Thermocouple Sensors and Cables

From my own experience, the readings you will get displayed will be out. I.E your exhaust temp may be hotter than normal. It's difficult to say by how much, but I suggest you quickly get to your local supplier and get Kx compensating cable.

please give me a specific answer I know that the Reading will be out but is the error on reading will be linear or non linear in order to process it in my PLC program ?

looking at the reference tables it's not linear. I assume you working at say 600C, a J t/c would have given you a mv reading of 33.11 and a K a reading of 24.91mv, if you reverse that a J at 24.91 gives you a temp of 453C and a K gives you a reading of 795C at 33.11mv. Big difference

I suggest you read the exhaust temp with portable t/c to ensure youre in the correct range. Take a mv reading at the termination point in your panel, check that against a reference table and offset your plc accordingly.

Or go get your cable whichever's easier.

J type cable with a K type thermocouple?--Can't work.

Or J type cable and J type cable?--Don't know.

Not always true:

"By using the Thermocouple Law of Intermediate Metals and making some simple assumptions, you can see that the voltage a data acquisition system measures depends only on the thermocouple type, the thermocouple voltage, and the cold-junction temperature. The measured voltage is in fact independent of the composition of the measurement leads and the cold junctions, J2 and J3.

According to the Thermocouple Law of Intermediate Metals, illustrated in Figure 2, inserting any type of wire into a thermocouple circuit has no effect on the output as long as both ends of that wire are the same temperature, or isothermal."

http://zone.ni.com/devzone/cda/tut/p/id/4237

Take the tables for J and K thermocouples.

At 0°C J and K give 0.000V

At 100°C J gives 5.268mV and K gives 4.095mV

At 500°C J gives 27.388mV and K gives 20.640mV.

If you do not take into the linearisation of the K module of the PLC, if you connect a J thermocouple to the K module of the PLC, the temperature that it is shown, instead of being 500°C, will be 658°C. This is not linear, for J thermocouples the nonlinearity span is larger than for K one, but IF YOU CAN LIVE with a +/- 10°C error, apply a correction factor of 0.76 in your PLC program and you will have it working.

If you make a look-up table with voltages for temperatures in degree increments, you can even have a precision of +/- one degree C.

I think I read the question wrong the first time. I thought the OP had a Type K TC, but only had Type J extension wire. After a reread, it sound like he is trying to use a type J in the place of a type K. With the correct conversion tables, you could probably figure it out. But type K TC's are so cheap, and who knows what standard the OP is working to, why screw around. Just go buy the right stuff.

You are right. I was just doing an exercise in futility.

Just like indel says compare the difference between the temperature cables, and program appriotly or just change to the PLC thermocouple module to Type J

It seems that the initial poster changed his mind of what he wants. So, dear Cho-Cho-San, we just had, as I said before, an exercise in futility.

BTW, I am manufacturing K thermocouple 4-20mA transmitters. Anybody interested?

Hi, All,

When only type J ( sensor, extension wire and card at the PLC) there will be only two points where voltages are produced by Seebeck Effect. One is at the thermocouple sensor, and the other at the terminals at the card. We intend to measure the voltage of the thermocouple. The voltage at the card is compensated (cold junction compensation) in the circuit of the card and will not affect the temperature measurement.

When type J wire is used in type K system, there will be two points of type J / K connections, one at the sensor and the other at the card. Each point has two wires positive and negative, there are actually four points where produce voltages.

Total of these four additional voltages will be added to the voltage of the thermocouple, then applied to the card.

The thermal voltages at two J/K points are not compensated yet. If the temperatures of the two J/K points are same, the total of the additional four thermal voltages are zero, because these voltages have opposite directions. The temperature measurement is also correct. However, generally, temperatures at the two J/K points are different, thus voltage measurement will be inaccurate.

The above is why different type such as J and K can not be mixed in a thermocouple circuit.

I was afraid I was mistaken until I read your post--This is what I understand and I believe you are right.

I have had Thermocouples (J type) that wouldn't work because the thermocouple wasn't shielded and the new wire was--They would would work perfectly until connected to equipment--It wasn't ground loop issues, either. We had the manufacturer define the problem that the probes were unshielded and some conditions (not all) they wouldn't work. The probes are not marked for shield condition either.

Thank you Akihito--Good Answer from me.

I can tell you that for sure you will get bad readings if using a different type cable that correspond to a specific thermocouple. Each thermocouple type is defined by the kind of metal material used as a conductor. If you want to use J type I'd say change your thermocouple to J type, or simply get a K type cable. As far as I know K type is the most common type of thermocouple so should not be a problem to find a supplier for that. If you are going to switch thermocouples from K to J, make sure your PLC input module can handle it (most modern modules do). You can see in the online Belden cable catalog, the model for a K type is 1018A, and the J type is 1000A. Any other questions just ask!