<< Click to Display Table of Contents >> Navigation:  Software Overview > RMS Monitoring Software > Working with the RMS Software > Using RMS > Calculations used in RMS > Enthalpy (H)

To obtain useful measurements of relative humidity, it is extremely important that the measurement probe and measured material have the same temperature. The specific enthalpy of moist air is an energetic property. It is composed of the specific enthalpies of the components in the mixture (dry air, water vapor) and is related to the mass fraction of the dry air. It is given in J/kg.

 

The enthalpy (or energy content) of moist air at pressure Pb, temperature t (°C) and mixing ratio r (g/kg) is defined by:

 

h [kJ / kg moist] = 1.00464 t + 0.001846 r x t + 2.5 r

 

Note: by convention, the enthalpy of dry air (r = 0) at 0°C is equal to zero. Negative values of enthalpy are possible and indicate that the energy content of the air / vapor mixture is less than the energy content of dry air at 0°C.

 

1 lb = 0.4536 kg

1 BTU = 1.05507 kJ

h [BTU / lb] = 0.4299 x h [kJ / kg] + 7.68

 

The value 7.68 is added to make the temperature of 0°F the reference for enthalpy expressed in BTU / lb.