CO2 does not go into solution after chilling

[MatthiasS]

I noticed a phenomenon which I cannot really explain. We are naturally carbonating our beers. We cap the fv at about day 3, the pressure rises to 1.8 bar and once fg is reached we set the tank on chill. Now I would expect the head pressure to go down to around 0.6 bar but that's not happening. The pressure stays at 1.8bar even after 4 days on chill. The beer is not overcarbonated which I confirmed with a zahm and nagel. Once we vent the tank below the saturation pressure it goes back up into equilibrium. But why does it only.do that when I vent the tank?
What do you think the cause of that is. We have 30 and 52hl fvs.

Matthias

[jebzter]

I would think that the headspace in the top of the tank is not actively cooled, so the residual pressure in the headspace does not drop as it would if the whole tank was cooled. This is what is incorrect about thinking that headspace pressure will drop when the beer is cooled to the same pressure that is the saturation point you are looking for. In actuality, the pressure will eventually go down to some equilibrium point, after the excess CO2 in the headspace dissolves into the beer after a week. Also, rough math on this since I do not know your process, 1.8 bar at fermentation temperatures, say 20C, if you took a fixed volume of the gas and reduced its temperature from that to say 1C, the new pressure would be 0.1 bar, way less than your saturation point. But basically, your headspace pressure will not rapidly change when the tank is cooled, it will take time, lots of it. A still tank probably would take a week, with agitation it would go faster.