FP-TC-120 ok at low temps but gives bad readings at >200 C

[Matt1]

I've been using the FP-TC-120 for years with no problems reading type-K thermocouples at 20 - 150 C temperatures or so.  Recently I wanted to read six (type-K) thermocouples in a furnace at 800 - 1200 C temperatures, but the unit gives me bad readings (in both LabView and in FieldPoint Explorer).  The six thermocouples are fine when I turn the furnace on, but about when the first thermocouple gets above about 250 C I start getting almost random values for all six, with occassionally the right values coming through.  If I disconnect all but one of the thermocouples, then I get a good, steady reading.  Some times it will work for two, but never for all six.  My firmware is "0030".  The FieldPoint unit and FP-1000 base are all at room temperature.

Thanks for any help.

[Hal_L]

Hi Matt,

I was wondering what revision of the FP-TC-120 you currently have.

Regards,
Hal L.

[Matt1]

My (firmware) revision is 30.  FieldPoint Explorer is version 3.0.2.

Further tests today (using a heat gun) gave no problems with a pair of thermocouples at 380 C and several others at room temp.  I am now wondering whether my problem was due to radiative heating of the FieldPoint module(s), since the furnace exterior (at about 125 C) would be partially visible by them.  I've moved the modules to a sheltered location, added some fans, and plan to try again in a couple of days.  The behavior suggests that the multiplexing chip is not functioning properly; perhaps that is due to it overheating.

Thanks for your interest.