RADIATION CALORIMETER FOR HEATING AND COOLING RAMPS USED FOR HYSTERESIS MEASUREMENTS AT PHASE-TRANSITION

Caloric data are usually obtained by measurement of the temperature ra mps or steps with electrical heating of a test cell. Electrical heatin g provides the greatest accuracy for measurement of the heat input to the cell, but this measurement can only be carried out in the directio n of increasing temperature. In special cases at the phase transition points, however, reversible heating and cooling runs are necessary in order to study hysteresis effects, and to carry out measurements into the metastable liquid-vapor and liquid-solid transition regions. Such a reversible calorimeter has been designed for the special use of c(v) -measurements in the critical region of fluids under microgravity. Tak ing into consideration the long relaxation time required for heating a fluid from the two-phase region through the critical point into the o ne phase region, a calorimeter was developed where the measurements st art in the homogeneous density region above T(c), and the sample is co oled down into the two-phase region. In microgravity the density will remain homogeneous down to T(c) and it is anticipated that the real va lue of c(v) can be observed not being influenced by density stratifica tion. A fluid sample of SF6 at the critical density is enclosed in a s pherical cell, which was produced by an electrolytic procedure. The sp here is mounted in a thermostat of various shells, which can be contin uously heated or cooled. The energy transport to the sphere takes plac e by radiation. By measuring the temperature difference between the sa mple cell and the surrounding shell, as well as the temperature ramp o f the sample cell itself, the heat capacity c(v) can be determined. Th e paper describes the design of the calorimeter, and the initial measu rements demonstrate the functionality of the calorimeter concept. Not only the hysteresis effect between heating and cooling runs, caused by the different density stratification under earth conditions, but also a remarkably good agreement between the experimental results and the theoretical model behavior can be observed.