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Bad Readings from Thermocouple with the SC-2071 Accessory

Hardware: Compact FieldPoint>>Connector Blocks

Problem: Why am I getting bad readings from my thermocouple on the SC-2071? In the test panels, the voltage I am reading is greater than 2 mV.

Solution: Ensure that the SC-2071 is selected as an accessory for your DAQ card with a gain of 100 and input range of +/- 0.1 V. The readings you should expect are 1.6 mV with a K or T type thermocouple. This voltage is the thermocouple voltage plus the cold junction voltage.

If you continue to read 2 mV, perform the following steps:
  1. Ensure that you are using RSE (Referenced Single Ended) on the board.
  2. Connect ACH1 to AIGND. This will short the inputs of channel 1 to make sure you are inputting zero volts.
  3. Read the voltage on the NI-DAQ Configuration Utility using the test panels, and you should read zero or at least a voltage in the order of microvolts. If you get almost zero in the order of microvolts, then everything is working fine and you should have no problems reading your signals.
If you continue to read a couple of millivolts on that input, then the problem is a grounding issue with the SC-2071. The solution for this is to ground the AIGND screw on the SC-2071 for each channel to a real ground. Connect the AIGND screw to the DGND screw. This will make your thermocouples use the digital ground as reference. Then you should be reading zero in the order of microvolts and your temperature readings will be correct.

If you continue to get the same nonzero voltage, you might be using a DAQCard-1200 connected to a laptop. In this configuration, you do not have a good ground on the digital ground screw and you will need to find an other way to ground that AGND signal. Follow these steps:
  1. Configure your DAQCard-1200 to use Differential Mode.
  2. Ensure the accessory is SC-2071.
  3. Connect pin AIGND to ACH1 and leave ACH0 free. (The CJC sensor will be using ACH0.)
  4. Read the CJC temp from channel 0.
  5. Connect your first thermocouple between channel ACH2 and ACH3, with the negative lead connected to ACH3. (Check your thermocouple specs for lead assignment.)
  6. Put a jumper wire between ACH3 and AIGND.
  7. On the rest of the thermocouples, connect ACH5 and ACH7 to AIGND. Connect the second thermocouple between AIGND and ACH4 and the third thermocouple between AIGND and ACH6.


Related Links: Products & Services: SC-2071 Online Catalog Pages

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Report Date: 07/20/1998
Last Updated: 03/03/2006
Document ID: 1BJA5K0O

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