MercuryiTC
©2014 Oxford Instruments Omicron NanoScience. All rights reserved.
Page 68
The simple diagram above shows the use of an ice bath to maintain the temperature of the
reference junctions. A calibration file with a zero voltage at the ice point must be used.
4.12.5 Heater control interlock
If a thermocouple is being used to control a heater and uses a cryogenic reference junction, it is
important to consider what happens if the reference junction cold bath runs out of cryogen.
If this occurs, the reference junction will warm up. Since the thermocouple voltage is a function
of the temperature difference between the reference and measurement junctions, the measured
voltage will decrease. The MercuryiTC assumes that the temperature of the reference junction
is fixed so the voltage reduction is interpreted as a reduction of the temperature of the
measurement junction.
If the iTC is using a heater to control at a set point, the reduction in the apparent temperature of
the measurement junction will cause the heater output power to be increased.
As the reference junction continues to warm up, the heater power will continue to increase.
To prevent this from happening, Oxford Instruments’ cryogenic systems include a
semiconductor device fitted to the cryogenic reference bath, together with the thermocouple
reference junction itself.
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