Measurement of Microkelvin Temperature Differences in a Critical-Point Thermostat

The density of a pure fluid near its critical point is extremely sensitive to temperature gradients. In the absence of gravity, this effect limits the fluidʹs homogeneity. For example, at 0.6 mK above the critical temperature, the microgravity experiment Critical Viscosity of Xenon (CVX) can allow temperature differences no larger than 0.2 uK, corresponding to a gradient of 10~5 K • m~-1. The CVX thermostat, which consists of a thick-walled copper cell contained within three concentric aluminum shells, was designed to achieve such a small temperature gradient. However, asymmetries not included in the thermostatʹs model could degrade the thermostatʹs performance. Therefore we measured the temperature gradient directly with a miniature commercial thermoelectric cooler consisting of 66 semiconductor thermocouples. We checked the results with a half-bridge consisting of two matched thermistors. The measurement was made along a thin-walled stainless-steel cell whose conductance was much lower than that of the copper cell, thus "amplifying" the temperature differences by a factor of 60. When the thermostat was controlled at a constant temperature, the steel cellʹs static temperature difference was 5 ± 1 uK.. (The value inferred for the copper cell is 0.08 uK.) Ramping the thermostatʹs temperature at a rate of 1 x 10~5 K • s~1 increased the temperature difference to 0.36 mK. These results demonstrate the feasibility of achieving extremely low temperature gradients.