Operator's Manual
Issue 14
/
Mar 2016 / UMC0071
Mercury
iPS
©2016 Oxford Instruments NanoScience. All rights reserved.
Page
73
6.6.3 Reference junction compensation
The MercuryiPS configuration options permit an internal or external reference junction to be
used. Each iPS temperature sensor circuit has a built-in temperature reference chip (ADT7310)
adjacent to the 9 way D-connector. The real-time temperature reported by this chip is used to
compensate the thermocouple voltage measurement.
The accuracy of the internal reference is limited by
the ADT730 device, which has an accuracy of 0.5C.
differences in temperature between pins 1 and 3 of the D-connector, which can be 0.5C or
more.
an assumption that the internal reference chip is 2.0C warmer than pins 1 and 3 in the 9 way D-
connector. This figure, derived from tests, is also limited by the accuracy of the ADT7310 chip.
Under some circumstances (eg. multiple heater cards installed and running at a high output)
this assumption may not be correct.
6.6.4 External reference junctions
For the above reasons, external referenced junctions are preferred for high accuracy
thermometry with thermocouples.
For cryogenic temperature measurement, the reference junction should also be at low
temperature. This is usually achieved with a liquid nitrogen bath. This is because the
thermocouple sensitivity drops off at low temperature, which has the effect of amplifying errors
from a room temperature reference.
