Author Topic: Hydrometer and refractomer don't agree on FG?  (Read 1638 times)

Offline Philbrew

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Hydrometer and refractomer don't agree on FG?
« on: May 06, 2016, 07:52:20 PM »
I brewed my first shot at a Belgian Saison two weeks ago (SG=1.057).  A week ago it was at 1.005 on the hydrometer and the Milwaukee 884 digital refractometer said 5.7 which calculates to 1.001 using the Northern Brewer online calculator.
I followed some good advice from the Forum and let it go another week.  Hydrometer now reads 1.002 and the refractometer now says 4.8 (calcs to 0.996).
It's a cheapo hydrometer but I calibrated it about a year ago with distilled water.

The two agreed on the SG but are far apart on the FG.  Do refractometers not work well down there?  Is NB calculator suspect in that range?  Something else going on?
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Offline toby

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #1 on: May 06, 2016, 07:59:40 PM »
Refractometers become much less accurate when ethanol is in the mix.

http://pintwell.com/2012/jan/21/calculating-finishing-gravity-refractometer-nuts/

Offline narvin

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #2 on: May 06, 2016, 08:00:32 PM »
Did you take an OG reading with the refractometer as well?  If the hydrometer is off by a couple of points, this is going to increase the discrepancy of the FG calculator.

I do think the traditional calculator tends to predict low for beers that are highly attenuated. 
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Offline narvin

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #3 on: May 06, 2016, 08:04:38 PM »
Sean made one based on empirical measurements that backs this up, and seems to agree more with your results than the NB calc.

http://seanterrill.com/2012/01/06/refractometer-calculator/
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Offline morticaixavier

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #4 on: May 06, 2016, 08:05:33 PM »
The refractometer correction calculators are all slightly suspect. Sean Terrills is pretty accurate over a wide range but you have to have the wort correction factor to make it work right. this requires measuring OG with a hydrometer and refractometer.
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Offline Philbrew

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #5 on: May 06, 2016, 08:12:57 PM »
Did you take an OG reading with the refractometer as well? 
Yes, the two (almost) agreed on the OG.  Hydr. read 1.056, Refrct said 14.1 (calcs to 1.057)
« Last Edit: May 06, 2016, 08:20:54 PM by Philbrew »
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Offline Philbrew

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #6 on: May 06, 2016, 08:15:46 PM »
Sean made one based on empirical measurements that backs this up, and seems to agree more with your results than the NB calc.

http://seanterrill.com/2012/01/06/refractometer-calculator/
Nice calculator.  Thanks.

What is the "wort correction factor"?
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Online kramerog

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #7 on: May 06, 2016, 08:29:04 PM »
I think Sean's site has an explanation of wort correction factor. IIRC, refractometers are calibrated for sugar solutions from fruits, which have a different sugars than wort, hence wort correction factor.

Offline Philbrew

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #8 on: May 06, 2016, 08:34:59 PM »
I think Sean's site has an explanation of wort correction factor.
I tried a search on his site and came up empty.
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Offline a10t2

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #9 on: May 06, 2016, 08:51:21 PM »
I think Sean's site has an explanation of wort correction factor.
I tried a search on his site and came up empty.

It seems to be working fine now. This was the fourth result: http://seanterrill.com/2010/06/11/refractometer-estimates-of-final-gravity/
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Offline Philbrew

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #10 on: May 07, 2016, 02:07:58 AM »
I think Sean's site has an explanation of wort correction factor.
I tried a search on his site and came up empty.

It seems to be working fine now. This was the fourth result: http://seanterrill.com/2010/06/11/refractometer-estimates-of-final-gravity/
Aahhhh, so "wort correction factor" is derived empirically.  I was looking for a "how to calculate".

So Toby is right.  Refractometers can show trends in highly attenuated beer but their accuracy sucks!
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Offline HoosierBrew

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #11 on: May 07, 2016, 02:24:08 AM »
  Refractometers can show trends in highly attenuated beer but their accuracy sucks!


Mine is pretty accurate to my hydro for OG readings. I just leave it out of the loop for FG. Hydro all the way for that.
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Offline Philbrew

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #12 on: May 07, 2016, 04:02:33 AM »
  Refractometers can show trends in highly attenuated beer but their accuracy sucks!


Mine is pretty accurate to my hydro for OG readings. I just leave it out of the loop for FG. Hydro all the way for that.
Yep, good for OG and you don't have to toss a lot of beer when taking daily FGs to see when a beer is finished fermenting.  But it looks like hydro if you want to know the FG.
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Offline narvin

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #13 on: May 07, 2016, 04:42:04 AM »
I like to look at it from a different perspective.  The wort correction factor is based on the fact that different sugars (carbohydrates) have different molecular weights.  Depending on your grist, mash schedule, composition of sugars in the final beer (unattenuated simple sugars vs unfewntable dextrines), the weight vs refractory index will diverge.

But, for a given style of beer, with similar grist, mash profile, and yeast, the refractive index should be a very valid comparison.  So you can think of it as a reading of its own, vs a means to convert to gravity.  If anything, the refractive index is less sensitive to alcohol and is a more definite measurement of sugar content, so it's more absolute.  If you have a Saison and raise the OG by 10 points, the same FG will actually have much more sugar since it's an overall average of density (high for sugar and low for alcohol).

Brix refractive index isn't a commonly published number, which is why I like to measure as many commercial beers as I can.  Comparing your Saison to Dupont or another classic style is very useful, even without converting to gravity.
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Offline toby

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Re: Hydrometer and refractomer don't agree on FG?
« Reply #14 on: May 07, 2016, 03:56:22 PM »
  Refractometers can show trends in highly attenuated beer but their accuracy sucks!


Mine is pretty accurate to my hydro for OG readings. I just leave it out of the loop for FG. Hydro all the way for that.
Yep, good for OG and you don't have to toss a lot of beer when taking daily FGs to see when a beer is finished fermenting.  But it looks like hydro if you want to know the FG.

That pretty much sums it up.