RTD lead extensions, can I use terminal blocks?

[scott1971]

Hi, all:

I've read a few posts and think I might have the answer, but thought I would run this for any experts who can confirm. I have an RTD whose leads are not long enough to reach my controller input (an Opto 22 RTD "puck&quot. My initial thought was to terminate the leads at a signal conditioner to turn the signal into a 4-20 mA to run into an analog input, but then I started to wonder if I can just run the leads to terminal blocks and then just continue onto the RTD input with stranded cable.

Can I do this? If so, I'm guessing I need to use "extension grade" thermocouple wire with the same ANSI type materials as used in the RTD leads.

Thanks to anyone who can provide more information.

 

[FredGarvin]

"In thermocouples, you have to use extension wires of the same type as of thermocouple like E, K, etc."

That's not really true. A typical TC circuit consists of the TC junction of the dissimilar metals going to a secondary junction. From the secondary junction copper wire can be used. However, that junction now is your reference junction. As long as the reference junction is at the same temperature as the measurement junction, no extraneous emf is induced. If the reference junction temp varries, it must be compensated for, usually electrically.

 

[ADMIRALKEN]

We have several dozen RTD's that are anywhere from 50-250ft from the controler, we run standard wire to the connection blocks of the RTD. The only requirement is that you have to use 3 or 4 wire RTD's, connect all the leads.

An alternative is to use a Temperature transmitter, which then outputs a 4-20mA signal. At my facility we use transmitters for critical applications, because you can get 4 wire redundant RTD transmitters, for everything else we use platinum 100 ohm RTD's, because they are much, much cheaper.

Ken

 

[buzzp]

I agree with all above posts. RTD's and thermocouples are totally different. RTD's change resistance with temperature and thermocouples generate a small voltage in proportion to the temperature due to two disimilar metals used for the junction. You can NOT use standard wire for thermocouple extensions unless you can recalibrate the electronics and even then the relationship will likely not be linear across the temperature span of the thermocouple. Each junction (terminal block connection) will generate another small voltage producing erroneous results.

 

[Gilli]

Can sb provide me any link to get the 10 ohm and 53 ohm RTDs resistance ~ temperature curve. I need to test the DCS channel by injection of resistance and these RTDs are installed in motor windings.

Thanks in Advance

 

[scott1971]

Thank you all... I believe the RTD being used was 3-wire, but I'll need to check with the technician. The RTD and controller are on the same skid, so even a small distance if it is two-wire won't be critical for my application.

I love this site.

 

[Ohmboy]

Posts to date appear accurate in information provided. If extending RTD lead lengths, use 3 or 4-wire and signal receiving device with lead length compensation. The 3rd & 4th wires are used to determine lead length net resistance, and factor it out of final resistance readings for best measurement accuracy.