CO2 Affecting Hydrometer Readings?

[specharka]

Hi guys, I’ve been experiencing some anomalies between my refractometer and (calibrated) hydrometer readings for final gravity, and I’m starting to think it has a good deal to do with the fact that my FG readings use carbed beer. Here’s the last two scenarios that occurred:

Imperial Stout
OG 1.095, 22.7 Bx
FG 1.022, 11.4 Bx

Roggenbier
OG 1.054, 13.2 Bx
FG 1.019, 7.1 Bx

According to a reputable refractometer calculator, the FG for both beers is about 5-6 points off, and I was wondering if this could be attributed to CO2 in solution (both beers are carbed). Barring some unusual anomaly like both measuring tools being out of cal, I really don’t know which measurement is correct. I used to achieve 0.001-0.002 similarity between the two methods, now I’m not so sure.

Any thoughts from the crowd?

 

[Smellyglove]

It does. Some guy from my local forum has done some trials and it's different from carbed to uncarbed beer.

 

[dmtaylor]

Yes, carbonation will tend to increase the reading. For example, a 1.008 FG beer might look like 1.010 with carbonation because the CO2 bubbles cling and float the hydrometer slightly. When taking FG readings, I always swish the sample around for a couple minutes to remove any carbonation.

Cheers.

 

[specharka]

Yes, carbonation will tend to increase the reading. For example, a 1.008 FG beer might look like 1.010 with carbonation because the CO2 bubbles cling and float the hydrometer slightly. When taking FG readings, I always swish the sample around for a couple minutes to remove any carbonation.

Cheers.

Yeah, I’ve done that with both of those samples and the readings definitely fell a little bit, probably 0.001-0.002 over 15 minutes, but they were still high compared to the refractometer readings. I’m just wondering if my methodology might be flawed and maybe the dissolved CO2 actually increases the specific gravity? Shooting from the hip here.

 

[dmtaylor]

Cold temperature will also increase gravity readings. Are your samples at 40-50 degrees F? That also could account for high readings, and with the CO2 also could be adding as much as 0.003-0.004. Then as the samples warm up over 15 minutes or whatever, the temperature impact could be another part of the reason you see a change.

I'm going to crunch some more numbers as I consider myself somewhat of an expert on refractometer use for FG readings. I'll let you know if I come up with something more. But for the time being, think about temperature. I think maybe I'm right about that.

Also... have you calibrated your refractometer in plain ~70 F water? If not, that could be another big part of the problem.

 

[RPh_Guy]

I wouldn't trust readings without running 2-point calibration on both instruments, at calibration temperature. ... Then I would start thinking about reasons for any differences between the two instruments.

 

[dmtaylor]

Yeah... I'm willing to bet on the temperature effect, and calibration problems. If your recalibrations find that your previous FG hydro readings are low by 0.003-0.004, and all Brix readings are low by 0.2 Brix or something like that, then the hydro and refractometer should be able to match up again within 0.001-0.002, which is the best margin of error that any refractometer calculator is able to do. I love my refractometer but I understand the FG calculations can only get me within that margin of error, and only if calibrations are stellar.

 

[specharka]

I wouldn't trust readings without running 2-point calibration on both instruments, at calibration temperature. ... Then I would start thinking about reasons for any differences between the two instruments.

Just checked it today. Both hydrometers (OG & FG) read 1.000 in 68F water and the refractometer reads 0.0 Bx as well. I brewed a black IPA today and both readings came in identically at 1.070 OG. So I’m still not sure where the discrepancy is coming from.

 

[RPh_Guy]

read 1.000 in 68F water

This is one calibration point.

Make a 22 Brix solution with fresh table sugar using a scale with 0.1g resolution or better, and then compare readings.

If you want lab-quality accuracy and precision with your instruments you need to start with lab-quality calibration technique.

 

[dmtaylor]

What temperature was your carbonated beer when you took the FG readings??

 

[specharka]

What temperature was your carbonated beer when you took the FG readings??

Both beers were at ambient temperature, around 72-74F.

I took another reading with 4 oz (old) table sugar dissolved in 2 cups of water and got readings of 1.079 SG and 19.0 Bx.

 

[BobBailey]

Degas the beer before taking a hydrometer reading. The bubbles will cling to the hydrometer and make it more buoyant. Also. you need to adjust for temperature. Most hydrometers are calibrated at either 60F or 68F.
I wouldn't expect a refractometer reading to be affected by the presence of CO2 as long as the correction factor was calculated properly. Never really experimented with this, so may be wrong here.

 

[dmtaylor]

Both beers were at ambient temperature, around 72-74F.

I took another reading with 4 oz (old) table sugar dissolved in 2 cups of water and got readings of 1.079 SG and 19.0 Bx.

Okay, so maybe it really is just a CO2 thing? I honestly don't know. Hmm. Interesting.

 

[Yooper]

Both beers were at ambient temperature, around 72-74F.

I took another reading with 4 oz (old) table sugar dissolved in 2 cups of water and got readings of 1.079 SG and 19.0 Bx.

There are ways to figure any wort correction factor (which sounds like what is going on here). The reason is simple- refractometers are designed to read the refraction of light in a sugar solution. Wort has other things, so you should always figure your wort correction factor.

The idea is to take a known wort (not sugar!!!!!) measurement. So, say you use 1/2 pound of light DME in 2 quarts of water. If the measurements are exact, your SG will be 1.044. Take it with a refractometer and with a hydrometer, and then apply a wort correction flavor to all other wort/beer readings when you compare.

To really make sure you're accurate, do more than one. Here's a good way to do it: https://www.brewersfriend.com/how-to-determine-your-refractometers-wort-correction-factor/

Once you plug your wort correction factor into a fairly accurate calculator, it should be closer.

by that I mean, not exact. It's close, but since they are different instruments reading different things, "very close" or "close" is about the best you can hope for. If you want accuracy, use a precision hydrometer only for FG readings.

 

[aprichman]

You have to decarbonate the beer before taking a hydrometer reading.

In brewing science we had an easy technique for this. Get two glasses, pour the beer back and forth between the glasses 20 times before pouring into the hydrometer tube. This will help break the carbonation out of the liquid.

Let the beer settle for a few minutes before taking your reading.

 

[specharka]

I think I’ve diagnosed at least part of my issue. I brewed a black IPA about two weeks ago, which has been sitting on dry hops for 10 days. A little long, I know, but the beer just wouldn’t seem to attenuate. So I performed an experiment with the gravity samples.

OG = 1.070, 17.2 Brix
FG = 1.019, 9.1 Brix

I de-carbed the beer and let the hydrometer sample settle for an hour. Initially, the reading rose to almost 1.020. As the hops began to settle and the residual CO2 dissipated, the reading came down to match the refractometer calculated measurements.

FG = 1.016

Readings were taken at approximately 0, 30, and 60 minutes. At the very least, this confirms that calibrated refractometer readings are still a good approximation of hydrometer readings, but I should let hydrometer readings settle before I read into them too much.

 

[dmtaylor]

Thanks for the intriguing update!