How to use a refractometer for final gravity reading?

Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum

Help Support Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum:

slayer021175666

Well-Known Member
Joined
Sep 24, 2020
Messages
442
Reaction score
216
I have a stout that had an OG of 1.080. It has fermented for a week and is only making a bubble about every 1 minute. I figured it's done fermenting so, I took another reading with the refractometer and it says 1.040! It gave me quite a fright! I figured, possible stuck fermentation. I got online and was told that there is some correction needed when using a refractometer to take FG readings. I used one of the online calculators and it said I'm actually down to 1.014. WHEW! Another calculator I found said I'm at 1.017. Either number would be acceptable but, is this really true and trustable? I haven't had the refractometer very long. Please tell me what I need to know and if this is all correct. The beer tastes finished to me. It's not sweet. I just want some others to chime in and tell me if its fermented out and its ok to go ahead and keg it.
Thank You, Guys.
 

DBhomebrew

Well-Known Member
Joined
Jun 18, 2020
Messages
2,285
Reaction score
4,350
Location
St Louis, MO
There are a few different formulae being used to compute refractometer measurements. Some are better than others depending on the OG, etc. For your question (Is it finished?) a consistent reading is more important than what the reading is.

Some calculators default a WCF of 1.04, others default 1.0. That may be a cause of some discrepancy when comparing two.

Comparing a refractometer to a hydrometer in your own brewery over the course of a few batches, with multiple calculators, will dial in your process and WCF.

I use 1.04 and the calculator in BrewCipher, Sean Tirrell's.
 

RM-MN

Supporting Member
HBT Supporter
Joined
Nov 26, 2010
Messages
15,123
Reaction score
6,223
Location
Solway
I have a stout that had an OG of 1.080. It has fermented for a week and is only making a bubble about every 1 minute. I figured it's done fermenting so, I took another reading with the refractometer and it says 1.040! It gave me quite a fright! I figured, possible stuck fermentation. I got online and was told that there is some correction needed when using a refractometer to take FG readings. I used one of the online calculators and it said I'm actually down to 1.014. WHEW! Another calculator I found said I'm at 1.017. Either number would be acceptable but, is this really true and trustable? I haven't had the refractometer very long. Please tell me what I need to know and if this is all correct. The beer tastes finished to me. It's not sweet. I just want some others to chime in and tell me if its fermented out and its ok to go ahead and keg it.
Thank You, Guys.
Do you want beer fast or do you want good beer? If fast is what you want, keg it. You can verify that it doesn't have excess pressure and vent a bit if it does.

if you want good beer, leave it in the fermenter for another 2-3 weeks. Being a high OG beer it will need some time to mature.
 
OP
OP
S

slayer021175666

Well-Known Member
Joined
Sep 24, 2020
Messages
442
Reaction score
216
I just tasted it. It's already good beer. I just want to know if these refractometer calculators are actually correct and if I can assume that the gravity is down to 1.014 or 1.017 as the other calculator said.
 

MaxStout

Supporting Member
HBT Supporter
Joined
Mar 29, 2013
Messages
13,625
Reaction score
14,256
Location
Inside a Klein Bottle
I don't trust a refrac for FG. For OG, it tracks within 1 point of the hydrometer, which is close enough. But FG, it's off by a couple points or more, depending on which formula I use.

I like a refrac on brew day, as I can take multiple readings in the process without wasting so much wort. For FG, I use a hydro.
 

jrgtr42

Well-Known Member
Joined
Oct 4, 2013
Messages
1,832
Reaction score
1,171
Location
Metrowest, Massachusets.
Personally, I trust my refrac. The first few brews I did, I backed the readings up with the hydrometer. After putting through the Brewers friend calculator, I'd read spot on, or close enough for government work. Since then, I don't bother with the hydrometer unless it;s reading funny. In most of those instances, it's an ID-10-T error.
But I'm not really worried about hitting an exact FG, as long as it's in the ballpark of where i'm expecting. I'll take the reading, I don't even bother with the calculations until I get my second check morning of expected packaging day, it's making the the reading isn't changing. As long as it's the same, then I'll do the calcs and move on to packaging.
 

dmtaylor

Lord Idiot the Lazy
HBT Supporter
Joined
Nov 8, 2009
Messages
5,286
Reaction score
4,185
Location
Two Rivers, WI
I've been tracking refractometer vs. hydrometer readings for dozens of batches. The most accurate calculation is on Brewer's Friend, linked below. I find this calculation to be accurate within 1-2 gravity points average, most cases. The actual equation on Brewer's Friend originates from a man named Petr Novotny, who provided an article to BYO or Zymurgy about 4 or 5 years ago, and has posted on the AHA forum in the past, see second link below. To use the calculator most effectively, it is best to record your readings in Brix. For your example, if I assume original 19.4 Brix and final 10 Brix, then you will indeed have an FG around 1.017, or possibly a little lower, depending on your particular "wort correction factor" (WCF). My own WCF here is about 0.99 or maybe 1.00. Others like Sean Terrill report an average of 1.04, which I have not been able to duplicate, at least not for MY refractometer. YMMV. I have found 3 other equations as well to compare against from Terrill, Bonham, Gossett, and they are all reasonably consistent with small range. In general, I find Terrill is most accurate when FG is less than 1.014, and Brewer's Friend when FG is greater than 1.014, but Brewer's Friend is most accurate overall regardless of FG.

So if you have no input to the contrary, I'd go with your current gravity being about 1.016 or 1.017. It's really close to that, shouldn't be lower than 1.015 or higher than 1.018. You can double check this with a real hydrometer if you want. Then over time, you'll be able to determine your own WCF.

Hope this helps.



And from the second thread, following is a direct link to my own guidance on how to take readings most effectively -- i.e., CALIBRATE! AND OFTEN!

 

jerrylotto

Well-Known Member
Joined
Apr 15, 2020
Messages
561
Reaction score
394
Location
North Chelmsford
If you haven't measured the wort correction factor for your refrractometer, you can still take measurements over time. When then don't change over a few days, the fermentation is done. You may not know the FG precisely until you take a reading with a hydrometer but the yeast won't get you any closer if it is done processing all available sugars.
 
Top