in the background on the eye chart to make sure your glasses prescription wasn’t over- or under- correcting your vision.
Could it be that incandescent light is biases toward the red end of the spectrum while natural light is actually “white” light?
IMO, incandescent light is biased towards the yellow/white, while some, but not all, of the fluorescent bulbs tend to be blue-ish or dim.  But, this could be changing.  The newer compact fluorescent bulbs might not pose as much of a variance as the older tube type fluorescent bulbs can.
One thing to consider is that in my refractometer, I don’t see any significant chromatic aberration which are the colored fringes around edges when using a cheap lens. If you see that it means that the color of the light caused a significant difference in the position of the transitional line that you use to read the refractometer.
Correct.  You should not see chromatic aberration with your refractometer unless something is misaligned inside (so don’t drop it!)
Kai
See my anwers above in bold
Title: Re: ATC Refractometers
Post by: richardt on May 28, 2010, 12:59:29 AM
Why is it necessary to sample the SG at full boil?  How do the results change what you do during a brew day?  And if it doesn't, then why do it?
I check the pre-boil gravity at a rolling boil because I've found that if I don't wait, the reading will be low due to the lower-gravity sparge runoff stratifying. Based on that reading, I'll adjust the length of the boil so that I hit my target OG exactly.
Maybe I’m a little lazy, but I like to keep it simple during the brew day.  I’ve not had a problem with missing my OG target as calculated by BeerSmith (using the 10 gallon Gott cooler/MLT and 20 gallon SS kettle).  I’m more interested in following the recipe to the letter.  Even if it occurred, being off by one or two (or, hypothetically, even five) gravity points doesn’t really bother me.  It’s just homebrew—not the profit margin of a commercial brew pub.
Are you mixing your 170°F wort to avoid stratification, or is that somehow unnecessary in your setup?
My “170 F wort” is the final runnings from the MLT (to make sure I don’t get below 3 Brix / 1.012 SG).  It’s a “clean-catch, mid-stream” sample  ;) obtained with the dropperette!  I simply don’t mix and sample the kettle contents on the way up to boiling, when it is boiling, or when it is chilling.  I only sample the kettle contents after wort chilling is complete.  I put the sanitized thermometer in the kettle and compare it against the waterproof digital thermometer in the output end of the immersion chiller.  When it gets close to each other, it is time to take a sanitized dropperette and take a reading (and I’m fully aware that the wort is stratified or beginning to stratify and that I’m probably drawing off the lower SG sample from the upper ½ of the kettle.  But, that’s just as flawed as transferring my wort to a fermenter and using the pint that remains behind in the trub for my OG reading—as it would likely have a higher SG from being in the lower 1/2 of the kettle).  Anyway, that is the way I do it.
Title: Re: ATC Refractometers
Post by: nyakavt on May 28, 2010, 02:32:13 PM
Richardt, from your last post it sounds like you don't take any measurements at elevated temperatures that are subsequently verified by a cooled post boil sample, or a mash gravity reading that is verified against the laboratory yield of the mash.  So if you do not verify any of your measurements, why are you arguing that you don't need to cool the sample in a sealed container???
Title: Re: ATC Refractometers
Post by: a10t2 on May 28, 2010, 05:28:48 PM
(and I’m fully aware that the wort is stratified or beginning to stratify and that I’m probably drawing off the lower SG sample from the upper ½ of the kettle.  But, that’s just as flawed as transferring my wort to a fermenter and using the pint that remains behind in the trub for my OG reading—as it would likely have a higher SG from being in the lower 1/2 of the kettle).

FWIW I don't think that would be true for either case.
Title: Re: ATC Refractometers
Post by: richardt on May 28, 2010, 08:16:35 PM
(and I’m fully aware that the wort is stratified or beginning to stratify and that I’m probably drawing off the lower SG sample from the upper ½ of the kettle.  But, that’s just as flawed as transferring my wort to a fermenter and using the pint that remains behind in the trub for my OG reading—as it would likely have a higher SG from being in the lower 1/2 of the kettle).

FWIW I don't think that would be true for either case.

I've done OG readings on '"slushy" trub sludge in the past when I didn't leave enough wort behind for a good hydrometer sample--the readings were ridiculously high--had to wait until the sludge settled hours later to get a more accurate reading with the hydrometer.  Even then, not so sure--it still seemed high for the targeted gravity.
Should be easy enough to test if someone felt like experimenting on their next batch (samples from upper 1/2 and bottom 1/2 after wort chill is completed and kettle wort residual+break material after transfer) measuring with both the hydrometer and the refractometer.
Title: Re: ATC Refractometers
Post by: a10t2 on May 28, 2010, 08:31:47 PM
I'll do that when I brew next week. It's a high-gravity beer (~24 Brix) so it should work well to test for even slight variations.
Title: Re: ATC Refractometers
Post by: richardt on May 28, 2010, 08:35:19 PM
Richardt, from your last post it sounds like you don't take any measurements at elevated temperatures that are subsequently verified by a cooled post boil sample, or a mash gravity reading that is verified against the laboratory yield of the mash.  So if you do not verify any of your measurements, why are you arguing that you don't need to cool the sample in a sealed container???
This is a forum to discuss differences in home brewing techniques and methodology, and to learn more about the principles of brewing science.  This discussion is proof of that.

Nyakavt, my reason is simple:  IMO, there is no need to take a HIGH TEMP sample during the boil, so I don’t.  Your statement that I don’t “verify any of my measurements” is misleading.  I calibrate my instruments and use them appropriately—therefore I have high confidence in their results.  If I use the instruments outside of normal operating conditions, I temper/adjust my interpretation of the results accordingly.  IMO, the cover plate on my refractometer is my “sealed container” [see additional discussion below].  OTOH, given the observations of others who do this, if you’re taking a larger sample from boiling wort, you’ll need a larger container and should seal it while it chills to minimize any evaporation.
 
Whether you take the sample at the beginning of the boil or when you‘re done chilling the wort, you still need to know two things to do your efficiency calculations:  wort volume and specific gravity.  A refractometer is just a quicker way to get a SG reading.  See “Efficiency Calculations” on http://en.wikipedia.org/wiki/Plato_scale (http://en.wikipedia.org/wiki/Plato_scale).

I’m suggesting that refractometer (or hydrometer) readings of already chilled wort might be a more sensible time to gather data for efficiency calculations, namely wort volume and specific gravity.  Markings on the brew stick or on the fermenter indicate the wort volume.   If done at that time, sample evaporation and temperature extremes are less of a concern with regards to measurement accuracy.  Still, there remains some inaccuracy which arises from using refractometers to measure mixtures other than pure sucrose (for which they were originally designed).  Nonetheless, the BYO article shows us how we can still use the refractometer with rather good accuracy if certain adjustments are made.

Why take a sample at the time you begin your boil (i.e., when the sample is at 212 F and must be chilled to get a more accurate reading)?  Why make more work for yourself?

My highest temp sample (<170F) is done at lautering to make sure I remain >3 Brix (> 1.012 SG), and not to calculate efficiency.  For purposes of estimating the SG of final runnings, I’m comfortable with a “fuzzier” result or ballpark number.  However, a sample at 212F is nearly 45F hotter.  A10t2 (Sean) says he does it at full boil because he wants to adjust his boil time, if necessary.
  
I did read on a Fresno, California Extension Service site that excessively warm ambient temps (e.g. >68-86F, 20-30C; California Valley) can warm up the refractometer enough to cause falsely elevated Brix readings (Source:  http://cefresno.ucdavis.edu/files/43066.pdf (http://cefresno.ucdavis.edu/files/43066.pdf) ).  According to the site, taking a reading at 50F (10C) can throw off your Brix reading by 0.89 (nearly 4 gravity points higher).  I suspect this is due to the excessive effect of the excessively high or low ambient temperature on the bimetal strip and the optical wedge it holds (the “ATC”, or automatic temperature compensator) within the refractometer and the effect of temperature on refractive index of the sample (RI is inversely related to temperature, i.e., RI decreases as temp increases).  Having said that, one or two drops of hot or warm liquid on the refractometer (and 30 seconds wait) probably does not have as much of a difference, as long as the refractometer temperature is within the ambient range (and preferably on the lower end of it).  I do know the sample is at ambient temp (cool) when I lick it off the refractometer surface after 30 seconds.

IMO, since the dropperette and the refractometer surface act as heat sinks (reducing the temperature closer to ambient temps), the temperature of my sample is well below the boiling point (reducing the amount of evaporation), and that I take two seconds or less to sample/smear/cover (very little opportunity for evaporation as I’ve created my own “sealed container” with the cover plate) with the refractometer, I’m less likely to experience temperature error.  That may not be the case if 212F wort were sampled or the ambient temps during the brew day were really cold or really hot.  Stratification may be a concern if I’m sampling from the boil kettle after chilling and may play a bigger role with darker and more dextrinous or proteinaceous beers, e.g. higher mash temps or wheat beers.  Stratification errors can be minimized by mixing prior to sampling (i.e., during a rolling boil, or immediately after transfer to the fermenter).  I may be making a mountain out of a molehill, but I think it can and does play a role—whether it is negligible or not, I just don’t know.  Some sites I've looked at seem to suggest that it does matter.

I’m a homebrewer, not a commercial brewer--I don’t care if I miss a calculated target by a small amount.  I also don’t care about my “efficiency”—which runs around 75% and sometimes higher.  I do care about following the recipe accurately with regards to the amount of water used, the temps of the rest(s), the times of the various hop additions, etc. so that I can reproduce it again.  I also have no doubts about the veracity of my measurements within the limitations of my instruments and the manner in which I use them.
Title: Re: ATC Refractometers
Post by: Kaiser on May 28, 2010, 10:52:39 PM
Richardt, you make some valid points but none of them disprove the idea that significant evaporation can occur between placing the sample on the refractometer and closing the lid of the wort is sufficiently hot. The question of why to take gravity readings at boil time has nothing to do with that.

Furthermore, there is NO stratification of wort after it has been mixed. I never saw anything like this reported and believe that we would have noticed that by now. If that was to happen it would also happen during the storage of beer and we all know that the first drop from a keg has the same gravity as the last.

Kai
Title: Re: ATC Refractometers
Post by: nyakavt on May 29, 2010, 05:46:32 PM
This is a forum to discuss differences in home brewing techniques and methodology, and to learn more about the principles of brewing science.  This discussion is proof of that.

Please don't take my post as an attempt to keep you from voicing your opinion, that's how we get the best discussions.

Quote
Nyakavt, my reason is simple:  IMO, there is no need to take a HIGH TEMP sample during the boil, so I don’t.  Your statement that I don’t “verify any of my measurements” is misleading.  I calibrate my instruments and use them appropriately—therefore I have high confidence in their results.  If I use the instruments outside of normal operating conditions, I temper/adjust my interpretation of the results accordingly.  IMO, the cover plate on my refractometer is my “sealed container” [see additional discussion below].  OTOH, given the observations of others who do this, if you’re taking a larger sample from boiling wort, you’ll need a larger container and should seal it while it chills to minimize any evaporation.

The purpose of a sample in boiling wort is to ensure that the sugar is evenly mixed throughout, and to be able to adjust the boil time to reach your target gravity.  To me making the beer that I set out to make is important so that I do not need to adjust the bitterness if I am 5 or 10 points off.  The other catch on my system is that I do not have a good way to measure volume post boil, since I use an immersion chiller which distorts the reading on the depth gauge after it goes in and I don't want to worry about sanitizing it.  Volume in the fermenter is no good either since I leave a 1/2 gallon or so behind in the kettle, so even though I would be calculating my 'true' efficiency, I'm more interested in the yield I got from the grains since the number is not dependent on the trub volume in the kettle because of varying amounts of grain or hops.

It's not that I'm trying to hit a specific number for my efficiency, but I want to hit the target gravity for the recipe and this is the best way I know to do it.  The 'verifying your measurements' comment is referring to the fact that you claim quickly closing the lid on the refractometer is 'good enough' for a sealed container, but you have no direct evidence to support this claim.  Every time I've tried this (that I can remember) the number was higher than expected.  In order to know that the 'warm' measurement you are taking is accurate, you need to compare it to some known accurate value, which can be done by comparing a pre-boil and post-boil (and cooled) gravity measurement with accurate volumes for each.

I certainly agree with you that it seems unlikely that a few seconds of exposure to air is enough to cause the change in gravity I am seeing, but nevertheless I cannot just ignore the discrepancy because it seems unlikely.  The surface area to volume ratio of the sample would be much higher than in a normal case of evaporation, whose surface area is only on top of the column of liquid.  It could be a combination of things going on, I really don't know.

Quote
My highest temp sample (<170F) is done at lautering to make sure I remain >3 Brix (> 1.012 SG), and not to calculate efficiency.  For purposes of estimating the SG of final runnings, I’m comfortable with a “fuzzier” result or ballpark number.

I can agree with this, since the number is inconsequential to reaching your target gravity, and any inaccuracy would put you on the safer side of avoiding the 3P mark.

Anyway, I'll be brewing today and will keep in mind some of the points you raised richardt.  Unfortunately it's a low gravity beer so the effects of stratification, if they exist, will be harder to detect.  Either way this does not solve the issue of inaccuracies at warm temperatures, and in my process I do have a need for them to hit the target gravity.
Title: Re: ATC Refractometers
Post by: nyakavt on May 30, 2010, 12:57:38 AM
Update on this, I'm brewing a scottish 70/- with kettle caramelization, so I was in the unique position to leave the wort sitting for a long time before firing the main burner.  The kettle sat still for about 2 hours with 25 liters of runnings, it was not stirred or disturbed other than running wort into the kettle.  After the wait (wort temp was ~125 F), I took a 5 mL sample in a 10 mL syringe, then stirred the kettle well for a minute (you could see the settled protein change the color of the wort) and take another sample from the same spot.  I then put both syringes in the freezer until it had chilled to approx. room temp, about 5 minutes.  I wiped down the outside o the syringes with a dry paper towel and shook them well.   Then I discarded the first 2 mL of sample to avoid any potential contamination by condensate on the outside of the syringe.  The unstirred sample read 6.7 Brix, and the stirred measured 6.8 brix.  This difference is within the resolution of the instrument (0.2 brix).  I did know which one I was sampling, so there is a potential for 'researcher bias' in interpreting the readings.   

This is only one observation and the wort is very low gravity, so it may not show stratification as well as another wort.  But the difference here is not enough to account for the difference I'm seeing when taking a warm sample.
Title: Re: ATC Refractometers
Post by: nyakavt on May 30, 2010, 05:20:19 AM
Yeesh, long day.  Ok here are the other measurements for this brew session.

First I took a pre-boil gravity by drawing a sample from the boiling wort with a syringe.  This time I put a drop on the lens and closed it almost immediately, probably 1/2 a second lapse (this is by far faster than I've ever done it before).  This reading was 9.3 brix.  I then placed that syringe in the freezer until it was chilled and it read 9.2 brix, again inside the instrument resolution.  So it appears that quickly closing the lens will get you very close to a chilled sample.  I also took and chilled a sample before the wort got to a boil which came in at 9.1 brix, but the wort boiled for a short amount of time before I could get to it so this is not directly comparable.  Still, very close.

The final test of the day was the top-bottom measurement.  After chilling and whirlpool the kettle was allowed to rest for 20 minutes.  Then I took a sample from the top of the wort and ran 1/2 liter into the fermenter and took a sample from that, effectively the bottom of the wort.  Here the top sample read 9.9 brix and the bottom read 9.8.  Oddly the bottom was the lower reading, but again this is inside the instrument resolution so it's probably just drift. 

So this demonstrates to me that on a wort of this gravity, stratification is not a concern, and quickly covering the lens gets you close enough to a sealed container reading.  I am going to continue testing on my next few batches to see if the trend holds.  In the long run, I will still prefer to chill the sample prior to reading, at least of boiling wort, to preserve the instrument as long as possible.
Title: Re: ATC Refractometers
Post by: Kaiser on May 30, 2010, 01:03:13 PM
Nyakavat,

Thanks for testing some of the theories. I assume you did not do a test where you kept the hop sample on the prisim w/o closing it immediately to see if we need to concern about evaporation.

I think stratification of run-off depends on how you collect the wort. If you have a hose that reaches all the way down to the bottom of the kettle you may get sufficient mixing action.

Kai
Title: Re: ATC Refractometers
Post by: nyakavt on May 30, 2010, 04:11:01 PM
Nyakavat,

Thanks for testing some of the theories. I assume you did not do a test where you kept the hop sample on the prisim w/o closing it immediately to see if we need to concern about evaporation.

No I forgot about this one, it will have to wait for next week.

Quote
I think stratification of run-off depends on how you collect the wort. If you have a hose that reaches all the way down to the bottom of the kettle you may get sufficient mixing action.

That's how it was, the hose was under the liquid level so I imagine it was stirring up the first runnings evenly well with the second.
Title: Re: ATC Refractometers
Post by: tygo on May 31, 2010, 05:35:14 PM
Ran the new refractometer through its first trials on my batch today.  I used it to measure the first and second runnings, and total boil gravity.  I also took a sample of the boiling wort halfway through the boil.  I took readings with my hydrometer as well for everything except for the boiling sample.  The boiling sample I dumped into a small glass jar and quickly capped then popped in the freezer for 5 minutes.

For all of the readings where I used both the refractometer and hydrometer the refractometer read 2-3 gravity points less than the hydrometer.  These were relatively hot samples (130F) where I'm factoring in a temperature adjustment for the hydrometer reading.

Overall a pretty neat tool but I think I need to run a few more batches using both tools to see if I can rely solely on the refractometer for the mash readings.
Title: Re: ATC Refractometers
Post by: richardt on June 03, 2010, 03:06:36 AM
Richardt, you make some valid points but none of them disprove the idea that significant evaporation can occur between placing the sample on the refractometer and closing the lid of the wort is sufficiently hot.
You are correct--and as Nyakavt pointed out, given the surface to volume ratio, there may be significant evaporation despite the short transfer time.  I just don't know how to prove or disprove it with what I've got laying around the house.  My knowledge of the evaporative process tells me that there must be energy put into the fluid (i.e., heat) or a reduction in surface tension or atmospheric pressure (i.e., agitation, air moving across the surface, or application of a vacuum).  None of those situations seem to be occuring.  And the quick application of the cover plate should negate any contribution towards evaporation that the change in surface to volume ratio may have.  But that's just my hypothesis--I could be wrong.  But, to be fair, if someone has the hypothesis that significant evaporation is occuring, let's try to support that claim experientially before we dogmatically say that it is indeed what is happening.

Quote
The question of why to take gravity readings at boil time has nothing to do with that.
I agree.  I explained my logic for low temp gravity readings earlier.  To summarize, IMO, it induces less temperature-associated artifacts with measurements (e.g, wort volume estimates, hydrometer and/or refractometer readings).

Quote
Furthermore, there is NO stratification of wort after it has been mixed. I never saw anything like this reported and believe that we would have noticed that by now. If that was to happen it would also happen during the storage of beer and we all know that the first drop from a keg has the same gravity as the last.
Kai

I disagree.  The only time stratification does not take place is in a true solution that is fully miscible.  A BMC light/diet/low carb lager product that has been fully fermented, devoid of proteins and dextrins as a result of low mash temps, adequate time in fermentation and conditioning tanks, followed by crash cooling in settling tanks, addition of fining agents, and use of filters, then... yeah, you'd probably be right.  Barring that, Wort and nearly all ales qualify as colloids, and therefore have stratification if given enough time and barring any agitation of the fluid (e.g., boiling, stirring, transferring from primary to secondary to bottling bucket, pouring from a bottle to a glass) or clarification/filtration.

Does anyone have experience with "Brew Balls"?  They're HDPE balls (HDPE SG = 0.94) with different weights w/in them so when the SG gets below the number on the ball, it slowly sinks below the surface.  I would be interested to know if it gradually rides the SG strata down over time, rather than entering a "free fall" and striking the bottom of the bucket or carboy with force like a rock dropped in a pool would if the wort were truly homogeneous.
Title: Re: ATC Refractometers
Post by: Kaiser on June 03, 2010, 03:43:19 PM
I disagree.  The only time stratification does not take place is in a true solution that is fully miscible.  A BMC light/diet/low carb lager product that has been fully fermented, devoid of proteins and dextrins as a result of low mash temps, adequate time in fermentation and conditioning tanks, followed by crash cooling in settling tanks, addition of fining agents, and use of filters, then... yeah, you'd probably be right.  Barring that, Wort and nearly all ales qualify as colloids, and therefore have stratification if given enough time and barring any agitation of the fluid (e.g., boiling, stirring, transferring from primary to secondary to bottling bucket, pouring from a bottle to a glass) or clarification/filtration.
]

What it comes down to is stratification that can be measured by our means of gravity measurements. I do agree that there are substances in wort and beer that will settle out over time. But the resulting change in gravity and the time it takes for that to happen cannot be used to explain the differences in gravity measurements that started this discussion. The majority of the beer and wort solid are truly dissolved and won’t stratify over time.

Kai



Title: Re: ATC Refractometers
Post by: richardt on June 03, 2010, 05:41:10 PM
In the interest of facilitating discussion, I’ve excerpted and condensed part of a wiki link below:

Total Dissolved Solids (often abbreviated TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized or micro-granular (colloidal sol) suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve the size of two micrometer.
 
Total dissolved solids are differentiated from total suspended solids (TSS), in that the latter cannot pass through a sieve of two micrometers and yet are indefinitely suspended in solution.

The term "settleable solids" refers to material of any size that will not remain suspended or dissolved in a holding tank not subject to motion, and excludes both TDS and TSS.[1] Settleable solids may include larger particulate matter or insoluble molecules.”
 
[source: http://en.wikipedia.org/wiki/Total_dissolved_solids (http://en.wikipedia.org/wiki/Total_dissolved_solids)]

10-20 micron filters are used for trap filtration (high solids capacity) while 0.45 micron filters are used to filter spoilage organisms, yeast and inert particulate matter.  The result is a bright (and stripped) beer.

Wort is a colloid, not a true solution.  This property affects our measurements, whether we use the hydrometer or the refractometer, as both instruments are calibrated to a true solution [sucrose and water], not a colloid.
Wort contains dissolved ions and solids, as well as suspended solids and settleable solids.  The collective densities of all of these in a thoroughly mixed liquid will affect the hydrometer reading.  Hypothetically, if the settleable solids are given enough time to settle and no mixing occurs, the SG could vary if samples were taken from different locations within the tank (probably negligible for 5 gal bucket compared to a 10 bbl tank).
 
The Tyndall effect (the scattering of light by colloidal sized particles) could easily be causing us to misread our refractometers as it would cause the blue line to read higher when the wort is more turbid (due to being thoroughly mixed, i.e., during a rolling boil).  This could be causing the refractometer to read higher than the hydrometer for the same sample.  See figure 7.14 in the link below for an illustration of that.  As I learn more about how a refractometer works and realize that a colloid scatters light while a solution does not, it makes even more sense why the scale reads higher for a refractometer.  Give it some thought and let me know whether or not you would consider this a possibility.  I think it is far more likely to be the cause than “evaporation.”

Evaporation theory is being tested to see if it has any significance at the homebrewing level (thank you tygo, nyakavt, and a10t2).

http://www.tutorvista.com/content/chemistry/chemistry-iv/surface-chemistry/colloidal-sols-properties.php
 (http://www.tutorvista.com/content/chemistry/chemistry-iv/surface-chemistry/colloidal-sols-properties.php)
Title: Re: ATC Refractometers
Post by: Kaiser on June 03, 2010, 07:06:30 PM
I don’t think light scattering can be used to explain a shift in the “blue line”. Since scattering doesn’t have a preferred direction it would cause the line to blur and not to shift. In addition to that the refractometers we are using are working on the principle of total internal reflection. This means the blue line we are seeing is the transition between being able to see through the prism and the wort (white) and being able to see the reflection of the blue prism background (blue). All that matters for this is the RI difference at the contact surface between the prism and the sample.  Suspended solids that settle on the prism can blur the transitional line since they create small areas of different RI differences.

There are refractonmeters that measure the RI by shining a beam of light through the sample. Those are more sensitive to sample clarity.

Kai

Title: Re: ATC Refractometers
Post by: richardt on June 03, 2010, 07:43:49 PM
http://www.grapestompers.com/refractometer_use.asp (http://www.grapestompers.com/refractometer_use.asp)

My refractometer looks and behaves like the one in this link.  Look at the photos and the examples and then consider the following.
The sample plate is blue, the cover plate is clear.  The wort is placed between the two.
Since all light passing through the refractometer is "filtered" by the blue lens, shouldn't the blue light indicate the prism and the wort [rather than the white as you've described above]?
If so, the white is simply the color of the reticule with no light falling on it as observed by us looking through the objective lens of the refractometer.

When calibrated with 1.000 SG distilled water and using a hypothetical 1.000 SG prism it will be calibrated to 0 since no deviation (refraction of light) occurs.
When a 1.040 SG sample (10 Brix) is placed on the sample plate and the cover plate applied, the light is refracted towards normal by the sample and then refracted away from normal when it passes from the sample into the prism.  This results in upward deviation of the blue line (which, I agree, is a little blurred rather than crisp).

There are some refractometer designs that use prisms with complex shapes (e.g., trapezoids and rhomboids) that utilize the principle of critical and total internal reflection, but I don't think that principle is what causes the blue line we see on the refractometer to move.
Title: Re: ATC Refractometers
Post by: a10t2 on June 03, 2010, 09:34:25 PM
I did some playing around during my brew day yesterday. First I took some pre-boil readings (actually about three minutes after the boil started and I was confident I didn't need to babysit it to avoid a boilover).

The first sample I drew was treated normally, which means I pulled ~2.5 mL using a syringe, and then immediately inverted it into a glass of ice water, so that only the tip remained in the air. It sat there while I took the rest of the readings, which ended up being about ten minutes. The first mL or so was discarded and the sample taken from the middle of the syringe. The rest of the samples were pulled with the cheap plastic dropper that came with the refractometer. In each case I placed three drops on the sample plate, waited a given amount of time (by watching a digital clock), then closed the plate, waited 30 seconds, and took a reading looking through the refractometer at the (overcast) sky. The plate and cover were then wiped dry with a paper towel.

RI in Brix (no wort correction applied)
chilled - 19.9 °Bx
<1 s - 20.2 °Bx
10 s - 20.7 °Bx
20 s - 20.9 °Bx
30 s - 21.1 °Bx
60 s - 21.0 °Bx
90 s - 21.1 °Bx
120 s - 21.1 °Bx

This compares pretty well to a refractometer sample that was covered with plastic wrap and chilled in an ice water bath to 20°C, then shaken to make sure it was uniform. That sample read 20.0 °Bx (uncorrected) and 1.079 using the hydrometer - a difference of about 0.0013 SG assuming a "wort correction factor" of 1.04.

Post-boil, I did my normal whirlpool/chill, which got the wort down to 18°C in 28 minutes. Then I killed the pump and let it settle for 36 minutes while I did some cleanup. A sample from the top of the kettle read 25.3 °Bx. I ran off about a quart into the fermenter to make sure the lines were clear, then pulled a sample from the stream, which also read 25.3 °Bx.
Title: Re: ATC Refractometers
Post by: Kaiser on June 03, 2010, 10:45:29 PM
Sean,

Thanks for taking the time to run these experiments and paying attention to factors that may reduce their value. One thing that may still have happened is a slight warming of the prism though the number of samples that were taken. However, this should have the effect that the sample reads a lower RI unless the RI of the glass changes more than the RI of the wort. I don't think that this is the case though.

Kai
Title: Re: ATC Refractometers
Post by: Kaiser on June 04, 2010, 12:19:18 AM
I have looked into the workings of those hand held refractometers as soon as I got mine. They do work on the principle of total internal reflection which simplifies a lot of things. For one sample clarity matters little. In addition to that the cover plate does not have to be perfectly parallel  to the prism surface and you also don’t need a light source that produces perfectly parallel light.

Here is some info on total internal reflection (http://en.wikipedia.org/wiki/Total_internal_reflection) which saves me some writing. When the RI of the sample changes the critical angle changes and with it the point at which the light will no more travel through the sample but will come from the blue background of the prism. In the illustration below this happens at the middle. A lens projects this light through a graduated scale and another lens allows you to see that scale and the projection. This is why you see a transition from white to blue.

I showed the rays of light that travel through the sample and are not internally reflected in orange while the internally reflected rays are shown in blue.  You can see how the angle between the ray of light and the prism surface gets progressively smaller as the ray hits the prism/sample transition closer to the tip of the prisim.  If you have particles lying on that transition they will scatter the light and cause a more blurred reading.

The refractometer is calibrated by shifting the 1st lens up and down (I believe that’s how it is done but I haven’t opened the refractometer it that far). The ATC function is done by a bimetal strip which moves the scale up and down depending on its temperature which needs to be the same as the prism and the sample for it to work.

(http://braukaiser.com/images/misc_forum/refractometer_workings.gif)

Kai
Title: Re: ATC Refractometers
Post by: richardt on June 04, 2010, 04:09:53 PM
I don’t normally quote myself, but my earlier comments need correction or clarification.

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The sample plate is blue, the cover plate is clear.  The wort is placed between the two.
Since all light passing through the refractometer is "filtered" by the blue lens, shouldn't the blue light indicate the prism and the wort [rather than the white as you've described above]?
If so, the white is simply the color of the reticule with no light falling on it as observed by us looking through the objective lens of the refractometer.


I am mistaken.   I took a good look at my refractometer with a bright light and realized that the prism itself is clear glass and that the lumen (the inside) of the refractometer is painted blue!  I now believe that Kai is correct in saying that the white light is the light that gets refracted by the sample and the prism.  See further explanation below.

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When calibrated with 1.000 SG distilled water and using a hypothetical 1.000 SG prism it will be calibrated to 0 since no deviation (refraction of light) occurs.
When a 1.040 SG sample (10 Brix) is placed on the sample plate and the cover plate applied, the light is refracted towards normal by the sample and then refracted away from normal when it passes from the sample into the prism.  This results in upward deviation of the blue line (which, I agree, is a little blurred rather than crisp).

Again, I am incorrect.  It turns out that the prism must always have a higher index of refraction than the sample.  As light passes from the sample into the prism, it is refracted (“bent”) towards "Normal" (defined as a perpendicular line that bisects the interface between two optically different media where the incident ray hits the interface).  The angles of incidence and refraction are drawn in reference to Normal, not the interface. 

As the refractive index of a sample increases (as would be the case with higher SG wort), there is less of a difference between it and the refractive index of the prism.  Therefore, the light would be refracted less.  This would cause the white “illuminated” portion of the reticule to increase (the ‘blue/white” interface line moves up).  As the SG of the wort declines, the refractive index of the wort sample declines (but the difference in refractive index between the sample and the prism increases results in a greater amount of refraction of the light.)  This results in the “blue/white” interface line moving down as the illuminated portion of the reticule decreases. 

Taking this concept further, as one approaches the refractive index of distilled water, the blue/white interface line reaches zero.  Given that air has a much lower index of refraction than water, the difference in refractive index is even greater, and we see no illumination of the reticule whatsoever because one of two things must be happening:  either the critical angle has been exceeded and white light cannot exit the back side of the prism, or the amount of refraction is so great that white light cannot exit the back of the prism (look carefully at the prism, the exit side is pretty small and situated quite high up on the prism.  White light will be refracted too far downward.)
 
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There are some refractometer designs that use prisms with complex shapes (e.g., trapezoids and rhomboids) that utilize the principle of critical and total internal reflection, but I don't think that principle is what causes the blue line we see on the refractometer to move.

I should clarify this.  The concept of Critical Angles and Total Internal Reflection (TIR) most likely do impact how the refractometer works, particularly in explaining why we see the color blue on the reticule.  For angles that are less than “critical” some light is reflected and some is refracted out of the prism (all 3 sides apply).  For angles that are greater than “critical,” TIR occurs (and the light is reflected). 

I presume that the blue color comes from refracted white light that had previously entered the prism, then exited the base of the prism, struck the blue paint [that lines the lumen of the refractometer], which absorbed all the colors of the white light while reflecting the blue color back into the prism), and that TIR occurs between the front and bottom sides of the prism until it exits the rear side of the prism and casts its blue light upon the reticule. 

Unfortunately, I cannot find a good, detailed schematic online for a good hand held refractometer, nor do I possess CAD skills.  As I roughly estimate the angles of my prism, the nose end seems to be around 15 degrees, the top back end seems to be around 30 degrees, and the bottom back end seems to be around 135 degrees.  Does anyone have a good explanation of how a refractometer really works? 
Title: Re: ATC Refractometers
Post by: Kaiser on June 04, 2010, 05:56:39 PM
Does anyone have a good explanation of how a refractometer really works? 

Yes, it is in my last post.

I got your PM and you're correct that the angles are measured against normal but that does not change how the refractometer works. I have not seen a good schematic on-line either.

BTW, why do you need to repeat what I already stated earlier? In particular why we see blue and white. You don't have to presume that the blue light comes from the blue background  it does.

Kai