Error from Type K Extension vs Thermocouple Wire


Thread Starter


When doing tests on batch annealing furnaces and refractory dryouts we often have to run several lines of thermocouple wire into the furnace and attach then to the load pile and/or refractory wall/roof. these are sacrificial as once at higher temps the lines will burn out, and the process is not critical that an error of a couple of degrees is going to cause issue.

in the past, junctions have been made from twisted and crimped ends, and then the wire run right out and direct to the device.

what i am trying to gauge is the accuracy, or lack there of. would it be better to use higher rated thermocouple wire for the tip? would be significantly better (to justify the cost) to do the whole thing with thermocouple wire?

about 1500 f is the limit of what we would be interested in. i know most wire is only rated for 900, but not interested in what they 'calibrate for' or guarantee.

any and all knowledge and experience welcome as always!
Extension wire is just that, it useful only at near ambient temperatures. Extension wires are designed to have a temperature characteristic comparable to the sensor at lower temps to reduce connection errors. Do not make the mistake of using extension wires for furnace measurements.

You may find type N thermocouples a bit more durable for the sensors and the junction needs to be fused.

Joseph Ugalde

I happen to work at a a thermocouple couple vendor and noticed your inquiry on here. The issue likely resides not in the type of cable or if it's extension grade, but moreover the insulation that the wire is comprised of. Let me know if you have any further questions. Always happy to answer on thermocouple.

TE Wire & Cable, LLC
A Marmon Wire & Cable/Berkshire Hathaway Company
on re-reading your post, you need to use large wire diameters 10-12 gauge for the sensing junction then you bring those out to a place where you can connect to more easily routed t/c wiring (smaller diameter wires), and connect to you monitor.

at 1500 Deg, you'll likely get only 50 Deg accuracy (2-3 %).
from my understanding extension wire is made of the same alloys as the t/c wire, just to a lesser specification. like i say, i know it works, i'm just trying to get an idea of the error.

compensating cable i know is is different alloys to produce an equivalent effect and can only be used at ambient stated.
We've used fiber glass sheathing in the past, burns out but normally last long enough to tell us what we need to know. often the tip may be 20ft from the door and then another 15ft to the recorder so using actual t/c's means connections, that fail/have error, and high cost.
you think the error difference between doing the same thing with type K t/c wire or type K extension wire would be negligible though? (+/- few degrees).
I opened the catalog of a T/C vendor and found a limit of error table for extension wire.

It appears that the standard limit of error for K wire is the same value as standard limit of error for thermocouple wire, but only for a limited range (200°C/392°F). As others have noted, the use of extension wire is for areas with ambient temperatures that are not at the hot junction temperature of a working T/C at 1500°F.

So I don't know what the error becomes beyond the stated range when extension wire is used for an actual hot junction measurement.

Why not run a length of extension wire with a hot end junction and measure the same point as one of your regular wire/thermocouples and see what the difference is? And come back and tell us.
I have used a similar arrangement to what you describe, but inside the furnace I use a MgO insulated sheath type T/C, instead of running insulated T/C wire and creating at junction by twisting. It will survive in that environment. I have used these in reducing atmosphere annealing furnace temps to 1700 degrees F. Then outside the furnace, you can make a transition connection to your compensated lead wire. Accuracy should be adequate if you set up like this. Just be sure to spec an appropriate sheath material.
What is important to understand is the voltage potential of the sensor is not from the junction - but rather developed along the length of the wire - so the key bit about extension wire is that the materials of its two conductors match very closely.

Both wires will develop a Seebeck effect voltage - due to the difference in material - two different voltages and this difference is the signal.

If the extension wire is not pure and one side differs from the other, one can create error signals.

To test the extension wire effect - simply short the end where you would normally connect the thermocouple and look for voltage with a meter that can measure very low voltages.

The error will be close to the noise floor - thus high RF noise can fool your meter.

Be sure that the junction to the extension wire has no temperature gradient between the two connections... physically close together in a closed box..