Spunding Valves: Gravity points to PSI conversion?

[isomerization]

I am just getting into using a spunding valve. With the first batch I used it on, I transferred to a corny once primary fermentation appeared to be slowing down (gravity was at 1.020, expected to hit 1.015-1.018, mash temp hit 156F urgh) and added the spunding valve.

So far, 48 hours later (at 70F), I have 8 psi built up. My questions is this, is there an easy way to convert between changes in gravity and psi generated? I realize temperature is a variable here as well, so could just be a rough approximation. Thanks!

 

[mredge73]

I don't think there is an easy way...
I am sure you could do it with some complicated math in a spreadsheet but you will need to have really good numbers on your head space volume.
Yeast also produce differing amounts of C02 based on what they are eating (maltose vs fructose vs glucose).
There is some science in the web on using yeast to produce C02 for aquarium use (growing algae) that may help.
If you build one, I would love to see it.

 

[dyqik]

Oops, misunderstood the question slightly, so editing.

Equilibrium carbonation works the same as for force carbing kegs, and gravity points turn to CO2 roughly the same as for priming with malt extract.

E.g. 8 psi at 65F gives an equilibrium carbonation level of 1.29 vols of CO2. Priming 5 gallons fermented at 65F to naturally carbonate to 1.29 vols requires 1.42 oz of DME. 1.42oz of DME adds about 40 gravity points per pound per gallon, so 1.42 oz in 5 gallons adds 0.00071 to the gravity. Therefore at 65F, a reduction of 0.00071 in gravity turns into 8 psi of Spunding pressure, assuming the headspace is much smaller than the beer volume. If it's not, you'll have to include the additional quantity of CO2 in the headspace in the calculation.

However, that's assuming that the sugars giving the final gravity points are as fermentable as the first sugars that are fermented, which is probably not really true, and you might have to make a correction depending on if maltose is being consumed or not. That's going to depend on whether your yeast is known to process maltose.

(I chose 65F as the temperature because the first forced carbonation chart I found only went that high...)

 

[isomerization]

Thank you both for the responses. It might be overly simplistic (due to differences in sugar-based CO2 production), but I think I can just use a force carbonation chart extended to 70F. For example (at 65F), in order to hit 2.4 volumes, 27 psi would need to be generated/trapped. That would correspond to less than 3 points of gravity in a 5 gallon batch, which seems about right (I think).

 

[dyqik]

Thank you both for the responses. It might be overly simplistic (due to differences in sugar-based CO2 production), but I think I can just use a force carbonation chart extended to 70F. For example (at 65F), in order to hit 2.4 volumes, 27 psi would need to be generated/trapped. That would correspond to less than 3 points of gravity in a 5 gallon batch, which seems about right (I think).

Yeah, I think that'll work pretty well, and sounds about right.