Author Topic: refractometer spreadsheet  (Read 1769 times)

Offline tschmidlin

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refractometer spreadsheet
« on: August 07, 2010, 04:23:31 PM »
Has anyone used the morebeer refractometer spreadsheet to monitor the gravity of their fermentations? 

http://morebeer.com/learn_vids/vids_refract

I broke my 100th hydrometer so I'm giving it a try, but I don't trust it yet.  My SG was 20 Brix, 1.083, and it's dropped to 10 Brix, which is 1.014 according to the spreadsheet.  The problem is that's an 83% apparent attenuation, and the yeasts I used are Wyeast 2308 and 2007, which have AAs around 70-75%.  Plus, the beer tastes much sweeter than I expect something that dropped from 1.083 to 1.014 to taste.

Obviously I need to get a new hydrometer or borrow one to test it, I'm just wondering if anyone has any experience with it.

Thanks.
Tom Schmidlin

Offline brushvalleybrewer

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Re: refractometer spreadsheet
« Reply #1 on: August 07, 2010, 05:05:34 PM »
I haven’t used that spreadsheet, but I do use a refractometer exclusively.

I use an iPhone app called BrewMath that does something similar.

If I plug in your Brix numbers I get 1.080 and 1.015 (close enough) and 81% ADF. Still pretty high.

Have you calibrated the refractometer? (Does it read 0 with water?)
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Offline tschmidlin

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Re: refractometer spreadsheet
« Reply #2 on: August 07, 2010, 09:09:42 PM »
Good idea - it's been a while since I calibrated it, but I found it was reading about .25 high.  So now it's even drier.  I remeasured and now it's reading sub-10 brix.

I gave a sample to my wife and, unprompted, she said she thought it was sweet.  Maybe it would be ok if it was colder and carbonated, but I want to check it with a hydrometer first.

At this point I'm seriously wondering what sources of error there may be, what might be in a beer besides water, sugar, and ethanol that can affect the refractive index.  I can't imagine there would be enough of any compound besides those to really depress the refraction, but . . .
Tom Schmidlin

Offline wingnut

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Re: refractometer spreadsheet
« Reply #3 on: August 07, 2010, 10:13:09 PM »
A couple things to think about.

One, I find the refractometor to not be an extremely precise instrument.  If for no other reason than the graduated lines and the refracted blue line bring some in accuracies.

Second, the ambient temperature around you makes a difference with the device.  While the refractomotor is “ATC” or auto temperature compensating, it does so because you place a very small amount of wort onto the device, and that essentially cools the wort instantly.  If you are like me and take reading in the summer for the OG outside in 95F heat, and then take the final reading in my basement at 68F, the device will have some error due to temperature.  (I try to take all my reading in my basement, which is consistent in temperature year round due to this)

Third, the wort tends to stratify sugar layers, so the wort at the top may read 20 brix, but half way down the reading may be 25 brix.  It is hard to be sure your wort is the same all the way through.  If you take a reading just after or during the boil, that should reduce the issues a lot. (Probably already your practice)

Fourth, most refractometors are only accurate to +/-  0.2 brix.  So they can be almost a quarter brix off when functioning correctly.

Lastly, I think your sheet may be off a bit.  When I use mine at 20brix I get 1.078 starting gravity, and with 10 brix final, I get 1.015 gravity.  That combined with some possible calibration issues you listed, and some rounding or accuracy in reading the refractometor, I think that may possibly bring attenuation back near 75%..  Perhapse mine is off, but at the very least, I findi it interesting that they are different.(Mine is from more beer too)

Regardless, the refractometor, with the small scale, is not a very precision instrument.  If you want to get accurate readings, the hydrometor is the way to go.  However, if you are looking for trends or “just close” readings, the refractometor is perfect.  I rarely use my hydromtoer any more, but that is also because I figure the beer is going to ferment how it wants to, and the gravity reading are more or less just data so  I can reflect on what is different about this batch than last batch.


Good luck and I hope the brew turns out great!
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Offline tschmidlin

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Re: refractometer spreadsheet
« Reply #4 on: August 07, 2010, 10:42:56 PM »
Hey wingnut, all good points, thanks.

I think the ambient temp is pretty consistent, around 60-70F.  If it's 95 here I'm unlikely to be outside brewing!  But you're right that these temp changes can make a difference, I'll have to note the ambient temp more carefully.

When I take my measurement it is post boil, post chiller,so there shouldn't be an issue there.

It's weird that your spreadsheet and mine differ.  The link to download the spreadsheet on the page I linked to says the spreadsheet is updated, maybe that's where the discrepancy is.

Ultimately I don't worry too much about FG, as long as the beer tastes right.  But since this one tastes under attenuated to me I checked to see how far it had to go, only to find the refractometer says it's more attenuated than expected, and here we are.  I'll let you know what a hydrometer says.
Tom Schmidlin

Offline tschmidlin

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Re: refractometer spreadsheet
« Reply #5 on: August 08, 2010, 03:55:10 PM »
I borrowed a hydrometer (thanks Tim!) and tested the beer, it's reading 1.020.  It's possible the SG was lower than I thought, so 10 brix is 1.020 in this case.  Or it's possible that something else is going on.  I'm going to get myself a new hydrometer and be more careful with my readings for a while, see what I can figure out.

But at least 1.020 is more about what I expected for this beer, a 76% apparent attenuation.  I'll see how it tastes cold and carbonated.
Tom Schmidlin

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Re: refractometer spreadsheet
« Reply #6 on: August 08, 2010, 06:44:14 PM »
I haven't found the FG correction used by that spreadsheet to be at all accurate. I think I've worked out one that is: http://seanterrill.com/2010/07/20/toward-a-better-refractometer-correlation/

If you check both refractometer and hydrometer readings for a few batches, I'd be interested to hear your results.
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Offline tschmidlin

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Re: refractometer spreadsheet
« Reply #7 on: August 08, 2010, 10:56:46 PM »
I'll let you know.   As of now yours is giving me more or less the same numbers as the morebeer spreadsheet, but I'll track it for a while and we'll see.
Tom Schmidlin