Author Topic: ATC Refractometers  (Read 7347 times)

Online Kaiser

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Re: ATC Refractometers
« Reply #15 on: May 25, 2010, 07:00:12 AM »
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

Offline a10t2

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Re: ATC Refractometers
« Reply #16 on: May 25, 2010, 08:37:31 AM »
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.
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Online Kaiser

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Re: ATC Refractometers
« Reply #17 on: May 25, 2010, 09:55:57 AM »
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

Offline richardt

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Re: ATC Refractometers
« Reply #18 on: May 26, 2010, 03:21:35 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.

Offline richardt

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Re: ATC Refractometers
« Reply #19 on: May 26, 2010, 03:23:30 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.
« Last Edit: May 27, 2010, 08:10:19 AM by richardt »

Offline a10t2

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Re: ATC Refractometers
« Reply #20 on: May 26, 2010, 04:29:18 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.
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Offline nyakavt

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Re: ATC Refractometers
« Reply #21 on: May 27, 2010, 06:08:40 AM »
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.

Online Kaiser

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Re: ATC Refractometers
« Reply #22 on: May 27, 2010, 07:26:36 AM »
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

Offline tygo

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Re: ATC Refractometers
« Reply #23 on: May 27, 2010, 07:48:36 AM »
So you just dip out a sample with the test tube, cap it, and chill it down somewhat?
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Offline richardt

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Re: ATC Refractometers
« Reply #24 on: May 27, 2010, 08:00:56 AM »
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

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

And this site has a nice schematic of the internal workings of an ATC refractometer:  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.


Offline a10t2

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Re: ATC Refractometers
« Reply #25 on: May 27, 2010, 08:48:46 AM »
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.
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Online Kaiser

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Re: ATC Refractometers
« Reply #26 on: May 27, 2010, 09:07:18 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

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


Offline bluesman

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Re: ATC Refractometers
« Reply #27 on: May 27, 2010, 09:43:59 AM »
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.
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Offline richardt

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Re: ATC Refractometers
« Reply #28 on: May 27, 2010, 10:40:17 AM »
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.

Online hopfenundmalz

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Re: ATC Refractometers
« Reply #29 on: May 27, 2010, 10:54:01 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.
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