Why is this one different?

This differs from other calculators in that it takes into account the headspace of the pressure vessel. Standard priming sugar calculators assume that the headspace is a couple of orders of magnitude less than the volume of beer. If you're filling a corny keg then this assumption is perfectly valid and should give good results. Also, if you're using forced carbonation, none of this matters because you keep the pressure fixed by topping up from a cylinder.

However, if you've got a plastic barrel (which tend to be about 25L) and you're only brewing 20L then you'll be slightly out with your sugar calculations. Use the above calculator with the defaults and just change the Headspace volume between 0.1L and 5L you'll see that the Mass of Glucose changes between 93.2g and 111.5g. That's a 19% difference, which equates to an actual carbonation of 1.82, not 2.

Of course, if you're only half filling a corny keg or barrel then these differences will be much greater. Above, if you enter Beer: 19, Headspace: 0.1 you get "88.6g". If you use 10/9.1 you get 80.4g. Not much difference? But you've got almost half the amount of beer! If you use the excellent Brewer's Friend priming calculator the former is 86.5g and the latter is 45.5g - much lower. 45.5g in a half empty keg translates to about 1.53 volumes of CO2, which is quite a way off your desired 2.

This calculator works out how much CO2 needs to be in the beer to reach the right carbonation level, the associated head pressure, how much CO2 is in the head then, from those, how much glucose is needed to make that mass of CO2. It also accounts for any dissolved CO2 from the fermentation. As a bonus it also works out what pressure you'd need to completely carbonate the beer in a single application of gas, rather than top-ups. In practice, unless you have a lot of head space and the temperature is low, your gas cylinder won't have enough pressure.

Initial pressure should be zero unless you're adding some CO2 from a cylinder. Sometimes Corny Kegs needs a bit of positive pressure to seal the lid.

See blog entry on priming for more details.

Modifications

• Correction added for the case where a keg needs some initial pressure e.g. to seal the lid.
• Henry’s Constant and the Van’t Hoff coefficient have been tweaked to match the Zahn-Nagel chart.

Assumptions

• The gases are 'ideal'.
• Only CO2 in the head space.
• Everything reaches equilibrium.
• A small temperature range (0-30C).
• Amount of CO2 converted to carbonates and bicarbonates in equilibrium is << 1% of dissolved CO2