A thermocouple is a device consisting of two wires of different metals welded together, insulated with PTFE, ceramic, or MIMS, and terminated in a plug with connectors made from the same material. They generate an output when a temperature gradient exists.
The Cold Junction Compensation (CJC) is a method used to compensate for the error in measured temperature by the equivalent of ambient temperature. It is used in temperature indicators to compensate for the ambient error. A Double Junction Thermocouple (DJT) is used to provide a test thermocouple with a Reference Junction (RJ) which is more accurate than using an electronic internal CJC system.
Another method is to use a Digital Voltmeter (DVM) without a dedicated temperature indicator. This type of instrument is unlikely to provide internal CJC, so the user requires additional equipment to measure the output of the thermocouple. The DJT consists of a length of thermocouple wire with copper wires welded to one end, known as the Cold Junction (CJ) or Reference Junction (RJ).
To establish the error associated with the DJT, the open end of the DJT must be welded together to create a measuring junction. A typical calibration set up involves placing the measuring junction into a stirred liquid bath and the RJ into ice/water or a thermocouple reference unit. The output of the DJT is measured on a calibrated DVM or on a microK.
The error associated with the double junction thermocouple can be corrected for during use. The calibration points should span ambient temperatures of 15°C to 25°C.
The Double Junction Thermocouple (DJT) is a versatile tool used for measuring thermocouples. It offers superior accuracy in reference junctions (RJ) and can be used with a thermocouple socket. The DJT can be switched on or off depending on whether the thermocouple is fitted with its own RJ. This method is ideal for accurate measurement of thermocouples, as low-quality RJ sensors can cause errors during calibration. Thermocouple measuring instruments can also switch the internal CJC on or off, requiring traceable calibration to quantify measurement errors.