Setup: K-type ungrounded thermocouples in 0.125" steel-sheathed probes -> NI-9219 Analog input module -> cDAQ-9174 -> lab computer running FlexLogger 2025 Q1.
I've noticed two problems:
-When these thermocouples are inserted into sand and then placed in a high-temperature (550*C) furnace, the initial measured response is a slow (minutes) downward temperature transient (contrary to what should happen, a period (minutes) of very slight temperature rise), followed by a gradual rise to furnace temperature. The thermocouples eventually read the correct temperature as the sand equalizes with the furnace, but the initial downward transient is concerning.
-When removing these thermocouples from a high-temperature environment (air or sand) to room temperature air, the measured response overshoots below room temperature by at least 10*C, and takes several minutes to equalize with room temperature. This overshoot and long equalization time is concerning.
The thermocouple probes are new, have proper polarity and read boiling water (the single calibration check point I have access to) accurately. The thermocouple cables are correctly attached to the NI-9219 with no severe kinks or bends. The thermocouples are equalized at room temperature prior to being inserted into the sand, and then equalized with the sand's internal temperature before being placed in the furnace. There are multiple opportunities during the handling of the sand for any static electricity to be discharged. The furnace uses resisitive heaters and inserting the bare probes in the furnace does not cause the downward transient (just in case it was magnetic induction from the heating coils).
I initially used an NI-9213 thermocouple module with the same response, and switched to the NI-9219 so that I could simultaneously run RTDs. I've tried using grounded thermocouple probes with the same response. RTDs have no issues.
Any help in understanding either of these problems would be welcome.
