The steam specific volume is defined by engine performance. The latter can either be measured or, in the Mollier, the steam evolution in the cylinder represented starting from steam chest steam conditions and drawing the enthalpy drop line. Due allowance should be taken for the internal thermodynamic efficiency obtaining in the cylinder. The latter should be taken as 0.8 for modern single expansion engines and 0.88 for compounds as an average figure at high powers. Should compressibility be accounted for, the pressure at the tuyère can be obtained deducting some 1.4 smokebox draught from the atmospheric pressure. This correction may reach 10% and due account should be taken of the adiabatic temperature drop.

The gas specific volume v₀ at STP (0°C, 760 mm Hg) is obtained by calculation of the performance of the boiler, better if actually measured in controlled road or plant tests. If compressibility effects are accounted for, the same corrections as for steam are applied.

The specific volume of the gas mixture v₁ can be obtained by a weighted average of both the specific volume of steam v and gas v_b. An additional correction is that due to the temperature increase obtaining because of friction and shock losses in the mixing chamber. These losses are:

[kg m² s^-3]

x = 0.1; and are to be reported to unit mass of mixture. This gives a temperature increase from which a corrected value of v₁ can be obtained.