Grounded Thermocouple MeasurementsHardware: SCXI>>Modules>>SCXI-1100
Problem: What is a grounded thermocouple measurement? What measurement options do I have to measure a three-wire/grounded thermocouple? How do I use an SCXI-1102 or SCXI-1100 with an SCXI-1303 to measure temperature from a grounded thermocouple? Solution: Background Your thermocouple is grounded when it is at the same electrical potential as the material being measured, and that material is grounded. In this case the material acts as an electrical conductor to provide a path from the thermocouple to ground. This occurs most commonly when the tip of the thermocouple is welded to the material to provide a fast thermal response time. Measurement Options National Instruments offers several options for thermocouple measurements (see Developer Zone Tutorial: Taking Thermocouple Temperature Measurements). Although external signal conditioning using the SCXI modules (external signal conditioning) will usually yield cleaner measurements, National Instruments thermocouple acquisition devices with built-in signal conditioning will still provide acceptable results for many. SCXI considerations To perform a grounded thermocouple measurement with the SCXI-1100/1102 and SCXI-1303, you need to remove the resistor network on the SCXI-1303. The resistor network that is installed in the SCXI-1303 module is a 10 Ω resistor network. For instructions on how to change or remove the resistor network, examine page 8 of the SCXI-1303 32-Channel Isothermal Terminal Block Installation Guide, which is linked below. The following table, from the SCXI-1303 32-Channel Isothermal Terminal Block Installation Guide, indicates when you should use the different resistor networks:  Non-SCXI acquisition device considerations In order to connect a three-wire/grounded thermocouple to a thermocouple acquisition device, please connect the positive terminal of the Thermocouple to the positive input of the acquisition device, negative terminal of the Thermocouple to the negative input of the acquisition device, and finally the ground terminal of the Thermocouple to the COM of the acquisition device. Also, check to make sure that the acquisition device has internal bias resistors. If not, add external bias resistors to provide a path to ground for the amplifier bias currents. Related Links: Developer Zone Tutorial: Field Wiring and Noise Considerations for Analog Signals Developer Zone Tutorial: Taking Thermocouple Temperature Measurements Product Manuals: SCXI-1303 32-Channel Isothermal Terminal Block Installation Guide (March 2000) Attachments:
Report Date: 05/08/2006 Last Updated: 02/15/2009 Document ID: 3X79MM6P |
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