TracerDAQ using Type K Thermocouples

[Henry Ballard]

I'm using a Type K TC to measure flame temperature, the TC is designed for this with chrome plated connectors on a ceramic base.  Running this through an Omega OMB-DAQ-2408 board.  When cold I get reasonable temperatures, but not when it gets hot.  The data file reads -9999 every time step for this TC.

Any suggestions?

 

[DAQman]

First, is the channel's data rate property set to 50 or 60 Hz, depending on your power grid?  Assuming it's set correctly, at what temperature does it go from reasonable values to -9999, which is an open thermocouple indication? 

[Henry Ballard]

Using TracerDAQ, I have the scan rate set to 0.8 Hz.  On cycle start all TCs read 78F  as soon as flame is detected its TC drops steadily to -40, -200, -9999F.  Upon shutdown, it reads -453F and increases steadily to +301F, then cools as expected.  Readings from other locations appear to be correct.  I am trying to measure the flame temperature with a type K which is advertized to be good to 2500F @ 55 mV, and well within the 75 mV limit of the DAQ.  Thinking the TC was bad I double checked all the connections.   I took another TC and setup a bench test with a candle to see what would happen.  The same behavior.   

Is there a trick to measuring flame temperatures with Type K TC's.  What am I doing wrong?

[DAQman]

You could have the thermocouple wired backward. That would explain the temperature drop when heat is applied. 

[Henry Ballard]

I was so sure I had them wired correctly, but I was wrong!  It amazes me that wired backward, a reasonable value is given up to about 200 F before it drops quickly into the negative range.

Thanks for all the input.