Thermocouple reader fails at high temp

[ecb1123]

Hello,

I'm working on a DAQ system to measure temperatures on our high-temp test rig (500-1000 deg C). I'm using two modules, NI9214 and NI9211. I find that the modules read normally at room temperature, but as the temp increases over a few hundred C, the readings get way skewed. For at least one thermocouople, I've found that having it plugged into the module makes all the other channels read errors. All the thermocouples read normally when plugged into a handheld reader.

Is there a max temp that the modules can read? I didn't see such a limit posted anywhere in the documentation.

Any advice is appreciated.

Thanks,
Elizabeth

[johnsold]

Elizabeth,

According to the dcoumentation the modules will measure over the standard range for the thermocouple type chosen. So that should not be the problem with Types B, K, R, and S. Which thermocouple type are you using?

Are the thermocouples grounded, isolated, or other? Are they connected to the device being measured by an electrically conductive path? Do the thermocouple lead wires run close to the furnace power leads or any high power circuits (such as blowers)?

My suspicion is tha you have a ground loop through at least one thermocouple - the one which causes the errors.

Lynn

[ecb1123]

Hi Lynn,

Thanks for your advice. I'm using Type K thermocouples. They're not probes, we just have raw TC wire with welded tips at the measurement end. The TC giving the most trouble is measuring the ceramic surface temperature on the heater, which is running at high voltage and also right next to the blower, but there shouldn't be any conductive connection to those.

I did some more testing today, and that worked fine up until it got to about 650C, then went into an error.

Thanks,

Elizabeth

[johnsold]

Elizabeth,

One possibility is that the ceramic becomes slightly conductive at those temperatures.   Another is that you are getting capacitive or inductive coupling between the high power circuits and the thermocouple. One way to distinguish between the two would be to move the thermocouple tip slightly so that it is not touching the ceramic (if mechanically feasible). Of course it will not accurately measure the temperature.  If the problem occurs with the thermocouple not touching, then capcitive or inductive coupling become the most probably candidates, although explaining the 650 C threshold becomes a question.

Look at page 9 of the NI 9214 Operating Instructions and Specifications. If you have an oscilloscpe or digital multimeter, try measuring the voltage between the thermocouple inputs, COM, and chassis ground. With the DMM measure both AC and DC voltages.  Note that the Channel to COM common-mode votlage limit is 1.2 V and the COM to chassis ground limit is 250 V. Be careful!

Lynn