At aprox 38” height, that’s about 1.3 PSI. So keeping it simple, at 1.4 PSI you’d be slowly pushing fluid back into the fermentor.
Again, it’s not about the amount of liquid. It’s the height of the column. 3 or 5 BBL fermentors come in all different shapes. The width is the main point I’m thinking of in this case. Cylinders are quite special in the way that doubling the diameter will quadruple the capacity that it will hold. Doing nothing to increase the PSI even though there’s more liquid. Since the height of the cylinder was not increased.
I’m mentioning this again so others don’t get the wrong impression about volumes and pressures. Aprox 38” under the surface in the great wide ocean will still be about 1.3 PSI.
To add to this with an engineering hat. The exact formula is:
p = 0.433 h SG
where
p = pressure (psi)
h = head (ft) (the total height of fluid)
SG = specific gravity of the fluid (water at 62 Deg. F. is 1.0)
So, 0.433 x 3.16 ft. x 1.03 SG (still fermenting) = 1.41 PSI
If you have a fermentor that is 30 feet tall with a water level of 25 feet it would be
0.433 x 25.0 FT x 1.03 SG = 11.15 PSI.
If you wanted to get really crazy you could add the valve if it isn't a full port valve which would have an equivalent pressure drop in foot of head.
Brew engineering fun. Enough of that I need a beer.
I like the product by the way.