Accurate Volumes of CO2

This question is probably more academic than practical, but here goes:

I'm using a Johnson A419 temp controller on my newly finished keezer. I have the set point at 42*F with a differential of 4*, which means the lowest temp the kegs will see is 38*F. I'm assuming an average temp of 40*, although maybe that oversimplifies the thermodynamics of the situation, I dunno.

So my question is, when using the volumes of CO2 tables (or Beersmith, in my case) to calculate the pressure to set my regulator at if I want, say, 2.5 volumes of CO2, should I be using the lowest temperature the kegs see to determine the correct pressure, or the average temperature they sit at? My gut tells me to go with the average, but if a little bit more CO2 gets into solution every time the keezer cycles down to 38*, I'm not sure how quickly it will equilibrate back out of solution as it warms back up to 42 after the compressor is done cycling.

Bottom line, I doubt this really matters enough to tell the difference in volumes of CO2 when actually drinking the beer, but my academic curiosity and futile pursuit of perfection in my homebrewing has gotten the better of me.

Gimlet said:

I'm using a Johnson A419 temp controller on my newly finished keezer. I have the set point at 42*F with a differential of 4*, which means the lowest temp the kegs will see is 38*F. I'm assuming an average temp of 40*, although maybe that oversimplifies the thermodynamics of the situation, I dunno.

Click to expand...

I thought when it comes to cooling - if your controller set point is 42 with a differential of 4, then the temp will range from 42 to 46. Not 38 to 42.

As for the carbonation differences... I'd set the pressure based on the average temp, like you said.

Seven said:

I thought when it comes to cooling - if your controller set point is 42 with a differential of 4, then the temp will range from 42 to 46. Not 38 to 42.

Click to expand...

According to my A419 manual (see figure 7), when the controller is in the cooling/cut-in mode as it came pre-programmed for me, the setpoint is where the controller closes the relay (powers on), and it powers off when it reaches the setpoint minus the differential. Having kept an eye on the temps over the past week of using the controller bears this out - the temp is always in the range of 38-42.

Thanks for your input about the average temp, though.

The difference in Co2 volume will be negligible but, if you adjust for the lowest temp, eventually the beer will find equilibrium at this low point and head pressure will increase at the higher temp to maintain Co2 levels in the beer. Assuming you are leaving the tank connected the entire time...