Thermocouple Extension wire and thermocouple type mismatch


Thread Starter


In Hydrogen compressor of petrochemical complex thermocouples are of j type, but we have only k type extension wire now. can we connect j type thermocouple with k type extension wire up to transmitter? Distance between sensor and transmitter is 1.2 meter. The transmitters are being used in critical interlocks related to tripping of compressor. Main problem is job has to be done urgently and new procurement will take at least one month.

the answer is "NO". Let see, your thermocouple is J type with iron (+) - constantan (-) (Fe-C) junction.

then you want to connect to K type wire which is Nickel-10% Chromium(+) - Nickel-5%(-). this configuration will made additional 2 additional junctions with each have different EMF generated dependent on temperature of these additional junctions.

So, final voltage measured by your instrument (temperature transmitter) will be accumulation of all 3 junctions EMF.

For detail explanation of this effect, please refer to basic
thermocouple theory:

Hope this help
Absolutely, fix it later.

you will get some error; don't forget even copper extension wires have been used in a pinch and don't have the polarity worries.

type k extensions wires are not type k t/c wire, they only provide similar millivolts at lower wiring temperatures.

in using type j extensions, make sure you match the polarities, with the k t/c.
you are absolutely correct, but in a critical situation it can be done,<pre>
is it recommended: No
is it desirable: No
is it safe: No</pre>
Can it be done? and are there situations that require an improvised fix? absolutely.

you do have to monitor or confirm the junction temperatures and allow for the measurement errors, but it is only a provisional fix.

Once the correct extension wire is available, the engineer is mandated to see that the provisional hook up is corrected, due to safety and operation concerns.
The bad data from mixing wire types makes me shudder. You can't dig up some J wire to do it right the first time?

How many points? How much J wire? More than 100 feet/30m total?

A local controls distributor doesn't have a roll of J wire you can buy with petty cash?

Here in the USA, it is not unknown for plant maintenance (I&E) guys to help out their acquaintances and associates at other plants when they're in a pinch. The cases I'm aware of have been "covert operations"; no paper work, just an exchange of whatever with a handshake on the understanding that the item will be replaced when 'the system' catches up.

It's hard to believe that there's any areas in the world, except maybe the polar ice caps, that don't have a roll of J wire that can be 'loaned' to a buddy until the goods get ordered and arrive.

Have you asked your neighbors at the smoke stack down the road for a favor?