For a pressure P, the equation used to find the temperature T is
1) T = aP + b
where a and b are constants determined from two fixed points, such as ice at 0oC and steam at 100oC.
As mentioned previously, the constant volume gas thermometer uses values from the triple point of water to calibrate other thermometers. To recall, 273.16 K (Kelvin) is the temperature where water exists in an equilibrium state as a gas, liquid, and solid. For equation 1, where a = 273.16 K, b = 0, Ptp is the pressure of the gas at the triple point of water, and P is the pressure of the gas at the temperature to be measured, we have
2) T = 273.16 K (P/Ptp)
For low pressure and high temperatures, where real gases behave like ideal gases, equation 1 becomes:
3) T = 273.16 K lim P/Ptp as Ptp → 0