thermocouple type K

http://cr4.globalspec.com/thread/23760/Type-K-thermocouple

Hi,

Normally, the first metal or element is identified as postive and the second one as negative, so for this case it is chromel. Eg Iron/constantan, iron wud be positive...

Thats the way i learnt from my elders which order me to climb up 100 ft columns to check for sensors in thermowell when the display in control room temp does not read "live zero" during my intern years.

Connect it to a read out, light a flame under the thermocouple (a lighter works) if it goes negative, you have it the wrong way.

of course, the best way is to look at the color of the wires. Negative is red, positive is yellow (in the US.)

You can identify the wires by the color code of the insulation. Yellow is chromel +, and alumel is red -. Also, carry around a small magnet in your pocket. Chromel is non-magnetic and alumel is slightly magnetic.

Sometimes, especially on bearing temperature thermocouples on rotating machines like turbines and generators, the t/c wires are not colour coded.

In furnaces they may be insulated by ceramic sleeves.

Connect to a thermocouple channel on the PLC , DCS or other monitor and hold on to it with your hand or place against a light bulb if you cant use an open flame.

I haven't come across any thermocouples myself that do not have some indication of how they are to be connected (not saying that none exist, just that I haven't seen them myself). The thermocouple wire we use to read aluminum bath temperature is type K and they are color coded. Some of our air thermocouples are also type K and they have a small + and - to indicate how they are to be connected. But basically, if you are reading temperature that is backwards then you have your connection backwards. One other thing, if you have your thermocouple attached correctly, and you are sure, and the temperature is still going the wrong way (especially on new installations) you could have some of your junctions going back to your temperature reading device installed backwards. This can happen when you have someone install the thermocouple that isn't familiar with them. I have had this happen that the electrician that installed thermocouple wire on a new system did what most electricians would do, he put red as positive.