Triple Point Immersion Cell Article

patent description

The invention is an integrated system for the realization of a single non-metal triple point via immersion-type fixed point cells. The base design can accommodate immersion-type triple-point cells of either argon, krypton, or xenon, or any other non-metal fixed point within an operating range between 75K and 175 K. This system is the first ever to implement a completely closed-cycle refrigeration solution to support immersion-type non-metal triple point cells. All previous such systems used liquid nitrogen pool boiling and were limited to argon cells only.

Our invention accommodates the highly practical immersion-type cell for use with common long-stem thermometers via a single externally accessible thermowell. In addition, our system accommodates capsule-type thermometers via three internal thermowells. In contrast, all previous work with non-metal triple-point cells in closed-cycle systems were limited to the use of the much less practical adiabatic-type cells that can only accommodate capsule-type thermometers are not directly accessible during use (i.e. while on the melt plateau). Having a system that accommodates immersion-type triple point cells allows users to direct access to the cell while on the melt plateau. Utilizing closed cycle technology allows the design to be readily adapted to argon TP cells or other alternative fixed points such as Kr and Xe TPs, for the calibration of long-stem SPRTs. This was never before possible due to the limitations associated with using designs based on the pool boiling of liquid cryogens.

The system is integrated into a single high-vacuum chamber and employs a four-zone design where active cooling is applied to the first two outer zones, and active control is implemented in all four zones. The refrigeration is provided by a set of two single-stage Stirling cryocoolers. These are commercial units that are mounted with passive vibration isolation and attenuation components to keep vibrations at acceptable levels for using standard platinum thermometers.

The system is designed around a single central 50 cm deep thermowell extending from an external port into the argon cell and continuously purged with pre-cooled helium gas. Three additional 46mm thermowells for capsule thermometers are accessible via the vacuum chamber. Having only one vacuum chamber greatly simplifies the design at the expense of inadequate cooling power available for freezing the argon sample in a reasonable time frame. This requires the insertion and use of an immersion cooler into the thermowell during the initial argon freeze.

The triple point cells are designed to be operated in the melting mode. During melting of the non-metal solid, the inner-most zone is set to adiabatic conditions, and thus limits the extent of any perturbative heat leaks that could otherwise affect the reproducibility of the melt plateau. An integral gas manifold and 9 liter storage volume permits the gas to remain at pressures below 300 psig when the system is at 300 K.

The invention provides a solution to the maintenance and performance limitations associated with liquid-nitrogen cooled systems in use for argon TP cells for the last 30 years. By eliminating the need for liquid nitrogen, three key problems are solved. First, the thermal stability, reproducibility, and duration of the melt plateaus are greatly improved since the thermal transients and variable gradients associated with the liquid pool are eliminated. Second, control zones' settings are not constrained by the vapor-liquid equilibrium of the nitrogen in a reservoir above the cell. In addition, our invention provides equally effective solutions for other higher-temperature fixed point cells, such as ones based on krypton and xenon triple point cells. These higher temperature fixed points were never before available for use as immersion-type cells.