Using Traditional NI-DAQ (Legacy) and Cold Junction Compensation Sensors

Issue Details

​Where can I find additional information on measuring and converting CJC sensor data when using Traditional NI-DAQ (Legacy) with a SCXI module?

Solution

Cold Junction Compensation (CJC) is necessary when making temperature measurements using thermocouples. The thermocouple itself relies on the principle that an electrical potential exists at the junction of two different metals. The amount of this potential varies with temperature. When using a thermocouple we measure the voltage potential across the thermocouple and can calculate the temperature causing this potential difference.

CJC becomes necessary because the junction between each end of the thermocouple and your measuring system (connector block, terminal block) also adds a potential difference to the thermocouple voltage. To compensate for this added potential we must know the temperature at the junction between the thermocouple and your measuring system. This temperature is measured using a CJC sensor. This is a temperature sensor other than a thermocouple; commonly used sensors are IC (integrated circuit) sensors and thermistors.

To make our actual thermocouple temperature measurement we measure the voltage from the thermocouple and CJC sensor. Then we convert the CJC sensor measurement into a CJC temperature and pass the thermocouple voltage and CJC temperature to our conversion function to calculate the thermocouple temperature.

Many of the SCXI and other terminal blocks include CJC sensors that are addressed using a special channel string. The following terminal blocks use an IC sensor for CJC: AMUX-64T, SCXI-1300, SCXI-1320, SCXI-1321, and SC-207X series. The following terminal blocks use a thermistor for CJC: SCXI-1303, SCXI-1322, SCXI 1327, and SCXI-1328. 

How you specify the CJC sensor in your channel string will depend on the device or SCXI module you are using. For terminal blocks directly connected to a data acquisition device such as the AMUX-64T and SC-207X series you configure the temperature sensor using a jumper on the terminal block and specify the onboard channel such as 0 to measure the CJC sensor. When you use an SCXI module and terminal block, you define a special channel ('mtemp', 'dtemp', or 'cjtemp') depending on the module (see below) you are using. The final channel string will include the chassis and module number as well.

e.g. ob0!sc1!md1!mtemp

'mtemp' is used by the following modules: SCXI-1100, SCXI-1120 (multiplexed), SCXI-1121 (multiplexed), and SCXI-1122. 

'dtemp' is used on the following modules: SCXI-1120 (parallel) and SCXI-1121 (parallel).

'cjtemp' is used on the SCXI-1102 module.

Additional Information

Note: This article is about NI-DAQ also known as Traditional DAQ (T-DAQ). NI-DAQmx replaced Traditional NI-DAQ in 2003. 
NI strongly recommends using NI-DAQmx for new applications.