Thermocouple Type?

[JT100]

I need to purchase several thermocouples for a proof loading test I will be performing in the near future, this is not anything I have had experience of in the past and was looking for advice as to what type would be best suited. I will be testing a motor and monitoring the temperature during the tests, from the research I have done a standard type K seems to meet requirements, I am planning on putting 3/4 across the motor, either by magnets/sticking on/drilling a hole. After doing several google searches this doesnt seem to narrow my choices down any, could anyone offer any advice? Thanks.

 

[roydm]

Type K are the most common. Another way to measure metal temperature is to spot weld each of the wires to the base metal, side by side. I have seen this done for weld stress relieving. The welder is a large capacitor, say 10,000 microfarad charged to about 90 Volts then discharged via an SCR.
Roy

 

[ScottyUK]

Type K is common and cheap. You don't seem to need the performance of the exotic types, so why complicate / add expense when there is no benefit?

You might want to investigate heat transfer greases and thermally conductive adhesives.


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If we learn from our mistakes I'm getting a great education!

 

[mcgyvr]

http://www.productsafet.com/products/tempa.html

Their thermocouple securement paste is the best I have used by far. No mixing and hardens like a rock and stays put. 125 grams goes a long way. A little pricey but well worth it. Just dip the tc tip in it and use some Kapton tape to hold it until it dries.. (A few minutes when heated slightly with a hot air gun.)
I have tried a few 2 part pastes from Omega and wasn't happy with them at all.

 

[JT100]

Thanks guys. They look like the sort of thing I need.

 

[hacksaw]

depending on you motor size you have winding and bearing rtds.

are you just using external enclosure measurements?

 

[JimCasey]

I like Hacksaw's suggestion about RTDs. You are likely to have fluctuationg magnetic fields on a motor because, after all, that's what makes them run. If/when you get induction into the T/C wires where you are only looking at a millivolt signal anyway, you could swamp the signal with motor noise. RTDs use 3 wires and a resistive element, so that the detector subtracts any noise detected by the thirf wire and only looks at the resistance across the element.

 

[ScottyUK]

Inducing anything into a low impedance source like a thermocouple takes some effort, especially if the t/c leads are twisted together. It's easier to induce current into an RTD circuit. That said, RTDs will certainly work: they are used in the bigger commercial applications such as motors and generators because they give better accuracy over the temperature range of interest. Both RTDs and t/c's can be filtered so heavily as to squash any line frequency noise induced in either sensor type.


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If we learn from our mistakes I'm getting a great education!