Thermocouple question

[fa50driver]

I purchased some tanks from China and they came with thermocouples. I previously had some controllers set up for K type thermocouples and wired them to the chinese thermocouples and "calibrated" them, but they don't seem to be working accurately. I suspect that they aren't the same type. Any way to identify the type (aside from asking the company for specs)?

I have attached photos, although they aren't the best quality.

[KaskaskiaBrew]

This could just be a 3 or 4 wire type thermocouple. 3 or 4 wire are supposed to be more accurate because they are able to compensate for the resistance through the actual wires themselves and not just the resistance across the thermocouple. If it is a 3 wire thermocouple that came with your system, your controller (depending on the brand) should be able to handle the change from a 2 to a 3 wire. You might have to re-run the wires that go from the controller to the thermocouple to change it from 2 to 3 wire but then it should hopefully work. That's about the extent of my thermocouple knowledge but hope it helps some.

Cheers,

[KaskaskiaBrew]

Should have added that you might be better off just getting rid of that one and switching back to a K type thermocouple. You can find them pretty cheap all over the internet and that might be easier than switching out your wires (if it is a 3 wire thermocouple).

Cheers,

[jherrington]

I thought the type K only used two wires.

[KaskaskiaBrew]

You're right. K type are only 2 wire and RTD's are 3 wire.

I thought the type K only used two wires.

[fa50driver]

Thanks guys. I do have a couple extra K types but the rod part of the mechanisms are too fat to fit into the thermowell. I thought that I read in another post that you can just twist the ends of the wires together and jam them into the thermowell (with some conductivity paste).

[CaptainEBC]

The 2nd photo with the specs shows "PT100." That's a common type of RTD.

[fa50driver]

Thx Captain. It appears that it is usable by my controller, but it appears to need three wires instead of two like the K type thermocouples.

[CaptainEBC]

Correct. I believe you can use any 3 conductor wire as long as the gauges and lengths are the same. I buy RTD wire from mcmaster for ours:

http://www.mcmaster.com/#rtd-wire/=i29ck3

[lhall]

You can just twist the thermocouple extension wires together and stick them in the thermowell. You might be off by a degree or two but it works fine. Make sure to use the correct wire though - I think type J is red and white and K is red and black?

[fa50driver]

Ok, so if I understand this correctly, I would just twist the ends of the correct thermocouple wire together to stuff in the thermowell, and program the controller for that particular thermocouple - IE, use type K thermocouple wire and program the controller for K type thermocouple and voilla, a poor man's thermocouple?

Next question, and perhaps a dumb one - if I have a 100 ft run of wire, should I use type K wire for the whole run or is there a different wire that should be used to carry the signal and one to measure the signal?

[lhall]

You can use what is called thermocouple extension wire. It's the same two types of metal as the actual thermocouple is made from, just not as high quality. There will be some errors but you can check the thermocouple in an ice bath and in boiling water and figure out the offset.

http://www.omega.com/techref/thermcolorcodes.html

[CaptainEBC]

The best poor man's thermocouple is an actual thermocouple. They're cheap and you don't have to worry about it falling apart or coming loose. If you have a thermowell, a spring loaded thermocouple will maintain better contact with the inner wall.

If you lose connection on your probe, your controller could malfunction and hold the glycol valves open.

Also, if you have the choice of thermocouple type, go with type J.