Author Topic: ATC Refractometers  (Read 10399 times)

Offline Kaiser

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Re: ATC Refractometers
« Reply #45 on: May 30, 2010, 06:03:13 AM »
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

Offline nyakavt

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Re: ATC Refractometers
« Reply #46 on: May 30, 2010, 09:11:01 AM »
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.

Offline tygo

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Re: ATC Refractometers
« Reply #47 on: May 31, 2010, 10:35:14 AM »
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.
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Offline richardt

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Re: ATC Refractometers
« Reply #48 on: June 02, 2010, 08:06:36 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.
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.

Offline Kaiser

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Re: ATC Refractometers
« Reply #49 on: June 03, 2010, 08:43:19 AM »
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




Offline richardt

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Re: ATC Refractometers
« Reply #50 on: June 03, 2010, 10:41:10 AM »
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]

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
« Last Edit: June 03, 2010, 10:43:35 AM by richardt »

Offline Kaiser

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Re: ATC Refractometers
« Reply #51 on: June 03, 2010, 12:06:30 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


Offline richardt

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Re: ATC Refractometers
« Reply #52 on: June 03, 2010, 12:43:49 PM »
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.

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Re: ATC Refractometers
« Reply #53 on: June 03, 2010, 02:34:25 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.
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Offline Kaiser

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Re: ATC Refractometers
« Reply #54 on: June 03, 2010, 03:45:29 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

Offline Kaiser

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Re: ATC Refractometers
« Reply #55 on: June 03, 2010, 05:19:18 PM »
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.



Kai
« Last Edit: June 03, 2010, 05:22:17 PM by Kaiser »

Offline richardt

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Re: ATC Refractometers
« Reply #56 on: June 04, 2010, 09:09:53 AM »
I don’t normally quote myself, but my earlier comments need correction or clarification.

Quote
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.

Quote
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.)
 
Quote
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? 

Offline Kaiser

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Re: ATC Refractometers
« Reply #57 on: June 04, 2010, 10:56:39 AM »
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