Using a Refractometer During Fermentation

[khiddy]

A few threads, and reading in the Wiki, have led me to believe that a refractometer is really only useful pre-fermentation to ensure that I'm hitting my numbers during the mash and boil, and then possibly again at the end of fermentation, to help calculate ABV and whether or not there's any leftover sugar to be fermented. As I understand it, the wort correction factor only applies to the pre-fermentation reading where I compare the actual hydrometer reading to my Brix reading on the refractometer, right? In other words, once fermentation has begun, this wort correction factor goes out the window, and you essentially have to use software (or go through a really complicated mathematical process) to get the real Brix reading, is that correct?

I've been taking hydro samples during the fermentation process of my lager and two ales I have going. Here is a little chart I've whipped up:

SG/SG in Brix/Brix Reading/
PDX Augustinerbrau (SG: 1.061 / 15 Brix)
1.036	9	9
1.022	5.5	7.2
		
Minute Wheat (SG: 1.049 / 12.4 Brix)
1.029	7.25	8.2
1.022	5.5	7
1.016	4	6
		
Mozart's Gold (SG: 1.043 / 10.6 Brix)
1.016	4	5.2
1.012	3	4.9
1.012	3	4.9

Here's the problem: When I enter my actual Brix reading (3rd column) into my software (I use BrewPal on the iPhone, but I've confirmed the same numbers in Dan Conley's online Refractometer-otron 5000) to perform the correction, it gives me a completely different number than the actual hydro reads.

For example, when I entered the 7.2 Brix of my latest PDX Augustinerbrau reading into the software, it tells me that with correction it should be an SG of 1.008 (2.1 Brix). My actual hydro reading is 1.022 (5.6 Brix).

Can anyone tell me what's going on here?

 

[dragon99]

I'm no expert on refractometers, but I do know that the accuracy of the corrected Brix reading taken during/after fermentation is dependent on the accuracy of the OG reading.

If you used the refractometer (and not a hydro) to measure the OG, it's probably a little bit off.

 

[khiddy]

My readings have all been hydrometer readings, supplemented by refractometer readings, since I just got the refractometer and want to learn its in's & out's before attempting to rely on it in any way. So the OG and SG readings here are actual readings from my temperature-corrected hydrometer.

Another possibility, I guess, is that my hydro is actually uncalibrated. I'll need to check it using distilled water this evening to see, that of course could make a huge difference!

 

[bendavanza]

I have found my readings brix vs hydro to be very different, but on the last batch I measured, the readings were different but the results were the same. I am also new to using a refractometer. Mine is an ATC bought at AHS.
The hydro gave reading of 1.082 -> 1.014 and the brix was 20 (1.077) -> 8.8 (1.009), so one is off and I really wish I understood it better. Both gave 8.9% respectively, using Brewpal on the iPhone, with their default 1.04 wort correction used on final gravity.

 

[The Pol]

Refractometers are highly accurate after fermentation. But, you need to have the actual Brix reading that you took for the OG so that brewing software can compensate.

 

[TipsyDragon]

real life will almost never match a calculation. calculations are just to get you in the ballpark.

 

[khiddy]

Thanks for the input, Pol. I do have the actual Brix readings from the OG (as well as the hydro readings), I've been putting them in my brew notes alongside the SGs. I should note that I'm entering the numbers into three different pieces of software, trying to learn more about how they might work - the aforementioned iPhone BrewPal software, the Refractometer-otron 5000, and the MoreBeer refractometer spreadsheet.

What I don't understand is that all of the calculators and the spreadsheet give me a crazy low alcohol-corrected SG number when compared to the actual hydro reading. In my existing example, with the PDX Augustinerbrau, my OG was 15 brix, corresponding to 1.061 (the same reading as my actual hydro OG). Yesterday's reading was 9 brix, which the spreadsheet tells me corresponds to 1.021, yet the actual hydro was 1.036. And today, I measured 7.2 brix, corresponding to an alcohol-corrected 1.010, yet the actual hydro reading is 1.022. What is the spreadsheet telling me?

 

[The Pol]

Thanks for the input, Pol. I do have the actual Brix readings from the OG (as well as the hydro readings), I've been putting them in my brew notes alongside the SGs. I should note that I'm entering the numbers into three different pieces of software, trying to learn more about how they might work - the aforementioned iPhone BrewPal software, the Refractometer-otron 5000, and the MoreBeer refractometer spreadsheet.

What I don't understand is that the spreadsheet, for example, gives me a crazy low converted SG number when compared to the actual hydro reading. In my existing example, with the PDX Augustinerbrau, my OG was 15 brix, corresponding to 1.061 (the same reading as my actual hydro OG). Yesterday's reading was 9 brix, which the spreadsheet tells me corresponds to 1.021, yet the actual hydro was 1.036. And today, I measured 7.2 brix, corresponding to 1.010, yet the actual hydro reading is 1.022. What is the spreadsheet telling me?

I have no idea... I just use ProMash and it is dead on all the time.

 

[malkore]

are you sure the spreadsheet is doing the calculations correctly?

i've had Excel do stuff to my formulas before that needed corrections or additional ( ) around parts of the formula.

have you compared your brix and hydro readings using online calculators to see if they come out proper?

is your refractometer Auto Temp Correcting?

does the OG brix from teh refrac. match the OG hydro reading?

 

[khiddy]

Well, the spreadsheet is the one that MoreBeer provides, and I don't see anyone else reporting issues with the calculations on that sheet, so while I can't be 100% sure, I doubt that's the problem.

The online and iPhone calculators are giving me similar (within 1 point SG) readings to the spreadsheet - I attribute that to rounding.

My refractometer is ATC, yes.

I don't have data on the OG brix for two of my batches, the refractometer arrived after I'd started those batches. But it did match the OG for the lager that I've been using as my example in follow-on posts (PDX Augustinerbrau).

 

[bendavanza]

Khiddy
Having the OG from a calibrated refractometer is crucial to having anything make sense.
even though my hydro and refract did not agree (yet) on the og and fg, the end result was the same. From what I understand, you do not use alcohol correction on the OG, because there is no alcohol yet, but you do on the FG, and having the correct OG numbers (from the same device) makes the numbers fall into place. If one of them is off by x amount, at least it should be consistent OG-FG giving the right abv.

 

[raceskier]

I have used the MoreBeer spreadsheet with the refractometer, taking measurements on a daily basis. I was able to get a very good hydrometer FG correlation. I have not tried the ProMash calculator. I will try both on a brew I just started fermenting.

 

[khiddy]

I'm definitely going to keep trying it, taking copious notes, and then perform some sort of correlation after a few brews are complete. As I mentioned earlier, this is my first time using a refractometer, and even my Master Brewer friend had never used one, so he's interested as well. Thanks for the suggestions, everyone.