The HPAK system is easier for me to understand (in terms of error management and uncertainty) because it uses a simple resistive Wheatstone bridge and a single differential amplifier for each thermistor in the sensor. It looks like the Tegam system uses multiple amplifiers to allow them to dispense with the two 1k bridge resistors. Presumably it relies on being able to cope with and cancel the offsets in the various amplifiers in their system as it auto nulls.
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It would be interesting to know how the amplifier offset issue is managed and how significant it can be.?
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There does appear to be a maintenance port at the rear of the 1830A. This isn't described in the manual and maybe this can support external DC substitution?
