What is cjc thermocouple

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. For terminal blocks directly connected to a data acquisition device such as the AMUXT and SCX 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. Open a service request. We also design and manufacture Temperature Calibrators. As always, if you have questions about this blog post, Thermometry technology in-general, or if you wish to talk to TEGAM about our products, please Contact Us here. View all posts by: Snowy DeCaen.

What Exactly is Cold Junction Compensation? Phone: Fax: Website: www. View Quote List Quote List. Recent Blogs. The Story of 4-Wire Measurement. All Blogs. Enter Email Confirm Your Email. If this instrumentation wiring is constructed of dissimilar metals, another thermocouple has been created, possibly creating an error. When multiple sensor input connections are spread out over a distance, temperature gradients over the terminal blocks are naturally present due to convective heating, transient air currents caused by opening and closing instrumentation cabinets, and local heat sources from adjacent electronics.

Only when an ambient temperature sensor is integrally mounted with the terminal block, and only when that temperature sensor has the same thermal response time as the terminal block, can precise cold junction compensation take place over static and dynamic ambient temperature conditions. Many competing products only have a single temperature sensor to compensate multiple input channels. Often this sensor cannot effectively measure gradients or respond with the same thermal characteristic as the multiple input terminal blocks.

Dataforth's initial engineering development, thermocouple prototype, used the standard pin pluggable terminal block used on other MAQ20 input modules, and it used a single, internal, CJC sensor. These modules are able to ensure such greatly improved accuracy because of their leading-edge design, which provides the precise terminal block temperature measurements that are essential for precise cold junction compensation.

Suppose a physics professor assigned a lab experiment to measure the temperature of a Bunsen burner. The class is given a burner, a thermocouple TC , an ice bath, digital voltmeter, and data table for the TC.

The class proceeds but soon remarks to the professor that the diagram in their text book shows connections of both ends of the TC to two copper wires and puts both of these joins into the ice bath.

Cut off two pieces from one of the thermocouple wires and use those. An even more clever student said that would work, but only one piece is needed. Hook it up as shown in Figure 2. He explained it this way. Now we have the same situation as the text book method; except we come back to the meter on two M1 wires. You need to somehow eliminate or compensate for the thermocouples created in the cold junctions.

There are different ways to do that. The connections need to be electrically isolated from the water in the ice-bath to avoid any leak currents causing errors, or possible corrosion being generated. It is anyhow not very practical on a process plant floor, so it is not normally used in process plants. Type N thermocouple is connected as presented in the picture. What is the measured temperature? Since the ice-bath was found to be impractical, you can also do the cold junction connection in some other known, fixed temperature.

You can have a small connection box that has a temperature control keeping the box always at a certain temperature. Typically, the temperature is higher than environment temperature, so the box needs only heating, not cooling.

When you know the temperature that your cold junction is in, and you also know the type of your thermocouple, you can calculate and compensate the cold junction thermovoltage. Many measurement devices or temperature calibrators have a functionality where you can enter the temperature of the cold junction and the device will do all the calculations for you and make the compensation. Please note that thermocouple calculations must always be made in voltage. A common error is to look for the table value for the measured voltage and add the cold junction temperature.

Calculation using temperature values would give You can then compensate the cold junction effect, but the compensation is a little bit more difficult as you need to measure the cold junction temperature all the time, and knowing your thermocouple type, make calculations to know the effect of the cold junction.

Luckily, many temperature calibrators provide a functionality to use a temperature probe to measure the cold junction temperature and the device makes all the compensations and calculations automatically. I mentioned that the previous example was difficult as you need to calculate the compensation at all times, but you could leave that to the measuring device to do it automatically.

The measuring device being a transmitter, DCS input card or temperature calibrator can be measuring the temperature of the cold junction all the time and automatically perform an on-line compensation of the cold junction error. Since the measuring device also knows the thermocouple type you select that in the menu , it can make the compensation automatically and continuously.

You just plug in the thermocouple wire into the device. Please take a look at the Beamex MC6-T temperature calibrator. It can be used also for calibrating thermocouples and it has an automatic cold junction compensation. It also offers a versatile connector where you can connect different thermocouple connectors, or bare thermocouple wires. Also, take a look at the Beamex MC6 calibrator for reference. Topics: Thermocouple. He started working for Beamex in and has, during his years at Beamex, worked in production, the service department, the calibration laboratory, as quality manager, as product manager and as product marketing manager.

Heikki's family consists of himself, his wife and their four children. In his spare time he enjoys playing the guitar.