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What is use of compensation cables in thermocouple
Assuming you really mean thermocouple extension cables:
You need to understand how a thermocouple measures temperature. A thermocouple is a junction between two dissimilar metals. That junction will generate a small emf (measured in millivolts) that is proportional to the temperature of the junction. In order to measure the millivolts the thermocouple wires will have to be connected to copper wire. The point at which that occurs becomes another junction that creates an emf. The temperature of this junction needs to be measured (usually via an RTD) so its value can be algebraically added to the measured millivolts to calculate the temperature of interest. This is called "cold junction compensation" because in the good old days an ice bath was used to keep the reference junction at 0 degC rather than measuring the temperature with an RTD. Note, the key requirement is to know the temperature where the junction is made. If you make the junction at the thermocouple head, you usually don't know the temperature there. Also note that it is usually OK to use terminal boards in junction boxes to interconnect the extension wiring as long as you use adjacent terminals since the 2 junctions across the terminal board they should be at the same temperature and will cancel.
If you want to avoid using thermocouple extension wire, the easiest solution is to use temperature transmitters that convert the millivolt signal to a 4-20 ma signal either at the thermocouple head or at a nearby junction box. The cold junction compensation is done inside the transmitter. That also avoids the need for thermocouple input cards, so all thermocouple inputs become regular analog inputs.
You need to understand how a thermocouple measures temperature. A thermocouple is a junction between two dissimilar metals. That junction will generate a small emf (measured in millivolts) that is proportional to the temperature of the junction. In order to measure the millivolts the thermocouple wires will have to be connected to copper wire. The point at which that occurs becomes another junction that creates an emf. The temperature of this junction needs to be measured (usually via an RTD) so its value can be algebraically added to the measured millivolts to calculate the temperature of interest. This is called "cold junction compensation" because in the good old days an ice bath was used to keep the reference junction at 0 degC rather than measuring the temperature with an RTD. Note, the key requirement is to know the temperature where the junction is made. If you make the junction at the thermocouple head, you usually don't know the temperature there. Also note that it is usually OK to use terminal boards in junction boxes to interconnect the extension wiring as long as you use adjacent terminals since the 2 junctions across the terminal board they should be at the same temperature and will cancel.
If you want to avoid using thermocouple extension wire, the easiest solution is to use temperature transmitters that convert the millivolt signal to a 4-20 ma signal either at the thermocouple head or at a nearby junction box. The cold junction compensation is done inside the transmitter. That also avoids the need for thermocouple input cards, so all thermocouple inputs become regular analog inputs.
control.com has this really neat feature called 'Search'. I actually typed 'thermocouple compensation cable' into Google and ended up with a result that took me to this thread in control.com:
http://www.control.com/thread/1026182245
So, there's two ways to find the same information: using an Internet search engine, such as Google, and the 'Search' feature of control.com.
http://www.control.com/thread/1026182245
So, there's two ways to find the same information: using an Internet search engine, such as Google, and the 'Search' feature of control.com.
