In high pressure calorimetry the pressure change is used to obtain the desired phenomenon (i.e. phase change) at constant
temperature. Two high pressure calorimeters have been developed to measure the latent heat of fusion of pure water (hexagonal
ice-type I) at subzero temperature. Both calorimeters used a constant pressurisation rate produced with a high pressure pump
driven by a step motor. The first calorimeter was a single cell calorimeter where mercury acted as the pressurisation fluid,
while the second one was differential (two cells) and was pressurised with pentane. Both calorimeters gave high accuracy data
of latent heat of fusion of pure water, which were determined taking into account that either the fluid used to pressurise
or the pressurisation rate affected the calorimetric signal.