New Giveaway - Wort Monster Conical Fermenter!

Home Brew Forums > Home Brewing Beer > Brew Science > Refractometer FG Readings




Reply
 
LinkBack Thread Tools Display Modes
Old 07-20-2010, 05:43 AM   #1
a10t2
Feedback Score: 0 reviews
Recipes 
 
Join Date: May 2010
Location: Leadville, CO
Posts: 557
Liked 14 Times on 13 Posts
Likes Given: 1

Default Refractometer FG Readings

I've mentioned it in a few threads before, but since it affects so many home brewers I thought it was worth a new discussion. For the past year or so I've been tracking my FGs using both a hydrometer and a refractometer, and found that the correlation used in JavaScript calculators, included in software, etc. is wildly inaccurate (at least for me). I've come up with a much better correlation, and I'm curious to hear what other brewers think. In particular, if anyone else has both SG and °Bx data for multiple batches, I'd love to see how well your findings correlate.

Blog post with more info: Toward a Better Refractometer Correlation



__________________
http://seanterrill.com/category/brewing/
Quote:
Originally Posted by monty3777 View Post
squeeze your sack like it owes you money.
a10t2 is offline
 
Reply With Quote Quick reply to this message
Old 07-20-2010, 10:19 AM   #2
HairyDogBrewing
Feedback Score: 0 reviews
Recipes 
 
Join Date: Feb 2008
Location: Mechanicsburg, PA
Posts: 580
Liked 4 Times on 4 Posts

Default

How do you launch the open office version?
I don't see a document in the zip, just supporting files.

Here are some readings from recent batches.
OG OG brix FG FG brix Beersmith FG
1.052 13.0 1.010 6.4 1.009
1.045 11.2 1.009 5.0 1.005
1.045 11.0 1.010 5.2 1.006
1.049 12.0 1.016 6.6 1.012
1.046 11.5 1.008 5.6 1.008



__________________
HairyDogBrewing is offline
 
Reply With Quote Quick reply to this message
Old 07-20-2010, 06:47 PM   #3
a10t2
Feedback Score: 0 reviews
Recipes 
 
Join Date: May 2010
Location: Leadville, CO
Posts: 557
Liked 14 Times on 13 Posts
Likes Given: 1

Default

Hmm, if it's downloading as a zip your PC may be doing some kind of processing. It's just a .ods file. (direct link) Maybe disable anti-virus software for a second? Or send me a PM with your email address and I'll email it to you.

Anyway, I plugged your data in and while it's an improvement, it isn't as good as I saw. The mean discrepancy for the old correlation was -2.9 points, and with the new one it's 0.7 points. The standard deviation is 3.8 points though, so it isn't a great fit.

One thing you might want to look into is that based on those five OGs, your "wort correction factor" looks to be 1.00, and you may have it set to the default 1.04. I don't know if you can change that in Beersmith.

__________________
http://seanterrill.com/category/brewing/
Quote:
Originally Posted by monty3777 View Post
squeeze your sack like it owes you money.
a10t2 is offline
 
Reply With Quote Quick reply to this message
Old 07-21-2010, 01:56 AM   #4
HairyDogBrewing
Feedback Score: 0 reviews
Recipes 
 
Join Date: Feb 2008
Location: Mechanicsburg, PA
Posts: 580
Liked 4 Times on 4 Posts

Default

I turned off Norton Internet Security and I still can't see a .ODS file.

I don't have a lot of confidence in my readings.
They just don't make any sense sometimes.

__________________
HairyDogBrewing is offline
 
Reply With Quote Quick reply to this message
Old 07-21-2010, 07:11 PM   #5
Brewerforlife
HBT_SUPPORTER.png
Feedback Score: 0 reviews
 
Brewerforlife's Avatar
Recipes 
 
Join Date: Dec 2009
Location: Marquette, MICHIGAN
Posts: 244
Liked 7 Times on 6 Posts
Likes Given: 106

Default

refractometer's are not very accurate for measuring finished
beer IMHO, even with the correction formula's.Great for measuring
wort and O.G during the brew. Just my .02$. Cheers!!!

__________________
Brewerforlife is offline
 
Reply With Quote Quick reply to this message
Old 07-22-2010, 12:13 AM   #6
ni*
Feedback Score: 0 reviews
Recipes 
 
Join Date: Jun 2008
Posts: 46
Liked 2 Times on 2 Posts

Default

You based your fit on 12 datapoints. Without wanting to be discouraging (from my experience, this is definitely something that needs looking into), your fitting equation has nine terms. I don't think a close fit with the data actually indicates much when you have only slightly fewer terms in your equation than datapoints to fit. ie, you could likely fit completely random data fairly well with that many terms. I don't have sufficient background in statistics to calculate the statistical significance of a fit like this, but I suspect there isn't much of one. With that many terms the closeness of the fit becomes pretty meaningless.

The equation will accurately predict the datapoints you have, but I don't think it's statistically clear that it will predict future datapoints with much accuracy.

__________________
ni* is offline
 
Reply With Quote Quick reply to this message
Old 07-22-2010, 12:37 AM   #7
Reelale
Feedback Score: 3 reviews
Recipes 
 
Join Date: Sep 2009
Posts: 17,738
Liked 1373 Times on 1331 Posts
Likes Given: 55

Default

Yep, what he said. I agree totally.

__________________
Reelale is offline
 
Reply With Quote Quick reply to this message
Old 07-22-2010, 01:28 AM   #8
a10t2
Feedback Score: 0 reviews
Recipes 
 
Join Date: May 2010
Location: Leadville, CO
Posts: 557
Liked 14 Times on 13 Posts
Likes Given: 1

Default

Yup, overfitting is the main concern at this point. That's why I'm looking for more data. The only reason I'm sticking with the full cubic set so far is that it makes it easy to drop terms and play around with fits later on, rather than do a new regression every time. That the linear equation remains a pretty good fit is encouraging.

Also, with these ranges (1.036-1.106 OG, 1.007-1.022 FG) hopefully there won't be too much need for extrapolation.

__________________
http://seanterrill.com/category/brewing/
Quote:
Originally Posted by monty3777 View Post
squeeze your sack like it owes you money.

Last edited by a10t2; 07-22-2010 at 01:32 AM.
a10t2 is offline
 
Reply With Quote Quick reply to this message
Old 08-06-2010, 09:12 PM   #9
ajdelange
Senior Member
HBT_SUPPORTER.png
Feedback Score: 0 reviews
Recipes 
 
Join Date: Aug 2010
Location: McLean/Ogden, Virginia/Quebec
Posts: 5,214
Liked 453 Times on 372 Posts
Likes Given: 13

Default

Here are some additional FG data. The first FG goes with the first nD etc. All nDs are corrected to 20 C (WITHOUT the use of the refractomer's ATC - don't use it for beer that has started to ferment!)

These (or rather the "points": 1000*(FG-1)) are fit quite nicely (r= 0.9499 - note that's r, not r-squared) by a linear function with an rms residual of 1.1 and a peak residual of -1.7 points which isn't half bad but 7 data points isn't a lot to draw global conclusions from. The fit is fg_points = -2046.4 + 1531.6*nD The beers include stout, ESB, bock,Vienna, weizen, Pils and alt.

FG's
1.009
1.009
1.02
1.011
1.013
1.01249
1.01525

nD's (Relative to Air)
1.34153
1.34237
1.34877
1.34429
1.34575
1.34363
1.34534

OG's
10.52
12.82
18.57
13.43
15.90
13.86
14.74

ABV's
4.20
5.45
7.44
6.20
6.68
5.63
5.68

[Edit] Added ABV and original gravity data. Note that OG's are calculated from ABV and true extract.

[Edit] If you want to convert the nD's to Brix you can use

Brix = ((4997.6025*nD - 21822.2749)*nD + 32237.6805)*nD - 16304.345

(Doubt all those decimal places are needed but thought I'd leave them in)

__________________

Last edited by ajdelange; 08-10-2010 at 06:11 PM. Reason: Brix polynomial corrected - noted that nDs are relative to air
ajdelange is offline
 
Reply With Quote Quick reply to this message
Old 08-09-2010, 02:03 AM   #10
a10t2
Feedback Score: 0 reviews
Recipes 
 
Join Date: May 2010
Location: Leadville, CO
Posts: 557
Liked 14 Times on 13 Posts
Likes Given: 1

Default

Thanks for sharing, AJ. Based on that data set, it seems like a correlation could be worked out based only on the final refractive index. That doesn't intuitively make sense to me - in a three-phase solution, I would think that at least two combinations of sugar and alcohol levels could yield the same nD. Do you have the OG data for those beers?

I definitely can't get as good a fit to my data without the second independent variable. It isn't terrible though (r^2 = 0.78).



__________________
http://seanterrill.com/category/brewing/
Quote:
Originally Posted by monty3777 View Post
squeeze your sack like it owes you money.
a10t2 is offline
 
Reply With Quote Quick reply to this message
Reply


Quick Reply
Message:
Options
Thread Tools
Display Modes