A negative volumesToAdd (the beer already holds more CO₂ than the target)
clamps the dose to zero and is reported as such.
There is no keg rate
Homebrewing folklore says to prime a keg below the bottle rate. We don't:
19 L in a Cornelius keg gets the same dose as the same 19 L in bottles.
This section explains why.
Published guidance.Brew Your Own's Mr. Wizard says to "use about
75 % of the priming sugar suggested in a recipe for bottle-conditioned
beer". Forum advice and kegging write-ups say one half to two thirds.
So the range is 0.50–0.75, which is a factor of 1.5 wide, and none of it is
derived.
The only attributable source doubts it. The same Mr. Wizard answer that
gives the 75 % says: "I have never been able to find the original source of
this advice. Most of the explanations for this are vague."
https://byo.com/mr-wizard/priming-when-keg/
Headspace doesn't account for it. The usual explanation is that some
of the CO₂ the sugar makes fills the keg's ullage and never dissolves. Nineteen litres at 2.4 volumes needs 2.4 × 19 × 1.96 = 89 g
dissolved. A 19 L Cornelius keg filled with 19 L of beer leaves ~1 L of
headspace, which at the equilibrium ~1.8 bar absolute and 4 °C holds
PV/RT = 1.8e5 × 1e−3 / (8.314 × 277) ≈ 0.08 mol ≈ 3.5 g. That's under
4 % of the dose, while the rule of thumb withholds 25–50 %. The same beer in
fifty-three bottles has ~795 mL of headspace in total, which at 20 °C holds
~2.6 g, so the two are within a gram of each other. Mr. Wizard does the
same calculation and reaches the same conclusion.
The stoichiometry is the same in either vessel.sugar → CO₂ is a mass
balance. Putting 89 g of CO₂ into 19 L of beer takes the sugar that makes
89 g of CO₂, whether the beer is in one keg or fifty-three bottles.
Real losses, but not a coefficient. A keg is pressurised to seat the lid
and then vented, often more than once to purge oxygen, and every vent loses
CO₂ the sugar made. Poppets and lid gaskets also leak a little over the
weeks of conditioning. Those losses are real, but they depend on the
brewer's routine and gaskets, not on kegs in general, and nobody has
measured them. That's why the published range is 50 % wide with no
derivation. A brewer who vents a lot can take a few grams off themselves;
we won't guess at a number the literature can't support.
The alternative. Skip the sugar and spund at
pressureFromCo2Volumes for the target and the fermentation
temperature, so the pressure sets the carbonation directly. That's the
reliable way to avoid over-carbonating a keg.
Priming sugar for a target carbonation.
A negative
volumesToAdd(the beer already holds more CO₂ than the target) clamps the dose to zero and is reported as such.There is no keg rate
Homebrewing folklore says to prime a keg below the bottle rate. We don't: 19 L in a Cornelius keg gets the same dose as the same 19 L in bottles. This section explains why.
Published guidance. Brew Your Own's Mr. Wizard says to "use about 75 % of the priming sugar suggested in a recipe for bottle-conditioned beer". Forum advice and kegging write-ups say one half to two thirds. So the range is 0.50–0.75, which is a factor of 1.5 wide, and none of it is derived.
The only attributable source doubts it. The same Mr. Wizard answer that gives the 75 % says: "I have never been able to find the original source of this advice. Most of the explanations for this are vague." https://byo.com/mr-wizard/priming-when-keg/
Headspace doesn't account for it. The usual explanation is that some of the CO₂ the sugar makes fills the keg's ullage and never dissolves. Nineteen litres at 2.4 volumes needs 2.4 × 19 × 1.96 = 89 g dissolved. A 19 L Cornelius keg filled with 19 L of beer leaves ~1 L of headspace, which at the equilibrium ~1.8 bar absolute and 4 °C holds
PV/RT= 1.8e5 × 1e−3 / (8.314 × 277) ≈ 0.08 mol ≈ 3.5 g. That's under 4 % of the dose, while the rule of thumb withholds 25–50 %. The same beer in fifty-three bottles has ~795 mL of headspace in total, which at 20 °C holds ~2.6 g, so the two are within a gram of each other. Mr. Wizard does the same calculation and reaches the same conclusion.The stoichiometry is the same in either vessel.
sugar → CO₂is a mass balance. Putting 89 g of CO₂ into 19 L of beer takes the sugar that makes 89 g of CO₂, whether the beer is in one keg or fifty-three bottles.Real losses, but not a coefficient. A keg is pressurised to seat the lid and then vented, often more than once to purge oxygen, and every vent loses CO₂ the sugar made. Poppets and lid gaskets also leak a little over the weeks of conditioning. Those losses are real, but they depend on the brewer's routine and gaskets, not on kegs in general, and nobody has measured them. That's why the published range is 50 % wide with no derivation. A brewer who vents a lot can take a few grams off themselves; we won't guess at a number the literature can't support.
The alternative. Skip the sugar and spund at pressureFromCo2Volumes for the target and the fermentation temperature, so the pressure sets the carbonation directly. That's the reliable way to avoid over-carbonating a keg.