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Old 09-13-2011, 08:54 PM   #1
deanocamino
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Default reading a hydrometer and hydrometer vs. refractometer

I have recently read we should read from the highest water line. Totally not what I have been doing for the longest time.

Also wonder what is there pros and cons using a refractometer vs. a hydrometer?

thanks

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Old 09-13-2011, 09:50 PM   #2
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A hydrometer should be read according to the manufacturers directions. Some instruct that it be it be read at the base, some at the top of the liquid run up on the stem.

A refractometer is a sometime thing. It is certainly easier to use and faster, you are less likely to drop it and it is a snap to clean. But it doesn't always agree with a hydrometer. If you are thinking of transitioning to a refractometer you should make many comparative readings between refractometer and hydrometer to determine the conditions under which the agreement is acceptable to you (and acceptability is an individual determination).

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Old 09-14-2011, 02:56 AM   #3
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I calibrated my hydrometers using known percents of sugar in water, and carefully created a calibration curve for each one. After using it a couple times I realized that reading at the top of the meniscus instead of the bottom as instructed by the manufacturer was the same as using the curve, for one of my 2 hydrometers. This won't work for yours, but the point I am trying to make is regardless of the manufacturer's instructions, calibrating your hydrometer is a good idea.

I did the same with my refractometer and happily it was right on from 0 to 20 B.

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Old 09-14-2011, 03:49 AM   #4
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Quote:
Originally Posted by ajdelange View Post
some at the top of the liquid run up on the stem.
Really? That obviously impairs accuracy, and from what I remember from chemistry class, is wholly non-standard. 100 ml measuring glasses for instance, are always calibrated to be read from the water surface base.
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Old 09-14-2011, 06:01 AM   #5
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My Hydrometer directions say read from the upper Meniscus. It's from France. Since I'm making beer for my personal enjoyment I'm not too worried if it's upper or lower as long as I read it consistently.

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Old 09-14-2011, 11:41 AM   #6
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Hydrometers that are read from the top of the meniscus are usually for specialist fluids of known and constant surface tension, such as milk; fluids in which you cannot reliably see the baseline through the fluid. Okay, I suppose, as long as the surface tension of the fluids are comparable.

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Old 09-14-2011, 12:00 PM   #7
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Originally Posted by G-Wheeler View Post
Hydrometers that are read from the top of the meniscus are usually for specialist fluids of known and constant surface tension, such as milk...
Or wort. Follow the manufacturers directions.

Also - making accurate sucrose solutions for calibration purposes is not so simple as it might at first seem. Tare a volumetric flask, add 1 gram of sugar, add enough water to make up to 10 grams, stopper and agitate. That will get you close to 10 °P but not spot on. To get spot on you would have to have lab grade sucrose, dry it in an oven, cool it in a dessicator and weigh it out somewhere other than Northern Virginia (where I live - the humidity is terrible). What I do is skip the oven and dessicator and make a solution of approximately 10 °P whose density I check with a more accurate instrument. Anyway, the "sensititivity" of a hydrometer (change in amount of stem out of the liquid per unit change of density) is wholly determined by the weight and geometry of the instrument so all you really need to do is check the 0 in distilled water. Note that the instrument will not read 0 in distilled water if it is calibrated for beer as the best brewers hydrometers are. This is that surface tension thing again.

Finally, a refractometer calibrated against sucrose solutions of known strength is great for measuring sucrose solutions (that what they are sold for) and will give an approximate reading in wort most of the time. But I've seen them read off by as much as 1 °P (1 Bx which, by the way, now a deprecated unit) in wort and of course as we all know they are way off in beer (i.e. once alcohol becomes a factor).
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Old 09-14-2011, 11:32 PM   #8
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Sounds like a lot of fuss for .001 accuracy in you measurements. I'd be willing to bet that actual wort temp vs. calibrated liquid temp of the hydrometer would throw your accuracy off more than the point on the meniscus at which you read. I just do it the same and call that good.

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Old 09-15-2011, 01:22 AM   #9
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aj,

Based on your knowledge and approach to these questions, I have to believe you were trained as a physical chemist. I am a simple synthetic organic chemist (i.e. a cook) and the way you poop out clear, correct and scientifically sound answers to all manner of questions here constantly impresses me.

I will continue to toss out my half-arsed answers to questions here, waiting eagerly for you to gently fill in the blanks, nudge me off the guardrails of chemistry, and complete the answer.

To illustrate, I did my calibration solutions on a lab balance I bought off eBay, good to 0.1 gram, calibrated with a set of weights from the lab at work, didn't bother to dry the sugar, and was happy to have made close enough calibration solutions that showed good agreement between my two hydrometers and one LHBS-sourced refractometer. I used RO water because I had some, to check 0 and since have found that my tap water is close enough to keep me happy.

Good enough for this old cook...

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Old 09-15-2011, 01:35 PM   #10
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Quote:
Originally Posted by aschettler View Post
Sounds like a lot of fuss for .001 accuracy in you measurements. I'd be willing to bet that actual wort temp vs. calibrated liquid temp of the hydrometer would throw your accuracy off more than the point on the meniscus at which you read. I just do it the same and call that good.
Certainly for the majority of home brewers 0.001 accuracy is more than enough but some of us are the geeky types. We own narrow range hydrometer sets that are easily read to 0.1 °P and with a little care to 0.05 °P which corresponds to 0.0002 SG. The lowest range member of the set starts at 0 °P and can, thus, be calibrated with DI water. The others do not start at 0 and thus must be calibrated against something other than water. I'm just pointing out that it is difficult to control the strength of a sucrose solution to 0.05 °P and that the only practical way to check such hydrometers is with solutions of approximately known strength against a more accurate means of measurement such as pycnometry or U-tube densitometry. The former is feasible for a home brewer who is interested in this (i.e. the lab/science) aspect of brewing. The latter isn't unless you have access to a brewery or distillery lab with the instrument.
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