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Automation Direct DL205 Thermocouple Error
Having issues with thermocouple signals connected to a DL205 PLC. All three are reading several hundred degrees high.

We have three type K TCs connected to an F2-04THM module. Thermocouples are reading 400-500 degrees low on the PLC input as compared to what they read with a TC calibrator. We have verified all jumper settings on the card and ladder logic in the PLC. TC extension cable is correct. The TC connections are difficult to get to - we have not verified but all readings are stable on either the PLC or the calibrator. We believe actual temp is approx. 1300 deg, PLC reads 1800 +/- for all 3 TCs. Has anyone encountered this issue on this card before?

By Steve Myres on 18 July, 2018 - 10:50 am

Might be a data encoding issue. Many 205 modules will send the data in either BCD or binary, and I know you can display/interpret the data in both ways. You may have a mismatch. Also, even if everything is correct in the PLC, if you're getting your PLC readings from an HMI, many HMIs default to BCD encoding for DL PLCs, so it might be misinterpreting the data.

By W.L. Mostia on 18 July, 2018 - 6:25 pm
1 out of 1 members thought this post was helpful...

Have you checked to see what the thermocouple are actually reading? Most multifunction process calibrators can read thermocouple signals.

William (Bill) L. Mostia, Jr. PE
ISA Fellow
WLM Engineering Co.

1 out of 1 members thought this post was helpful...

An input configured for J type, but wired with a K type thermocouple will produce erroneous high readings, where the higher the temperature, the larger the error.

Is the input really configured for a Type K?

>Is the input really configured for a Type K?

Good call, looking through the t/c tables the Type K produces 37% more voltage at 1300(F), pretty close to the reported error.

Thanks for your responses

- Based on the module manual, jumper settings are correct for 4 channels, Type K TC, and for degrees F.

- Actual temperature reading was taken with the TC disconnected from PLC and connected to TC calibrator input.

- I previously was confident of the BCD/Binary configuration, but am looking at that again.

By Steve Myres on 19 July, 2018 - 8:40 pm
1 out of 1 members thought this post was helpful...

How does that module do CJC? 0x series modules have a thermistor permanently installed on the wiring connector. Don't know how the 205's do it. Maybe the local reference is damaged.

>- Actual temperature reading was taken with the TC
>disconnected from PLC and connected to TC calibrator input.

Simulate a known Temperature value of same operating range of the same TC type to verify readings.