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I use a Refractometer when I brew mostly for the boil off phase of an all grain batch. This helps me make sure I get to the exact FG I want for the specific type of beer. It works well and I don’t have problems.

One day while drinking one of my Märzen beers I wondered what the gravity of my beer would be now. Even though I’d taken the FG before racking and knew it was on point. So I took a few drops adjusted for temperature and took a reading.

My FG for this beer was 1.013 - I racked to keg and then force carbonated. What was the reading of my beer now (?) 1.025

I was shocked. This must be a mistake I thought - I knew the bubbles would increase the buoyancy and so the gravity but that would have a bigger effect on a hydrometer surely?

I’ve subsequently brewed and tested all through the forced carbonation process and yes the gravity rose. I estimate that a carbonation level of 2.5 - 3 gives you a gravity reading of 1.025 - 1.035.

Has anyone else experienced this? It is an adequate measure of carbonation levels? Where could one find out more about the relationship between gravity and carbonation levels?

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    How did you measure the FG of the beer? If you measured it with a hydrometer, then the refrac will definitely show you a higher gravity due to the presence of alcohol. I'd be curious to hear what else you come up with - my initial suspicion is that this wouldn't be a reliable way to measure carbonation, but I'd loved to be proven wrong! – Frazbro Nov 25 '19 at 21:31
  • I always use an Hydrometer for final gravity. The refractometer gave me the same reading - everything was calibrated correctly. What astounded me was I took readings during the carbonation process and I could see the SG rise. I agree that it might not be best way. But I don’t actually know another way to ‘measure’ carbonation levels. – zatbusch Nov 26 '19 at 5:12
  • I don’t think you are going to get an accurate reading. Refrafractometry relies in coefficients for the mixture in question. In this case, wort. When alcohol is produced this skews the results. Carbonation will affect this as well. However, since a carbonated beverage is not a homogeneous mixture, I’m not sure there is an accurate way of developing a coefficient for this. – mreff555 Nov 26 '19 at 21:14
  • Something also doesn't seem right if you got the safe FG reading from your hydrometer and your refrac. The way to measure carbonation is to measure the temperature and pressure of a sample at equilibrium, then you can either do the maths yourself or consult a lookup chart. Granted, that's not always practical (like if you want to see if a bottle is carbed), but it is accurate. If you're measuring carbonation in a keg though, it's by far the best way. Unhook the gas and leave it be for a while, then measure temp and pressure. – Frazbro Nov 27 '19 at 21:49

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