Temp Probe Drift using UIS-WCB1?

[alexmoskos]

Hi,

I am using two Type K Thermocouples (https://www.automationdirect.com/adc/Shopping/Catalog/Sensors_-z-_Encoders/Temperature_Sensors_-a-_Transmitters/Thermocouple_Sensors/Probes_(with_Hex_Nipple)/THMK-H06L01-01), and then wiring another 15ft of Thermocouple wire from the sensors to my Unistream PLC expansion module (UIS-WCB1) to the RT2+ and RT2- and RT3+ and RT3- terminals. The PLC is mounted in a NEMA 4X enclosure (very well sealed).

I have two issues,

1. When I plug these Thermocouples into the PLC and then select the Type K thermocouple option for the module, my numbers are off by +20 Degrees F. This is true at Ambient Room temperature of about 70F and when the probes are hotter at 170 F. Any thought as to why these values would be off?

2. What I am noticing is that after my temperature probes have been sitting in the same environment for a long time, so the temperature should be really stable, and then turn on my PLC, the reported temperature starts drifting upwards 6-7 degrees over the course of 20-30 minutes. Why would this happen?

The expansion module installation guide talks about thermocouple isolation. I am not using any analog inputs and only controlling two digital outputs from the PLC right now. I am a little confused what the installation guide means by "you may use up to one exposed-junction thermocouple per UIS-WCB1 module". If both of these probes have 15ft extension wires, is that two exposed-junctions? I'm confused what "exposed junction" means.

 

Any help is appreciated, thank you.

[Joe Tauser]

I'm guessing the picture is representative, as it shows a type J probe (red and white wires).  The part number you specified is for a type K so it should be OK.

After reading Unitronics' definition of "exposed junction" I think what they're trying to say is "grounded junction" which connects the junction to the metal of the probe.  Your part is an ungrounded isolated junction so that should be OK, too.

You realize that the value reported by the module is 0.1F resolution, right?  (Just asking) 

How exactly did you connect the extension wire to the thermocouples?

What happens when you disconnect one of the thermocouples?

What happens when you disconnect both thermocouples and put a shorting jumper right on the module between RT2+ and RT2-  and another one between RT3+ and RT3-?  This should give you a stable ambient temperature reading.

Joe T.