Low temperature calibrator (LTC) for the range -190 to -100 °C

The project's purpose was to enable transferring of the ITS-90 temperature scale to industry reference instruments in the range -190 °C to -100 °C. The project has constructed an LTC, low temperature calibrator, adapted for comparison calibration in this temperature range. The equipment can be used for calibrations of industrial sensors at any temperature from -190 °C to -100 ° with low measurement uncertainty. The equipment is also adapted to the industry's need to calibrate sensors of varying sizes, especially short sensors.

The equipment consists of a cryo container supplemented by a comparator unit (a large copper cylinder). The cylinder was mounted in the cryo container  by being suspended in a holder surrounded by a tight pressure vessel. In the holder there is also a heating element that allows the temperature to be varied. Liquid nitrogen is used as a coolant in the cryo container, which causes the temperature in the comparator block to drop sharply, down to -190 °C. The temperature level in the block is then regulated with the electric heater connected to an external regulator and by reducing the pressure around the block, which reduces the cooling process.

Tests and simulations

In the initial phase of the project, a simulation was made of the temperature distribution inside the LTC equipment, in order to be able to choose a suitable insulation method, quantify the supplied electrical power and assess vertical and radial temperature gradients. These simulations were then validated in the built-up equipment and minor improvements were made to minimize the temperature gradients in the block. Tests were also carried out that showed the system's long-term stability.