OT - Thermocouple Wiring Question

[skyfox]

Can standard terminal blocks be used for extending TC wires? Total run length is less than 4 feet. TC wire + extension wire. Extension wires will be of high quality TC wires. Temperature deviation of +/- 2 Deg. C is tolerable. Is it OK to use terminal blocks to extend the length of TC wires instead of using male/Female TC connectors?

Thanks.

 

[bernie_carlton]

I believe that there are thermocouple terminal blocks

 

[skyfox]

I believe that there are thermocouple terminal blocks

Hi Bernie,

You are correct. There are. But they are not always readily available or easy to come by and cost a bit more. Yet another specialized part to add to a BOM. I am trying to find out how drastically different temperature readings would be when using a standard TB as opposed to a TC compatible connector pair or a TB. Looking to see if someone had already carried out this experiment?


Cheers!!!

 

[parky]

Because Thermocouples by design are made of two different metals (including wires) it is important to keep that through the whole cable length, yes it will work but there will be losses in accuracy, as the thermocouple works by the differences in the two metals any other metals introduced will give false readings.

 

[TurpoUrpo]

Can standard terminal blocks be used for extending TC wires? Total run length is less than 4 feet. TC wire + extension wire. Extension wires will be of high quality TC wires. Temperature deviation of +/- 2 Deg. C is tolerable. Is it OK to use terminal blocks to extend the length of TC wires instead of using male/Female TC connectors?

Thanks.

Yes, but you shouldn't.

IF you still are going to do it, make sure to put both TC wires in from the same side so that the wires are actually "bonded" as well as possible.

There is a valid reason why the TC wire is of the same stuff as the TC itself all the way up to the what ever is reading that TC voltage.

 

[Rson]

No Thermocouple terminal blocks, no worky.

The terminal blocks have to be made of the dissimilar metals. Also - you need thermocouple wire as well. Not standard wire*

If you have to - don't run to a terminal block, run right to the I/O of the PLC or controller - but use the thermocouple wire at a minimum with thermocouple connectors.

*I see you are already doing this

 

[travispedley]

Yes, but you shouldn't.

IF you still are going to do it, make sure to put both TC wires in from the same side so that the wires are actually "bonded" as well as possible.

There is a valid reason why the TC wire is of the same stuff as the TC itself all the way up to the what ever is reading that TC voltage.

+1 Use the same side of the terminal block and you'll be fine.

 

[danw]

>I am trying to find out how drastically different temperature readings would be when using a standard TB as opposed to a TC compatible connector pair or a TB.

It will work as long as there is no difference in the temperature of the connection screws on the two terminal blocks.

There is a thermocouple thermometry principle known as the "Law of Intermediate Metals, which says that when metals that are not the thermocouple alloys are used in a thermocouple circuit that the those metals must be isothermal, meaning, both sides of the thermocouple wire must be at the same temperature because any difference in temperature from one side to the other will generate an EMF that will 'appear' at the cold end as a gradient difference and result invan error.

This concept is in play with EVERY junction on every commercial AI card because the terminal screws are typically nickle plated copper. Nickel is not a native alloy of any of the common thermocouples. The assumption is that that the terminal screws are at the same temperature.

I have experimented. It was about November 1978 and I sprayed one side of the terminal connections with a aerosol spray coolant that watched the reported temperature dropped about 30 Deg F. As the terminal block slowly warmed to ambient, the temperature came back down.

So it's the difference in temperature between one side of the terminal block and the other terminal block that will produce an error.

The terminal blocks made from T/C alloys eliminates the concern of uneven temperatures.

 

[RET]

It will be fine for what youre trying to do. The terminals on the plc cards are just normal nickel plated materials anyway.

 

[g.mccormick]

It will be fine for what youre trying to do. The terminals on the plc cards are just normal nickel plated materials anyway.

But the card has a cjc to know the temperature of that junction.

The analog input card reads the MV from the the thermocouple, however, that mv value is not meaningful unless you know the reference (cold junction) temperature at the card. If you use non-type specific thermocouple wire and connections all of the way from the hot junction (thermocouple) to the input card, you will get a reading but it will be wrong. How much depends on lots of stuff. I would never do it, but people do ....

 

[NetNathan]

If the terminal blocks have a straight through design, where you can extend the length of the TC wires so they are compressed together you will be okay.
Here is an example of this type.. you will notice there is only one screw in center of TB.
https://www.automationdirect.com/adc/shopping/catalog/terminal_blocks/dinnector_din-rail_terminal_blocks/thermocouple_terminal_blocks_-a-_accessories#sort=undefined%20asc&start=0

Otherwise use these..which are made of thermocouple material.
https://www.omega.com/en-us/search?text=thermocouple%20terminal%20blocks

 

[danw]

If you use non-type specific thermocouple wire and connections all of the way from the hot junction (thermocouple) to the input card, you will get a reading but it will be wrong.

You are correct, but the first post in this thread says that the extension wire is thermocouple wire, not non-alloy (copper) wire.



When thermocouple extension wire is used the CJ works as it should and any error would only be in the difference in temperature between the two terminal blocks.

 

[James Mcquade]

i had a vendor do the same thing you are suggesting. there was a 7 degree c difference in temperature. yes, thermocouple terminals are more expensive, but will save you a lot of grief in the long run.
james

 

[skyfox]

Thank you ALL for your feedback. I got my Thermocouples from OMEGA today and was able to run a quick test at room temperature.


1. Thermocouple with 36" lead directly terminated at PLC TC Card.

2.Thermocouple with 36" lead + 24" extension wire connected via a a standard set of terminal blocks, but connected to the same screw terminals, and then the extension leads terminated at PLC TC Card.

Both sensing elements were placed next to each other in a room. After an hour or so,

#1 Read 22.12 Degrees C
#2 Read 22.10 Degrees C

I could have gone with longer lead lengths, but to get one in custom lead lengths, there was a 26 week long lead-time. At some point, I would like to do a test where the test would ramp up to temperatures in excess of 100 C and see what the delta might be with hook-up #1 & #2. Unfortunately, that would have to be at a later date as I do not have the luxury of a spare TC card at the moment and the system that this is going in to, has no provision to attach a 2nd TC.


NATHAN,

Thanks for the OMEGA links. I swear, sometimes searching thru Omega can be challenging. They won't bring up some of their own stuff during a search.I specially like the din-rail mount that accepts the plug. If they have that in stock and I can get it within a week, I will go that route.


Thanks again to all of you for your help. A Happy Thanksgiving to all of you and your families.


CHEERS!

 

[skyfox]

Because Thermocouples by design are made of two different metals (including wires) it is important to keep that through the whole cable length, yes it will work but there will be losses in accuracy, as the thermocouple works by the differences in the two metals any other metals introduced will give false readings.

OT-OT.

OMG!

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Cheers!