THERMODYNAMIC ANALYSIS OF HELIUM BOIL-OFF EXPERIMENTS WITH PRESSURE VARIATIONS

A thermodynamic analysis by calorimetric experiments in a system with changing pressure is presented. A general equation is derived for use in calculating the rate of heat loss from measured mass flow rate. The results show that the largest contribution from pressure variation is the sensible heat of liquid helium in a Dewar. A dimensionless parameter that was identified provides an indication of the importance of pressure variation relative to the latent heat of vaporization during an experiment. This dimensionless parameter is a function of system pressure (and the thermodynamic properties of helium), rate of change of system pressure, liquid helium inventory in the Dewar and measured mass flow rate. In the special case when the effect of pressure variation is small compared to the latent heat of vaporization, the heat loss rate is the product of the measured mass flow rate and the latent heat of vaporization, multiplied by a correction factor that is a function of the ratio of vapour density to liquid density. This correction factor is significant for helium at pressures near or above 1 atm and should always be included in the calculation.