Thermocouple parallel connections

Ronnie Sullivan · Jul 31, 2014

Hi
I've been asked if I can monitor the temperature from 3 Type K thermocouples going into a panel to 3 normal temperature controllers. (West 6100+)

The panel has a Mitsubishi (FX1n) plc and a HMI (I have to buy an FX2n-4AD-TC Thermocouple extension board)

The 3 temperature probes are wired in compensating cable direct to the controllers.

The temperature controllers at the moment just control the temperature at working temperature and they need a 'cold start' routine whereby the temperature is gradually raised over a 48 hour period 100 degrees at a time.

They have done it manually before by adjusting the setpoint 100 degrees at a time but it means it has to be watched over by an operative for 48 hours.

My question is; can I just parallel the connections from the temp controllers (in compensating cable) to the A/D unit and still get a accurate reading in the plc?

widelto · Jul 31, 2014

I do not know the answer to your question, but i know that you can install more than one TC in the same package like attached figure.
Please ask if they can package more than two TCs.

osmanmom · Jul 31, 2014

www.termoaparatura.com.pl/files_mce/czujniki_temperatury/tc_glowicowe/43a_tt_g_en.pdf

osmanmom · Jul 31, 2014

www.omega.com/temperature/Z/pdf/z021-032.pdf

paulcs · Jul 31, 2014

If you want PV monitoring only, you can install a dual thermocouple. TC1 feeds the West controller, TC2 feeds the PLC. This also provides additional alarm capability, if one TC is faulty. Simple, isolated, low cost.

Otherwise, you can have the West EIA485 communicate to the PLC. The you can monitor the TC, monitor the SP, transmit a SP, etc.

Ronnie Sullivan · Jul 31, 2014

Thanks for the info chaps - plenty options to consider.

I think before I try any suggestions, I will wire one in parallel to the Special TC A/D unit and see what figures I am getting.

I will let you know the results.

As an aside, if the thermocouple goes out of range or open or closed circuit the A/D unit triggers an automatic alarm to be used in the plc program. (I know the west 6100 can be configured to do that also but there is no alarm relay on these so they just flash a red led)

paulcs · Jul 31, 2014

An issue is how the thermocouple break circuit works.
That can confuse a paralleled TC device.

TConnolly · Jul 31, 2014

Why not replace the temperature controllers with some that have a ramp to setpoint capability?

bimmersix · Jul 31, 2014

I've seen it done with no issues on equipment that a)wasn't built by the brightest people and b) didn't have extreme accuracy requirements. That said, the readings across several temp controllers being supplied by the same thermocouple only varied by a few degrees in the 600-1000F range. Dual element TC is a good option if you can swing that.

Ronnie Sullivan · Jul 31, 2014

TConnolly said:
Why not replace the temperature controllers with some that have a ramp to setpoint capability?

Three reasons. (1)They would be throwing away perfectly good temperature controllers to replace them with (2)considerably more expensive units.

(3) Plus at the lower temperatures on a cold start up the temperature runs away and cannot be controlled by a set point. The burners have to turn off completely.

TConnolly · Jul 31, 2014

I see West 6100 controllers online for around $270 US. I also see West models that have a single setpoint with ramp to setpoint online for around the same price, one model is even less. So its going to be about $800 US to replace the controllers. How does that compare to the price of an analog input card, dual element T/Cs, extra T/C extension wire, and the extra work?

Just trying to give you options here. Sometimes we all have a tendency to tunnel in on one solution when scratching the controller might just be better.

Ronnie Sullivan · Jul 31, 2014

of course you are and I appreciate it and hope I didn't come across as being rude.

But for a cold start (which I'm trying to achieve) the profile is something like up to 100 C hold for 6 hours - up to 200 C hold for 12 hours etc.

The burners have high and low fire but the low fire races to over 200 C and so they have to be turned off at 100 and 200 not just high and low fire. Above that they can be controlled normally by the setpoint.

osmanmom · Jul 31, 2014

info,

www.omega.com/toc_asp/subsectionSC.asp?subsection=A04&book=vhpc

www.rkcinst.com/english/product/ramp-e.htm

www.west-cs.com/products/l2/programmable-temperature-controllers/

industrial.omron.ca/en/products/catalogue/control_components/temperature_controllers/digital_process_controllers/e5_n-ht/default.html

shooter · Aug 1, 2014

west has nice ramp controllers, so you can put a CAM function with several times and temps in it. so best option here is just replace the controllers with a modern one, although they are 10 years old or so.

Ronnie Sullivan · Aug 2, 2014

My question is; can I just parallel the connections from the temp controllers (in compensating cable) to the A/D unit and still get a accurate reading in the plc?

I have looked into profile temperature controllers and for what I want it would be overkill and far more expensive. (At cold start, I need the burners to turn off - not modulate)

I also need the plc to know what the temperatures are for future developments.

And nobody gave me a definitive answer as to whether I could double up the connections from the probes. But, I shall connect one to the A/D TC unit and see if the figures I get in the plc are accurate.

Thermocouple parallel connections

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