In an underground ventilation system where there is a change in the moisture content of air, sigma heat is useful to identify and quantify the heat transfer processes. The literature on psychrometry suggests that the sigma heat is a function of barometric pressure and wet-bulb temperature. However, a direct expression of this relationship has been elusive. This paper presents a direct and straightforward expression for sigma heat as a function of wet-bulb temperature and barometric pressures within the range generally encountered in the underground mine environment. Also, a direct equation for wet bulb temperature as a function of sigma heat and barometric pressure is presented, which otherwise require lengthy calculations involving iterations. The difference between calculated and predicted sigma heat and wet bulb temperature is within +1.32 kJ/kgda and -1.39 kJ/kgda; and +0.28 °C and -0.64 °C, respectively. The efficacy of these prediction equations is quantified with some examples involving the performance parameters of cooling tower and spray chambers. Moreover, a psychrometric chart with sigma heat as a parameter in place of enthalpy is produced with the help of MATLAB code and presented in this paper.
