Taking an accurate FG.

Mon Mar 06, 2017 7:38 pm

Entrained gasses may affect our gravity readings. Any liquid at equilibrium with the atmosphere will have entrained gasses which would cause a gravity reading to be (very?) slightly lower than it actually is. We can assume that OG readings using freshly cooled wort that have little or no entrained gasses will be accurate within the tolerance of our instruments, but what about an FG reading?

A finished brew will have considerable CO2 dissolved in it. And let's say that you forget to take an FG, and have to use a carbonated sample from the keg. What then?

This is one of those trivial things that I did not concern myself with in earlier times. It still does not greatly concern me, but my nature causes me to wonder what correction I should make when taking a reading from a finished brew or a carbonated sample.

Charlie
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Re: Taking an accurate FG.

Sun Mar 26, 2017 2:18 pm

Funny you should post this Charlie.

For my last five brews, I have been trying to keg spund them with about 6 gravity points remaining in fermentation to capture the natural CO2 produced by the yeast. A couple I have hit, and others not so much. Either way, an actively fermenting beer has lots of CO2 dissolved in it making final gravity readings a PITA. Here is what I have done to overcome it for hopefully a more accurate reading.

Collect my sample into a large glass and simply stir the crap out of it with a spoon. Remove any head that forms and keep stirring quickly. Let it rest for a few minutes, come back and continue. Within a few solid rounds of this, most of the CO2 has been "knocked" out. At least for a better, closer reading than before when all the CO2 was just pushing my hydrometer sky high above the liquid.
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brewinhard
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Re: Taking an accurate FG.

Thu Apr 20, 2017 7:54 am

brewinhard wrote:Funny you should post this Charlie.

Collect my sample into a large glass and simply stir the crap out of it with a spoon. Remove any head that forms and keep stirring quickly. Let it rest for a few minutes, come back and continue. Within a few solid rounds of this, most of the CO2 has been "knocked" out. At least for a better, closer reading than before when all the CO2 was just pushing my hydrometer sky high above the liquid.


This is exactly what I do. Also if your sample is warmer, the C02 will come out of the solution quicker. Just make sure to account for the temperature difference if above 60 degrees! :jnj
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Re: Taking an accurate FG.

Sat Apr 22, 2017 1:45 pm

I have read that pouring through a coffee filter will pull the CO2 out of it as well. I have been on the spund wagon as well, and generally just put the sample out and then go do something else for a while so it warms/decarbonates. If I do decide to whip it out, I'm thinking chopstick.

wait...not that what I 'whip out' is like a chop stick, that thing is a full stack of dimes.
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Re: Taking an accurate FG.

Sun Apr 23, 2017 2:25 pm

NateBrews wrote:wait...not that what I 'whip out' is like a chop stick, that thing is a full stack of dimes.


Holy shit! Nate, that is hilarious man!

Wait a minute, aren't a chop stick and a full stack a dimes pretty much the same thing? :lol: :lol:
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Re: Taking an accurate FG.

Sun Apr 23, 2017 8:45 pm

Cool. I appreciate the info.

I hate to be a worry wart, but it concerned me that on the rare occasion when I did not take an FG I would get a bad reading with a carbonated sample. It seems that my concern was not unfounded.

When I was at GRB we had a thing for reading volume of dissolved CO2 in finished beer. You filled it with brew right out of the bright tank, shook the crap out of it, and then took the reading off a pressure plunger. Seems like the principle is the same.

Charlie
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