tank pressure gauge reading after carbonating

[barleyfreak]

Been reading about changing my carb method to eliminate venting while carbonating and I started wondering what the pressure gauge on the BT should read when carbonation is at correct volumes, and CO2 is disconnected. For example, if I correctly carbonate (however I do it) to 2.6 volumes, and my head pressure reads 12 psi, if I disconnect CO2 from the tank, then shouldn't the gauge still remain 12 psi days later?

A couple batches ago, I carbonated a batch, but didn't have a tap ready, so I disconnected the CO2, and a couple days later the gauge dropped to 8psi. Does that mean that that is the actual absorbed CO2, or do I have a leak? Stupid question perhaps, just wondering what the gauge should read, if no CO2 is applied after carbing.

[soia1138]

Sounds like a leak more than anything to me. If there are no leaks then our tanks will hold saturation pressure for as long as it sits. A carb tester/zahm wil tell you for sure but my money is on leak

[dick murton]

It might not be a leak, though this is a common problem. If the system is completely gas tight, it also depends on the beer temperature.

If 2.6 vol only requires a top pressure of 8 psi at that temperature, then more CO2 will dissolve out of the headspace into the beer over a period of time. If you are going to hold beer for that length of time without the dissolved CO2 content changing, then you need to adjust the head pressure so the temperature, top pressure and dissolved gas remain in equilibrium.

Download a temperature / top pressure / volume chart. I got mine from Meheen, and assume they still make it available. According to this, at 12 psi, any temperature below 37 F will hold more than 2.6 vol in solution. At 8 ps1, 2.6 vol is in equilibrium at 30 F. So my guess is that if your beer was cold enough, some of the top pressure gas will have dissolved. Also consider that when you thought the whole of the tank even carbonated, it is not impossible there is some, away from your sample point that is undercarbonated. And finally, your instruments may be giving you a slightly high pressure reading unless perfectly calibrated, leading to possible discrepancies when carbonating.

[barleyfreak]

Thanks for the replies. I am also guessing there is a leak. But so I understand it then, if there is no leak at all, and the gauge is accurate, and the beer has been evenly carbed to 2.62 vols at 37, the head pressure reads 12psi, and I shut off the gas to the tank, then the gauge should remain at 12psi. If it is carbed at less than 2.62 vols, and the head pressure is at 12psi when gas is shut off, then the pressure reading will gradually decrease a bit as it reaches equilibrium?

I do use a carb chart to make sure that dispense pressure and vols of CO2 match. Don't have a Zahm though.

[TonyT]

Sounds like what's happening is that you're assuming 100% efficiency from your carb stone (chart says 2.6vols at 37F/12PSI, must be 2.6vols...right?). Well most carb stones don't work at 100% efficiency. That means that it's possible that a portion of the co2 you expected to go into solution just went into the headspace. So at 12 psi at the end of carbonation, you may only be at 2.4 vols actual dissolved co2, which is why head pressure drops over a few days as the beer comes to equilibrium.

Without a zahm or some other co2 testing device, you can't accurately measure the volume of co2 in solution. Judging carbonation levels immediately after carbonation is complete the by using just temp/pressure without accounting for stone inefficiency will always be an educated guess rather than an exact number.

[barleyfreak]

Thanks TonyT, that is what I was guessing was going on (other than a leak).

[soia1138]

All good points but I just don't see a 4psi drop in only a couple of days. Also that would be a difference of like .4 volumes which is quite a bit and definitely something you would be able to detect without a Zahm. My leaning toward leak is the speed at which you drop from 12 to 8. That would mean a seriously inefficient stone/carbing procedure and still would likely take more than a couple of days for that head pressure to work into solution. The other option is that you had a temperature drop during those couple of days, not sure what your glycol/chilling system is.

[barleyfreak]

Temp change is unlikely -- all BT's in the walk-in, and with the 7bbl volume minor cooler temp fluctuations won't change the beer temp appreciably. Looks like a leak. Had another beer I finished carbing yesterday. It dropped from 11.5 psi head pressure to 11 in 24 hours w/ gas line shut off. Looking forward to using gitcheegumee's flowmeter process.