Normal "Leak-Down" During Carbonation... Or leak?

[doug293cz]

Ok, let's look at the OP's situation:

A corny keg has a total volume of about 5.3 - 5.35 gal, so let's assume 18.9 L (5 gal) of beer, and 1.5 L (0.4 gal) of headspace volume (which includes the CO2 tubing and internal regulator volume.) If the beer was fermented at 68°F (20°C) then it will start with a carbonation level of 0.84 volumes, and the total CO2 mass in the beer will be 31.34 g. If we purge the headspace of air, and pressurize to 12 psig (26.7 psia CO2 partial pressure), chill to 4°C (~40°F), and shut off the CO2 tank, then we will have 5.39 g of CO2 in the headspace.

Now the beer starts to absorb CO2 from the headspace, since at 4°C and 26.7 psia CO2 partial pressure (12 psi gauge pressure) the equilibrium CO2 volumes in the beer would be 2.52 volumes (which is way more than the initial 0.84 volumes.) As the beer absorbs CO2 the beer carbonation level increases, and the headspace pressure decreases. When equilibrium is reached, the beer carbonation level will be 0.93 volumes (34.76 g of dissolved CO2), and the headspace pressure will be -4.8 psig (9,9 psia CO2 partial pressure) and the mass of CO2 in the head space will have dropped from 5.39 g to 1.97 g, for a reduction of 3.42 g. For the gauge pressure to drop to 0 psi only requires the beer to absorb about 3.42 * 12 / (12 + 4.8) = 2.44 g of CO2 to be absorbed by the beer.

It's very plausible that the beer could absorb 2 - 2.5 g of CO2 overnight.

Brew on

 

[doug293cz]

Ok, let's look at the OP's situation:

A corny keg has a total volume of about 5.3 - 5.35 gal, so let's assume 18.9 L (5 gal) of beer, and 1.5 L (0.4 gal) of headspace volume (which includes the CO2 tubing and internal regulator volume.) If the beer was fermented at 68°F (20°C) then it will start with a carbonation level of 0.84 volumes, and the total CO2 mass in the beer will be 31.34 g. If we purge the headspace of air, and pressurize to 12 psig (26.7 psia CO2 partial pressure), chill to 4°C (~40°F), and shut off the CO2 tank, then we will have 5.39 g of CO2 in the headspace.

Now the beer starts to absorb CO2 from the headspace, since at 4°C and 26.7 psia CO2 partial pressure (12 psi gauge pressure) the equilibrium CO2 volumes in the beer would be 2.52 volumes (which is way more than the initial 0.84 volumes.) As the beer absorbs CO2 the beer carbonation level increases, and the headspace pressure decreases. When equilibrium is reached, the beer carbonation level will be 0.93 volumes (34.76 g of dissolved CO2), and the headspace pressure will be -4.8 psig (9,9 psia CO2 partial pressure) and the mass of CO2 in the head space will have dropped from 5.39 g to 1.97 g, for a reduction of 3.42 g. For the gauge pressure to drop to 0 psi only requires the beer to absorb about 3.42 * 12 / (12 + 4.8) = 2.44 g of CO2 to be absorbed by the beer.

It's very plausible that the beer could absorb 2 - 2.5 g of CO2 overnight.

Brew on

Nerdy math alert:

For those curious about how the calculations are done for the above, I submit the following derivation for determining the equilibrium pressure in a sealed system if you know the beer volume, headspace volume, temperature, and mass of CO2 in the system. The first equation allows you to calculate the starting mass of CO2, if you know the fermentation pressure (blue PA) and the current headspace pressure (green PA.) In OP's case they are not the same, but in other cases they may be.

The first thing we do is calculate the total CO2 mass. Blue PA = 14.7 psia, and green PA = 14.7 + 12 = 26.7 psia. We can then use the last equation below to calculate the new CO2 partial pressure when everything comes to equilibrium.

In the above equations, the meaning of the variables are:

CO2 is the mass of CO2 in grams​

VolB is the volume of beer in liters​

VolH is the headspace volume in liters​

PA is the CO2 partial pressure in psia​

TK is the absolute temperature in °K​

The blue part of the first equation comes from equation 2.1 in the attached .pdf converted for absolute (rather than gauge) pressure, and temp in °K (rather than °F.) 1.977 g/L is the value and units for 1"volume" of carbonation - i.e. the density of CO2 at STP. The green part of the first equation is just the ideal gas law.

Brew on

 

[Nagorg]

Thats awesome... Thanks @doug293cz !