Hi. I have been using an scxi-1102 with scxi-1300 terminal block in scxi-1000 chassis with scxi-1600 usb daq device to measure 28 type R thermocouples. All bar two of the thermocouples were spot welded to a metal structure, through which a ground connection is presumably being found as the readings go unstable if I ground the negative terminal in the scxi-1300 block (ground loop). The readings are stable without grounding in the scxi-1300 terminal block, and calibrate to within a couple of degrees using a handheld source to inject thermocouple voltage........but when reading the actual thermocouples the results can be as much as 30 degrees in error (not all errors in the same direction). The error was only spotted when the thermocouples were checked on a calibrated hand held reader. I am using virtual daqmx channels, set to type R with a 0 to 1600degC signal range and using the built in cold junction compensation.
Can anyone thing of a reason for this error? The same technique with an SCC chassis an SCC-TC02 modules works fine. Do I need to ground the scxi chassis or 1300 terminal block to the same ground as the metal structure? Are the two couples that weren't grounded at either end causing the problem (even though the readings were stable)? Would the scxi-1303 terminal block fare any better?
Cheers.
