Calgen™ and Conditioning of Cryogenic Sensors

Cryo-con's CalGen™ procedure generates high accuracy sensor calibration curves by fitting standard curves to selected temperature reference points. This provides a method for obtaining higher accuracy temperature measurements without the expense of full-range sensor calibrations.

CalGen™ can be used with Silicon Diode sensors at one, two or three temperature reference points as follows:

1. One point near 300K. The portion of a Diode sensor curve above 30K will be fit to a specified point near 300K. This is a two-point fit where the 30K point is taken from the existing calibration curve. The portion of the curve below 30K is unaffected.
2. Two points: 300K and 77K. Here, two user-specified points are taken to fit the diode curve region above 30K. The entire curve is offset to match the 77K point, then, the >30K region is fit to the two points.
3. Three points: 300K, 77K and 4.2K. Two points above 30K are fit as in the selection above. Then, a third point is used to fit a single point in the high-sensitivity region below 20K.
4. One point near 4.2K. This is a two-point fit where the 20K point is taken from the existing calibration curve. The portion of the curve above 20K is unaffected.

CalGen™ can be used with Platinum sensors at one or two reference points:

1. One point near 300K. The Platinum sensor curve is offset to fit the specified point.
2. Two points: 300K and 77K. The entire sensor calibration curve is fit to the two specified points.

Silicon Diode or Platinum temperature sensors can be ordered from Cryo-con with Calgen™ calibration. The reference points used are:

1. About 4.2K, Liquid Helium temperature.
2. About 77.35, Liquid Nitrogen temperature.
3. 310K using a precision bath.

These temperatures are established to an accuracy of ±10mK by using calibrated reference sensors.

Resistance thermometers will generally exhibit a shift in their calibration as the result of exposure to low temperatures. This is due to the difference in temperature expansion coefficients between the sensor element and its mount which causes a plastic deformation of the element. This deformation increases with decreasing temperature. Temperature cycling, or conditioning the sensor stabilizes the deformation and permits a consistent measuring accuracy over a reduced temperature range.

Thin film Platinum thermometers will exhibit a hysteresis when used below -50°C and then later used at temperatures above 150°C. At 150°C or below, the shift in the temperature curve remains stable; at higher temperatures, the curve returns to its original calibration value. Cryogenic conditioning by temperature cycling in Liquid Nitrogen will provide a stable calibration curve as long as the conditioned sensor is never exposed to temperatures above 150°C.

If an extended temperature range is required, a wire-wound Platinum sensor should be used since the sensor element is attached to a minimum strain mount.

All sensors offered by Cryo-con are conditioned for cryogenic use by cycling over their specified temperature range.