When we considered low temperature comparison calibration, we had to take into account health and safety considerations of both the liquids used in the compressors to generate low temperatures, and the liquids used in the calibration volume itself. The cost of chillers increases considerably for very low temperatures. We concluded that using chillers for very low temperatures was expensive, unsafe, and unreliable.
Our solution is a simple, heated metal block design using a single, safe, surrounding liquid nitrogen.
An apparatus immersed in liquid nitrogen will cool to approximately -195°C. Our design, using a controller and heater, permits the cryostat to be set at any temperature above liquid nitrogen temperature.
The Cryostat comprises an insulated machined copper equalizing block inside an 80mm diameter tube 480mm long, attached via a flange to a lid, giving access for three thermometers, a vacuum port, and a Lemo connector for the temperature sensor and heater. A cable runs to a controller, which sets the temperature. An RS422 connector permits the calibration to be automated using one of our Software programs.
The most used range for the Isotech Cryostat is between -80°C and-180°C. This temperature range is selected because oxygen condenses at -186°C, and if this is then accidentally coiled off, it can cause a health and safety issue. The performance of the Cryostat actually improves as the temperature is lowered, because the temperature difference between Cryostat and its surrounding liquid nitrogen is smaller. These measurements were made with two 670 model thermometers.
The Cryostat is connected to a vacuum pump. It is pumped for five minutes, and then sealed. The Cryostat is connected to the controller, and switched on to check the connections. The cryostat is then lowered into the container of liquid nitrogen, which should come to between 25mm and 75mm below the flange. Three thermometer tubes exit the cryostat, and have nylon compression fittings. The standard PRT and unknown thermometers to be calibrated are lowered into the wells, and the nylon fittings are hand-tightened until the nylon grips the thermometer without damage. The controller is set to the required temperature, and the Cryostat left to stabilize before comparison readings are made.