chances are you have a fault in the circuit somewhere... The fact that it is reading so low...
most theromcouples are rated for a certain temp range (the most common is copper constantan, -180C to 400C) and -198 sounds like the low end of your particular probe.
Here is how a thermocouple (diode) works: Two dissimilar metals are fuzed together. Since they are different metals there is a voltage drop (contact potential) accross the interface. This voltage drop increases with increasing temperature.
so your temp probe is just a calibrated voltmeter. which also means if the metal in your thermocouple is touching a charged surface you will get erronious readings.
sorry if that was too disjointed...
Tokamac
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