Measurement Issues

During my last 2 brews I've had some inconsistencies in measurements i've seen between my refactometer pre-boil, post boil, and my hydrometers. So I decided to perform a 2-point calibration check on all my equipment.

I used distilled water as a base for both samples. The first sample was straight distilled water. The second sample was a 12% w/w solution - 36g sucrose in 264g distilled water (a.k.a 12 Brix/1.048 SG) of sucrose and distilled water. The 12% solution was prepared on scale that weighs to 0.01g and was calibrated immediately prior to preparing the sample. All hydrometer tests performed at 60F. Refractometer tests performed at room temp (72F).

Measurement results as follows:

Hydrometer 1 (60F/60F) - very old

• Distilled Sample - 1.006
• 12% w/w Sample - 1.053

Hydrometer 2 (60F/60F) - new

• Distilled Sample - 1.002
• 12% w/w Sample - 1.050

Refractometer w/ ATC (scale in Brix)

• Distilled Sample - 0 Brix
• 12% w/w Sample - 11.6 Brix

So the conclusion I come to is that both of my hydrometers have shifted, the newest by 2 points, and the oldest by 5-6. I am comfortable that can be explained. But I am more confused by the refractometer reading though. How can it be explained that it is spot on at 0, but off by 0.4 Brix at this low of a measurement? The only calibration option for the refractometer shifts the scale up or down.

I'm further confused by my last brew where i measured 11.2 Brix pre-boil, 12 Brix post boil, and only 1.042 in the Hydrometer. That's not even in the ball park. I can't explain that at all.

Any thoughts?

Sucrose is used in these sugar scales as it is easy to refine and more immune to moisture than the other sugars but nevertheless for precise work the sucrose should be dried at 102 °C for an hour or so before weighing.

Your new hydrometer appears to have a bias error of 2 points. Perhaps you are reading the wrong part of the meniscus (top vs base)? Is it a precision (narrow range) hydrometer? If not is it really possible to read it to 1 point?

As to the refractometer: It is not at all unusual to see a 1 - 2 Bx error in the measurement of wort as compared to a precision hydrometer. That's why my brewing refractometer is in a drawer. Add to that the fact that yours appears to be off by 0.4 Bx against a calibrating solution and therein lies the probable explanation. As to why is is so far off against the calibrating solution the possibilities lie in the ATC or that this particular instrument is faulty. Keep fiddling. Make up another test solution and try again. Sometimes it takes a while to develop ones lab techniques to the point where he has confidence in the numbers he is getting. Get a set of precision hydrometers (you'll be glad you have them) and use that, rather than weighings, to determine what the °P of your test solutions. It is possible to read them to better than 0.1 °P. Remember that the strength of your solution starts increasing the instant you make it up as water is evaporating all the time. I have found that it is best to weigh the sugar and water into a tared volumetric flask (which can be sealed to minimize evaporation loss and loss from spatter of solution onto the side of a beaker etc.).

ajdelange said:

Sucrose is used in these sugar scales as it is easy to refine and more immune to moisture than the other sugars but nevertheless for precise work the sucrose should be dried at 102 °C for an hour or so before weighing.

Your new hydrometer appears to have a bias error of 2 points. Perhaps you are reading the wrong part of the meniscus (top vs base)? Is it a precision (narrow range) hydrometer? If not is it really possible to read it to 1 point?

As to the refractometer: It is not at all unusual to see a 1 - 2 Bx error in the measurement of wort as compared to a precision hydrometer. That's why my brewing refractometer is in a drawer. Add to that the fact that yours appears to be off by 0.4 Bx against a calibrating solution and therein lies the probable explanation. As to why is is so far off against the calibrating solution the possibilities lie in the ATC or that this particular instrument is faulty. Keep fiddling. Make up another test solution and try again. Sometimes it takes a while to develop ones lab techniques to the point where he has confidence in the numbers he is getting. Get a set of precision hydrometers (you'll be glad you have them) and use that, rather than weighings, to determine what the °P of your test solutions. It is possible to read them to better than 0.1 °P. Remember that the strength of your solution starts increasing the instant you make it up as water is evaporating all the time. I have found that it is best to weigh the sugar and water into a tared volumetric flask (which can be sealed to minimize evaporation loss and loss from spatter of solution onto the side of a beaker etc.).

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Thanks for the input. Narrow range hydrometers on are on my to-buy list. I only use the refract for a ballpark figure of pre-boil gravity to catch if there are any gross errors on gravity. I am well aware of its limitations with respect to different sugar ratios.

I was under the impression that sucrose didn't absorb much moisture from the air, but its easy enough for me to re-do the tests with a dried sample. I'll give that a shot.

Your impression is correct. The fact that it does indeed adsorb a bit of moisture is not enough to explain your error but a real scientist would dry everything he weighs.

ajdelange said:

Get a set of precision hydrometers (you'll be glad you have them) and use that, rather than weighings, to determine what the °P of your test solutions. It is possible to read them to better than 0.1 °P.

Click to expand...

I thought I remembered one of your posts in another thread where you indicated you don't trust a hydrometer reading unless the hydrometer has been calibrated. I can't find the post I'm thinking of, so maybe I'm remembering wrong. From this post it seems that you trust a precision hydrometer to be dead on. Is this correct? If so, would that also apply to a bottling hydrometer that's not advertised as "precision", but rather simply "for precise monitoring" of finishing gravity?

ncbrewer said:

I thought I remembered one of your posts in another thread where you indicated you don't trust a hydrometer reading unless the hydrometer has been calibrated. I can't find the post I'm thinking of, so maybe I'm remembering wrong.

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You are probably remembering correctly. I don't really trust any instrument unless it has been calibrated and hydrometers are no exception. But there is calibration and calibration. We all have readily available to us a substance of known specific gravity and that is deionized water. Thus we can easily calibrate one point on the SG scale thus getting the offset or intercept. The problem is to find something else from which we get another point on the scale and thus the slope.

ncbrewer said:

From this post it seems that you trust a precision hydrometer to be dead on. Is this correct?

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No, not dead on but there are some other considerations here. If you had 3 volt meters, one that you got on sale at Radio Shack and two others from Fluke one of which had a current metrology sticker on it which would you be most confident in? The point being that precision hydrometers are intended to be used for precision measurements and should be sold with some sort of calibration certificate and should have some specified accuracy whereas the plastic full range hydrometer that came with you first home brewing kit has no calibration certificate nor is any information about accuracy supplied with it. As with the volt meters, I would have more confidence in the precision hydrometers than the typical full range one given solid specs. This confidence is bolstered by the fact that comparisons I have done between precision hydrometers and digital densitometers (which are calibrated against dry air and DI water) have all made the precision hydrometers look darn good,

ncbrewer said:

If so, would that also apply to a bottling hydrometer that's not advertised as "precision", but rather simply "for precise monitoring" of finishing gravity?

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This could be the place to insert stuff about the difference between precision and accuracy but let's not do that. The physics behind a narrow range hydrometer are simple enough that making the scale is easy enough to do and it is then a matter of putting it at the right position in the stem. How they do that I don't know (comparison with a digital density meter perhaps) but here accuracy is definitely enhanced by the higher precision.

AJ,

Do you have any specific suggestions for a precision lab grade hydrometer? Looks like northern brewer sells a set of 3 narrow range hydrometers for $30.

Great answers AJ. Thanks.

schematix said:

AJ,

Do you have any specific suggestions for a precision lab grade hydrometer? Looks like northern brewer sells a set of 3 narrow range hydrometers for $30.

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No, sorry, I don't. I bought a set many years ago from a brew supply shop in one of the Carolinas (I don't remember which) and was smart enough at the time to get 2 of each so while there have been mishaps I still have a complete set of 3. Were I in VA I could go look at them to see if there is any manufacturer info on them but I'm not.

How convenient. LHBS carried narrow range hydrometers with built in thermometer for temperature compensation. All with NIST certificate. I tested 2 solutions in the cross over ranges (plus DI water) and everything was dead on.

View attachment ImageUploadedByHome Brew1470605059.241559.jpg

I'm a sucker for nice tools so I got all 3 and a glass hydrometer tube.

Love at first sight.