KB88975
Published:
Revision: 1.0

Current Outputs and Discrete Outputs Can Reduce Thermocouple Accuracy


Applies To:

GRV-R7-MM1001-10

Versions Affected:

All versions


Symptoms:

On GRV-R7-MM1001-10, thermocouple readings may be inaccurate when other channels are configured as current outputs or discrete sinking outputs.

Current output or DC sinking output channel types can add error to thermocouple measurements. This error increases with the proximity of either of these channels to the thermocouple channels. While any configuration with both thermocouple channels and current output or DC sinking output channel introduces inaccuracies, the most severe inaccuracies occur with the following configurations:

  • For current outputs, when channels 0-3 are configured as Type J Thermocouples and channels 4-7 are configured as 0-20 mA Analog Outputs, channels 4-7 drive 20 mA into a 100 Ohm loop resistance. The resulting thermocouple error is lowest on channel 0 (4 °C) and increases with each channel, reaching 7 °C on channel 3.
  • For DC sinking outputs, when channels 0-3 are configured as Type J Thermocouples and channels 4-7 are configured as 5-30 VDC Digital Outputs, and then channels 4-7 are turned on with 1 A of current flowing through each, the resulting thermocouple error is lowest on channel 0 (0.6 °C) and increases with each channel, reaching 3 °C on channel 3.

Workaround:

To reduce this error, choose the lowest possible channel numbers for thermocouples and the highest possible channel numbers for current output or DC sinking output channels. For example, choose channel 0 for your thermocouple and channel 7 for your output channel.

Resolution:

Opto 22 is investigating this issue.

Questions?

Contact: Opto 22 Product Support.
Phone: 800-835-6786 or 951-695-3080
Email: support@opto22.com


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