Rf. Berg et al., MEASUREMENT OF MICROKELVIN TEMPERATURE DIFFERENCES IN A CRITICAL-POINT THERMOSTAT, International journal of thermophysics, 19(2), 1998, pp. 481-490
The density of a pure fluid near its critical point is extremely sensi
tive to temperature gradients. In the absence of gravity, this effect
limits the fluid's homogeneity. For example, at 0.6 mK above the criti
cal temperature, the microgravity experiment Critical Viscosity of Xen
on (CVX) can allow temperature differences no larger than 0.2 mu K, co
rresponding to a gradient of 10(-5) K.m(-1). The CVX thermostat, which
consists of a thick-walled copper cell contained within three concent
ric aluminum shells, was designed to achieve such a small temperature
gradient. However, asymmetries not included in the thermostat's model
could degrade the thermostat's performance. Therefore we measured the
temperature gradient directly with a miniature commercial thermoelectr
ic 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 c
onductance was much lower than that of the copper cell, thus ''amplify
ing'' the temperature differences by a factor of 60. When the thermost
at was controlled at a constant temperature, the steel cell's static t
emperature difference was 5 +/- 1 mu K. (The value inferred for the co
pper cell is 0.08 mu K.) Ramping the thermostat's temperature at a rat
e of 1 x 10(-5) K.s(-1) increased the temperature difference to 0.36 m
K. These results demonstrate the feasibility of achieving extremely lo
w temperature gradients.