KB88975
Published: June 9, 2020
Revision: 3.0

Analog Current Outputs and Discrete Outputs Can Reduce Thermocouple Accuracy


Applies To:

GRV-R7-MM1001-10

Versions Affected:

All versions prior to 2.0.1

Resolved In Version:

2.0.1


Symptoms:

On some early release modules of GRV-R7-MM1001-10, thermocouple readings may be inaccurate when other channels are configured as analog current outputs or discrete sinking outputs.
 
Channels configured for either analog current output or discrete sinking output can add error to channels configured as thermocouples. This error increases with the proximity of either of these types of channels to the thermocouple channels. While any configuration with both thermocouple channels and analog current output or discrete sinking output channels introduces inaccuracies, the most severe inaccuracies occur with the following configurations:

  • For analog current outputs, when channels 0-3 are configured as Type J thermocouples and channels 4-7 are configured as 0-20 mA Analog Outputs, and then 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 discrete 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.
Serial Numbers Most Likely Affected
The likelihood that your module may be affected by this issue is as follows:
  • Serial numbers 774770 to 775320: Unlikely to be affected by this issue.
  • Serial numbers 773262 to 774769: Somewhat likely to be affected by this issue.
  • Serial numbers 773261 and lower: Very likely to be affected by this issue.
Note: Modules with serial numbers greater than or equal to 775320 are not affected by this issue.

Workaround:

To reduce this error, do the following:

  • On channels 0–3, do not combine thermocouple input channels with discrete sinking output or analog current output channels.
  • If you want to combine thermocouple input channels on your module with either analog current output or discrete sinking output, set them up as follows:
    • Assign the thermocouple input to the lower-end of the channel range, beginning with channel 0.
    • Assign the analog current output or discrete sinking output to the higher-end of the channel range, beginning with channel 7.

Resolution:

Opto 22 has resolved this issue.

Questions?

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


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