Author Topic: Accurately measuring gravity  (Read 1452 times)

Offline Visor

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Accurately measuring gravity
« on: July 02, 2017, 12:02:59 AM »
  I increasingly been having serious doubts about the accuracy of the tools I use to try to measure gravity, so this afternoon I decided to set up a test to see what was most accurate. Ray Daniels' "Designing Great Beers" says that 1 lb. of table sugar dissolved in water to make 1 gallon yields a Specific gravity of 1.046. I scaled that down to 1 oz. of sugar and water to make 1 cup, which equals 1 lb. per gallon, and proceeded to test the Vee-Gee refractometer I recently purchased to replace the POS I originally bought, and an assortment of hydrometers I have collected.
   The 1st test at a concentration that should be 1.046, the VeeGee gave a brix of 10.8 = 1.043 s.g., hydro #1 said 1.047, hydro #2 said 1.049, #3 said 1.050, #4 which I just bought as part of a 3 hydro set that is supposed to be super accurate as they each cover different gravity ranges said 1.049 [it covers a range from 1.000 to 1.070].
   Next I heated the water to about 100*, added another oz. of sugar which should put the s.g. at 1.092, stirred thoroughly and cooled to room temp. The results corrected for temp were VeeGee - Brix of 20.5 = 1.085, Hydro #1 - 1.089, #2 - 1.089, #3 - 1.090, & #4 which covers a range from 1.060 to 1.130 said 1.088.
   I then repeated the process with another ounce of sugar which should put the actual s.g. at 1.138 and tested all again. Results were VeeGee - Brix 28.4 = 1.122, #1 - 1.024, #2 - 1.124, #3 - 1.125, and the same #4 as the previous test - 1.124.
   Needless to say I was anything but impressed. Clearly the refractometer was the least accurate instrument tested. Hydrometers #1 & #2 are the most accurate at most "normal" brewing gravities, and #3 is the most accurate above about 1.060 or so. The narrow range hydro's were generally the least accurate, even if they are the easiest to read. The refractor is now destined for the trash can, and the set of hydro's I just bought are going back to Midwest, unless they won't refund my purchase, in which case they're traveling with the VeeGee.
   Maybe I am expecting too much from my tools, but this BEER for Pete's sake, not friggin horse-shoes, close isn't good enough for me.
   Has anyone else found a more foolproof [or perhaps tool-proof] way to measure gravity? I look forward to everyone's input, including critique of my testing protocol [gotta luv them fancy words!].
   Thanks.
I spent most of my money on beer, tools and guns, the rest I foolishly squandered on stupid stuff!

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Re: Accurately measuring gravity
« Reply #1 on: July 02, 2017, 01:30:49 AM »
  I increasingly been having serious doubts about the accuracy of the tools I use to try to measure gravity, so this afternoon I decided to set up a test to see what was most accurate. Ray Daniels' "Designing Great Beers" says that 1 lb. of table sugar dissolved in water to make 1 gallon yields a Specific gravity of 1.046. I scaled that down to 1 oz. of sugar and water to make 1 cup, which equals 1 lb. per gallon, and proceeded to test the Vee-Gee refractometer I recently purchased to replace the POS I originally bought, and an assortment of hydrometers I have collected.
   The 1st test at a concentration that should be 1.046, the VeeGee gave a brix of 10.8 = 1.043 s.g., hydro #1 said 1.047, hydro #2 said 1.049, #3 said 1.050, #4 which I just bought as part of a 3 hydro set that is supposed to be super accurate as they each cover different gravity ranges said 1.049 [it covers a range from 1.000 to 1.070].
   Next I heated the water to about 100*, added another oz. of sugar which should put the s.g. at 1.092, stirred thoroughly and cooled to room temp. The results corrected for temp were VeeGee - Brix of 20.5 = 1.085, Hydro #1 - 1.089, #2 - 1.089, #3 - 1.090, & #4 which covers a range from 1.060 to 1.130 said 1.088.
   I then repeated the process with another ounce of sugar which should put the actual s.g. at 1.138 and tested all again. Results were VeeGee - Brix 28.4 = 1.122, #1 - 1.024, #2 - 1.124, #3 - 1.125, and the same #4 as the previous test - 1.124.
   Needless to say I was anything but impressed. Clearly the refractometer was the least accurate instrument tested. Hydrometers #1 & #2 are the most accurate at most "normal" brewing gravities, and #3 is the most accurate above about 1.060 or so. The narrow range hydro's were generally the least accurate, even if they are the easiest to read. The refractor is now destined for the trash can, and the set of hydro's I just bought are going back to Midwest, unless they won't refund my purchase, in which case they're traveling with the VeeGee.
   Maybe I am expecting too much from my tools, but this BEER for Pete's sake, not friggin horse-shoes, close isn't good enough for me.
   Has anyone else found a more foolproof [or perhaps tool-proof] way to measure gravity? I look forward to everyone's input, including critique of my testing protocol [gotta luv them fancy words!].
   Thanks.

Did you calibrate all of them? I check my refractometer before each batch and it's always dead nuts.

Offline Visor

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Re: Accurately measuring gravity
« Reply #2 on: July 02, 2017, 04:16:16 PM »
  I discovered after posting that the set screw on what I'm guessing is the refractometer's adjustment band was loose, so I'll have to figure out how to re-calibrate and try it again. As for the hydrometers, I'm not aware of any way to calibrate them.
 
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Offline rob_f

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Re: Accurately measuring gravity
« Reply #3 on: July 02, 2017, 04:30:23 PM »
  As for the hydrometers, I'm not aware of any way to calibrate them.
 
Put each hydrometer in 60F water and see how far it deviates from 1.000. I usually find them reading high from the paper inside sliding down. Then I wrap tape around the top until it reads 1.000.
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Offline Stevie

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Re: Accurately measuring gravity
« Reply #4 on: July 02, 2017, 04:39:48 PM »
  As for the hydrometers, I'm not aware of any way to calibrate them.
 
Put each hydrometer in 60F water and see how far it deviates from 1.000. I usually find them reading high from the paper inside sliding down. Then I wrap tape around the top until it reads 1.000.
Distilled or RO water for increased accuracy.

Offline mabrungard

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Re: Accurately measuring gravity
« Reply #5 on: July 02, 2017, 05:08:22 PM »
Were you measuring 1 oz or were you measuring 28.35 grams? Hopefully your scale enabled you measure in grams and not oz. That would be a potential source of error. However, it doesn't diminish your work in measuring the differences between your hydrometers!
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Offline Visor

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Re: Accurately measuring gravity
« Reply #6 on: July 02, 2017, 05:41:28 PM »
   I weighed out an ounce on a gram scale I just purchased, it is very sensitive and measures to tenths of a gram or thousandths of an ounce, but that doesn't of necessity mean it's accurate. Why would measuring in grams be better than measuring in ounces?
   The tape idea might work - sort of, except that a couple of the hydro are right on the gnat's ass in distilled water, start adding sugar though and the wheels apparently start coming off.
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Offline Visor

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Re: Accurately measuring gravity
« Reply #7 on: July 02, 2017, 11:09:01 PM »
   So I recalibrated the refractometer to zero [with distilled water Stevie - thanks for the reminder] and re-tested it with the 1.138 solution and it came up with a Brix of 29.5 which equals 1.127 S.G. That's not even in the ballpark, I'm wondering what is the point of continuing to measure gravity, especially on big beers when my measuring instruments are this worthless [ but I know I'll continue to do so]. I'm also thinking that a hell of a lot of other home brewers are getting measurements that are just as inaccurate, but don't know it, after all, they probably bought a lot of the same junk I bought from the same suppliers, because we all foolishly thought the stuff would work as advertised. I would be very interested if a few other folks would repeat my testing with their hydro's and refrac's and report their results. Maybe I'm just extremely unlucky, and always wind up with the 1 piece of crap out of a thousand off the assembly line, but the statistical likeliness of that being the case is miniscule at best.
   rob_f, your suggestion about taping the hydro's might work if they were reading high as it would lower them in the test jar, adding weight at this point would only exaggerate the inaccuracy, thanks for the idea though.
I spent most of my money on beer, tools and guns, the rest I foolishly squandered on stupid stuff!

Offline mabrungard

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Re: Accurately measuring gravity
« Reply #8 on: July 02, 2017, 11:39:38 PM »
Why would measuring in grams be better than measuring in ounces?


If a scale is only reporting in ounces and not in tenths or hundreths of an ounce, it would not be suited for measuring small quantities. If you were using a scale that only reported to the nearest gram, it would still be 28 times more sensitive than a scale reporting in whole ounces. 

The fact that your scale reports in those fine divisions, your measurements should be good.
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Offline Bilsch

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Re: Accurately measuring gravity
« Reply #9 on: July 03, 2017, 12:42:20 AM »
Also I'd toss in here that using a cup for measuring the water probably isn't all that accurate either. Your best bet is to weigh the water on the same scale your using for the sugar. 1 ml water = 1 gram.

Offline yso191

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Re: Accurately measuring gravity
« Reply #10 on: July 03, 2017, 02:16:18 PM »
I have ranted about inaccurate hydrometers in the past on this very forum.  It hasn't done me any good...

Now I don't care (I'm telling myself).  Since it is for homebrewing purposes it is more about ballparking it and noting changes.  In other words the primary information you need is relative.
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Offline hopfenundmalz

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Re: Accurately measuring gravity
« Reply #11 on: July 03, 2017, 04:16:34 PM »
  As for the hydrometers, I'm not aware of any way to calibrate them.
 
Put each hydrometer in 60F water and see how far it deviates from 1.000. I usually find them reading high from the paper inside sliding down. Then I wrap tape around the top until it reads 1.000.
I like that idea.
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Offline mabrungard

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Re: Accurately measuring gravity
« Reply #12 on: July 03, 2017, 06:01:58 PM »
Another way to assess the density of your solution is to weigh a mass of known volume. If you have lab-quality volumetric cylinders or a calibrated pycnometer (big bucks!), you can use your scale to assess what the solution's density is.

PS: you can check the accuracy of a volumetric cylinder's marks by measuring the mass of distilled water filled to that level.
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Offline Visor

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Re: Accurately measuring gravity
« Reply #13 on: July 03, 2017, 10:13:03 PM »
   I'm far too anal of a tool junkie to ever be content with crappy tools Steve, probably not quite anal enough to invest in the kind of lab required to perform the tests Martin just suggested. Let me see, didn't I pick up a pycnometer at a garage sale a few years ago, where on earth did I stash that darned thing? ;)
  One thing this does mean, my brewhouse efficiency is a good bit better than I thought, and my heavier beers have a higher ABV than I thought [and I though I was just turning into a weenie].
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Offline leejoreilly

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Re: Accurately measuring gravity
« Reply #14 on: July 04, 2017, 01:34:23 PM »
I have ranted about inaccurate hydrometers in the past on this very forum.  It hasn't done me any good...

Now I don't care (I'm telling myself).  Since it is for homebrewing purposes it is more about ballparking it and noting changes.  In other words the primary information you need is relative.

Steve, I've got to agree with you; for me, great accuracy on gravities doesn't bring much to my party. "Somewhere near 1.060" is close enough to tell me I likely didn't screw up too badly. I do recognize that "I just want to know" is a perfectly valid reason for pursuing greater accuracy, just not for me. If I get an OG of 1.059 or 1.061 rather than 1.060, I'm not likely to do anything about it (I COULD, but I'm not likely to), so the info is not worth much to me. YMMV