I've found that a better option to a thermocouple is to use a type of sensor called an RTD. Most PID's will accept this type of input device. There are two benfits- better accuracy and the fact that the PID will detect a failed sensor and stop your process in this event. I'm not sure if a broken thermocouple would be detected.

While you are correct about the fact that an RTD is more accurate than a thermocouple, the increased accuracy is insignificant for our purposes. In laymans terms, its like buying a new speedometer for your vehicle because its 10% more accurate and will read 55 Mph to a +/-0.001 MPH resolution. It doesnt make a difference for the purpose of the application.

On your second point about malfunctions, I do believe a broken thermocouple would also be detected.

An example of RTD use:
A good place to use an RTD would be an incubator.. A type of device where a difference of 0.5 degrees can have an effect on the process.

Hope that helps a bit...

Also, many RTD's are made with an integral stainless steel sheath- unlike thermocouples that require a separate thermowell. They come in convenient diameters and lengths that allow mounting with compression fitting through the tank wall (or top)

quote:

Originally posted by moelarrycheese:
Also, many RTD's are made with an integral stainless steel sheath- unlike thermocouples that require a separate thermowell. They come in convenient diameters and lengths that allow mounting with compression fitting through the tank wall (or top)

Both RTD's and Thermocouples have the same options for mounting. You can get either the RTD or a Thermocouple with, or without an integral sheath.

RTD's are generally more expensive than standard thermocouples.

Yep what Murphy said.
Also a thermowell is not necessary for a standard TC, A hose clamp and some insulation is sufficient in my opinion...
I use TC's like this;
Jon

my personal preference are PT100's. They are nice because by changing the wiring configuration you can change the accuracy you can get out of them. (i.e. 2, 3, or 4 wire). Most PID's will support PT100's.