ReadonlypressureGauge pressure the beer sat under at tempC, psi, or null when
the schedule finished open to atmosphere.
ReadonlytempThe temperature the figure was read at, °C.
ReadonlyvolumesVolumes of CO₂ still in solution when the beer is packaged.
The CO₂ a finished ferment leaves behind, and the conditions it was computed at.
The two cases use different published fits, so a caller showing "0.86 volumes" needs the temperature and pressure to say which one produced it. See scheduleResidualCo2.