Hello.
I use honeywell UDC 3000 temp controller connected to Type K high thermocoupler and this connection is also connected to PLC Automationdirect DL405 440CPU via F4- 08 THM module. The question is the display PV on UDC 3K and F4-08THM is quite different showing only small deviation at lower temp between 3K and PLC, but large difference i.e. PV is 300, PLC 45. I am trying to setup test bed to test and there is 3K calibration process too. Is there any experience you have been this issue before? Thanks in advance for your input.

 

The AD F4-08THM module is a thermocouple (T/C) input module so presumably the connection is made with the Type K thermocouple wire in parallel from the UDC 3000 to the AD T/C module (not copper wire, not some other thermocouple wire).

Either unit can be misconfigured for the wrong type thermocouple which would produce erroneous temperature values, with the error increasing with temperature.

Depending on which generation the UDC 3000 is, it might need external cold junction (CJ) compensation, and device that screwed into a terminal with a wire that connected to a different terminal, but the absence or lack of a CJ compensator would make the reported temperature too low, not too high.

There is some chance of the thermocouple break/burnout circuit on one device interfering with the reading on the other device.

Either device's analog input can be faulty.

Substituting a signal from thermocouple calibrator will show which unit is reading correctly.

 

Thanks David
This is first time for me joining a kind of open forum but getting a feedback is very excited and appreciate for you.
I like to express a little more detail wiring conditions and reply to your comments.
1. There are two different thermocoupler wires, TC K sensor to UDC 3K (a little thick wire and this cable looks like a solid copper cable. I will verify and let you know soon ) and UDC 3K to F4-08THM (thin wire but this cable is shielded and thermocoupler type K cable). The distanace from sensor to UDC is 20feet, and UDC to PLC is 10feet.
2. F4-08THM modules have few models different in terms of dip sw setting. The current module is old and hard to find out the exact manual to set the Type K TC, So I am going to change to a module which has a dip sw setting to Type K TC. I will post after test.
3. The temp reading on UDC at high temp zone (200 ~ 400 degree C) is very higher than the F4-08THM PLC reading but at low temp is pretty similar. Actual reading at UDC is 300 deg C and 45 deg C at PLC. I will check all configuration and program at PLC for F4-08THM module once again.
I will consider all your valuable feedback to find out what caused this value difference and let you know accordingly.
Regards,

 

I think your biggest problem is using the same TC for two different devices and having different wiring.
If it were me I would either go with an RTD or the best option, in my opinion, is go with a different temperature controller that can retransmit the PV value. Automation Direct sells this for around $550 and it is basically a Yokogawa temp controller.
Adding a second K type thermocouple and running one to each device might also get you where you need to be.
From what I understand you are basically setting the mV signal up in series. I personally have never had very good luck doing this.

 

For the record, the US commercial heat treating industry used paralleled thermocouples between a controller and a recorder for at least 4 1/2 decades 1950's through the mid 1980's and there are some that still maintain the practice today. It can work. The continuous process industries that use temperature transmitters for every temperature signal gape in unbelief at the concept, but it was and is done.

But, the thermocouple connections need to be in parallel (+) to (+), (-) to (-), wire needs to be the same type thermocouple wire, the common mode between the hot junction and the two devices has to be within the tolerance of either device's AI so a ground loop doesn't offset the readings, and the thermocouple break detection circuits can not interfere with the other device's reading.

Connecting thermocouples in series is incorrect because that's a 'thermopile' of additive signals, not the same EMF voltage signal in parallel to two different devices.

I would agree that a dual element thermocouple assembly is a valid work-around.

Re-transmitting the temperature signal from the device connected to the thermocouple to the second device is a valid work-around.

Substituting an RTD for a thermocouple avoids any inaccuracy due to the cold junction compensation, avoids the prospect of thermocouple drift and offers the opportunity to periodically check the calibration of the RTD which not feasible with thermocouples. But in an of itself is not likely to solve this particular problem

But all that said, I still have my money on a misconfigured/miswired PLC input.