Lower Than Expected FG - But OG is Correct

[thomer]

I have been brewing for almost 2 years so still consider myself a noob and learning. I BIAB, use Brewsmith software and TILT's to measure gravity. As I BIAB I have a brewhouse efficiency of about 70% so manipulate the amount of grain used to give me the correct OG. However I quite often find that on brewday while the OG is within a point of where its supposed to be, my FG quite often ends up 5 points lower than expected. What might the reason be?

 

[Franktalk]

Well, to answer the question, we're going to need more information. What is the grain bill, typically? What yeast? What mash temps and times? Fermentation temps?

 

[thomer]

Grain bill is around 15-17lb.
Yeast normally Safale 04 or 05.
Mash temp 152-154.
Mash time 60-90 mins.
Fermentation temps start at 68 go up to 72.

 

[Franktalk]

Safale 05 usually has an apparent attenuation of about 80%, and 04 just a bit less. How are you figuring your FG? With software or are you doing the math?

 

[thomer]

Safale 05 usually has an apparent attenuation of about 80%, and 04 just a bit less. How are you figuring your FG? With software or are you doing the math?

Thanks for taking the time to answer. I use a refractometer while brewing, but then swap to a TILT in the fermenter. So my OG and FG are basically taken from the same source (TILT).

 

[Franktalk]

Yes, but how or where are you getting your "expected FG?"

 

[Bobby_M]

If it's consistent, the primary driver of lower FGs is an uncalibrated mashing thermometer that reads higher than actual. Lower mash temps make more fermentable wort and vice versa. The other option might be that the TILT is a little off on its reading. I'd confirm that number with a hydrometer to rule that out.

 

[thomer]

Yes, but how or where are you getting your "expected FG?"

A lot of my recipes recently are from clone books with FG listed, of from BeerSmith.

 

[Franktalk]

Gotcha! Bobby raises a good point about mash thermometers.

 

[thomer]

If it's consistent, the primary driver of lower FGs is an uncalibrated mashing thermometer that reads higher than actual. Lower mash temps make more fermentable wort and vice versa. The other option might be that the TILT is a little off on its reading. I'd confirm that number with a hydrometer to rule that out.

Bobby, I am actually using your eBIAB kit with probe and an Auber Cube. Maybe I should send it back for a refund...
I have two TILTs and they both report lower than expected FG, so I don't think that is the issue.

 

[doug293cz]

I trust my Tilt for rate of SG change, but not for accurate SG measurements. I second the recommendation to check FG with a calibrated hydrometer (and don't forget to correct for measurement temp vs. calibration temp.)

Brew on

 

[bwible]

I trust my Tilt for rate of SG change, but not for accurate SG measurements. I second the recommendation to check FG with a calibrated hydrometer (and don't forget to correct for measurement temp vs. calibration temp.)

Brew on

Second this. I use mine for approximation. No way this is dead accurate. It gets stuck in foam, gets stuff stuck to it. And I’ve had numerous issues trying to calibrate this thing. It is what it is - and one thing its not is a very accurate scientific instrument.

 

[RogerGrahl]

I trust my Tilt for rate of SG change, but not for accurate SG measurements. I second the recommendation to check FG with a calibrated hydrometer (and don't forget to correct for measurement temp vs. calibration temp.)

Brew on

I think doug293cz has it right. My Tilt is accurate at OG, but FG is usually high by about 4 points. The Tilt is a good tool for tracking the end of fermentation, but it doesn’t seem to be a good tool for taking actual gravity measurements.

 

[SRJHops]

+ 1 on using a hydrometer for FG. I actually use my refractometer, Tilt, and hydrometer to get the OG. Mostly because the Tilt almost never gets the OG correct. Then I use the hydrometer to check against the Tilt for the FG.

I do find that the Tilt is better at FG. But I agree with the others that a Tilt is mostly useful to track fermentation and know when you hit FG.

Another thought I had is that if your software is ALWAYS 5 SG's off, then that's a pretty easy fix. Either adjust the software if you can, or just know it will be off. For whatever reason, I can't get the Water Calculator in Brewer's Friend to predict the mash ph very well. It's always off by the same amount every time (too low). So I just set it for a higher ph target and that usually works.

P.S. I assume your grain bill does not include sugar? That will usually drive the FG down lower than the calculators seem able to predict, at least in my experience.

 

[AlexKay]

++ on the Tilt not being accurate on FG. Once you open up the fermenter and see all the gunk deposited on it, it’s clear why. If you care about exact value of FG (for consistency, or to validate your process), you need to make a separate measurement. Since you have the refractometer already, I’d use that! Just be sure to adjust your result with a refractometer calculator, otherwise we’ll all be posting in the “why is my FG too high” thread.

 

[thomer]

Thanks for all the input. Next time I brew I will check the FG of the TILT against my refractometer as a test.

 

[hotbeer]

Thanks for all the input. Next time I brew I will check the FG of the TILT against my refractometer as a test.

You could do that, but that's going to bring up questions as to whether you are correctly calculating the corrections for your refractometer. Refractometer doesn't measure SG.

So you might just get a hydrometer for beer and know what your SG really is. Then all that would have to be known is did you take the reading when the wort or beer was at 68°F or 20°C.

 

[AlexKay]

You could do that, but that's going to bring up questions as to whether you are correctly calculating the corrections for your refractometer. Refractometer doesn't measure SG.

So you might just get a hydrometer for beer and know what your SG really is. Then all that would have to be known is did you take the reading when the wort or beer was at 68°F or 20°C.

There's a bias toward a gravity measurement over a refractive index measurement, but I don't know why that is.

If you're looking to determine consistency, the refractometer measurement is just as good. If you're looking to determine ABV, the refractometer measurement is just as good. If you want to know how heavy your beer is, then I suppose the hydrometer is the way to go.

 

[Teufelhunde]

. The other option might be that the TILT is a little off on its reading. I'd confirm that number with a hydrometer to rule that out.

The tilt can also be recalibrated using distilled water.

 

[Bobby_M]

Bobby, I am actually using your eBIAB kit with probe and an Auber Cube. Maybe I should send it back for a refund...
I have two TILTs and they both report lower than expected FG, so I don't think that is the issue.

Does the Auber display show the correct temperature when boiling (given your elevation)? If it's reading higher than boiling, that would mean it just needs a correction in the menu.

 

[thomer]

Does the Auber display show the correct temperature when boiling (given your elevation)? If it's reading higher than boiling, that would mean it just needs a correction in the menu.

Sorry for the late response. Yes it does read the correct boiling temperature for my elevation.

 

[TheMadKing]

There's a bias toward a gravity measurement over a refractive index measurement, but I don't know why that is.

If you're looking to determine consistency, the refractometer measurement is just as good. If you're looking to determine ABV, the refractometer measurement is just as good. If you want to know how heavy your beer is, then I suppose the hydrometer is the way to go.

That bias exists because you have to do a calculation using a wort correction factor to determine an accurate specific gravity using a refractometer in the presence of alcohol. Unless you know that number for your wort very precisely it's easy to come up with a number that's off by a couple thousandths.

A hydrometer is a direct measurement and in general a direct measurement and reading is better than a converted measurement.

 

[AlexKay]

The hydrometer is only a direct measurement if you care how much your beer weighs. Otherwise you’re converting to get ABV, in which case you’re choosing one of several formulas to do so and you’re making assumptions about the sugar profiles of your wort and beer. Just like a refractometer.