Evaluation of phase transition errors in heat capacity calorimeters using SPICE simulated RC models.

A technique is presented which allows the development of extremely complex (> 5000 components) RC models of calorimeters and provides simulation under a wide variety of inputs. A commercially available circuit simulation program (SPICE) is used to 'build' RC models of relaxation and scanning calorimeter designs used to measure heat capacity. The instrument models are constructed using subcircuits to represent small elements of the materials used in the designs. Only simple linear equations are needed to evaluate the RC values in the subcircuit. The subcircuits also use voltage controlled switches to simulate phase transitions where lambda, C, and delta h can change value instantaneously at predetermined temperatures. The resulting simulations of the two designs provide an ability to predict instrument sensitivity to lambda and C independently. Simulated outputs agree with measured outputs within 10%. Model simulations show serious errors in the heat capacity determinations from both designs during phase transitions. Interactions between lambda, C, and delta h are shown for both designs. The technique provides a means to construct, evaluate, and optimize a calorimeter design completely in software.