When we considered low temperature comparison
calibration, we had to take into account Health and Safety
considerations, both of the liquids used in compressors to
generate low temperatures and also 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, 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. |