Incorrect Thermocouple Voltage Readings with SCXI-1303

Updated Nov 17, 2017

Issue Details

When I connect a thermocouple to the SCXI-1303, on certain connections there appears to be an offset in my measurement caused by a ground loop. If I remove the thermocouple from the terminal block and connect it to a high precision DMM or voltmeter, the thermocouple produces a different voltage than it did when connected through the SCXI-1303 to the SCXI-1102 or SCXI-1100 analog input modules. What am I doing wrong?

Solution

There are several kinds of thermocouples available on the market. Thermocouples can be floating or ground referenced depending on how they are constructed. Floating thermocouples are more common, so the SCXI-1303 is configured in the factory to work with this kind of thermocouple. Floating thermocouples must be referenced to the SCXI module's ground using a bias resistor. The bias resistors are located in the SCXI-1303 as socketed resistor networks that can easily be removed and/or replaced with a resistor network of a different value.

Note: Different bias resistor values are recommended for use with the SCXI-1102 and SCXI-1100.

If you are seeing an incorrect voltage caused by a ground loop, you should simply remove the bias resistor networks from the SCXI-1303. You most likely have a ground referenced sensor and have inadvertently connected the sensor to the SCXI-1102 or SCXI-1100 module using an SCXI-1303 configured with bias resistors for floating signal sources.

The recommended resistor network configurations for the SCXI-1303 32-Channel Isothermal Terminal Block can vary based on your system setup and can be found on page 7 in the Product Manuals: SCXI-1303 32-Channel Isothermal Terminal Block (321923B-01). Important details include the kind of signal source you are connecting, lead lengths, and several other factors. The recommendations made in the SCXI-1303 32-Channel Isothermal Terminal Block Installation Guide are based on typical systems, and if your system setup differs from the ones described, you should try configuring the bias resistor networks in a way most suitable for your application.

Please read the Product Manuals: SCXI-1303 32-Channel Isothermal Terminal Block (321923B-01) for more information about these bias resistor networks and recommendations on what to use with your sensor. The specific bias resistors that are specified to be used with this terminal block and 1100\1102 modules can be found on pages 2-3.

Note:  National Instruments devices can only read measurements directly from these following thermocouples: J, K, N, R, S, T, B, E.  

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