43-TV-25-35 GLO Iss.4 Dec 06 UK 247A5E01001767-04Appendix C: Thermocouple ConnectionsHow Thermocouples workThermocouples produce a voltage dependant upon the temperature gradient between thehot and cold junctions. The hot junction is the measuring junction of the thermocouple, thecold junction is the point where the thermocouple wires are connected, usually the connec-tion block on the back of the recorder.If the hot (measuring) junction is at 100°C and the connection block of the recorder is at 20°Cthe thermocouple will produce a voltage corresponding to 80°C, the difference between thehot and cold junctions.For the recorder to correctly report the 100°C it needs to know the temperature at the coldjunction. There are 4 options of cold junction compensation (CJC) on the recorder that aredetailed below. The simplest and most common is to use the “Internal Automatic” compen-sation. This uses a CJC sensor built onto the AI board to measure the connector tempera-ture. In this case it will measure 20°C, this will then be added to the 80°C measured by thethermocouple to give a displayed reading of 100°C.The 4 methods of cold junction compensation are described below.All diagrams show passive burnout, for active burnout replace the + for *._+CJCColdJunction20°C100°CHot Junction
