In operation, liquid from the test facility is circulated through the RHS core. The liquid is heated by passing over the resistance heaters. If the test facility's calibrations are under investigation, the RHS input power and temperatures are compared with those recorded by the test facility. If a collector is under test, the RHS is placed in series with the collector. The power input to the RHS is placed in series with the collector. The power input to the RHS core heaters is adjusted to give a temperature rise through the RHS equal to some fraction (usually one-half to one) of the temperature rise across the collector. Since the same mass flow passes through both the RHS and the collector, the energy gain in the collector is simply the RHS input power multiplied by the ratio of the temperature rises.