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CJC Jumpers: Why are they needed in Thermocouple PLC Modules?

CJC jumpers are cold junction compensators used in analog thermocouple PLC modules. To better understand the concept of Cold Junction Compensation (CJC) with CJC jumpers, let’s first review what is a thermocouple and its working principle.

What is a Thermocouple? 

A thermocouple is a sensing device that measures temperature, it’s basically a high-temperature range thermometer. It is called a thermocouple because it works by coupling two wires of different materials (metals or metal alloys), and thermoelectric properties at one end. Heating or cooling the connection point of the two thermocouple wires, creates a voltage that can be correlated to the temperature measurements of the thermocouple.

Thermocouples are often manufactured in a wide range of technical specifications and a variety of models/styles, such as infrared thermocouples, thermocouple probes, bare wire thermocouples, transition joint thermocouple probes, thermocouple probes with connectors, or just a thermocouple wire. All these thermocouple styles can be used to measure temperature, and when a transmitter is included in between, the thermocouple sensor can be used with a Programmable Logic Controller (PLC). You’ll often come across temperature control loops in Ladder Logic programs for PLCs.

Some of the key benefits of thermocouples are: they can measure very high temperatures, they are relatively cheap, and very robust sensors which don’t break easily. Also, unlike Resistance Temperature Detectors (RTDs), thermocouple measurement circuits do not require an excitation current, so their circuits are simple to make. Moreover, you can optimize different types of thermocouple sensors for specific applications.

Alluding to the aforementioned benefits, thermocouples are widely used in a variety of temperature measuring applications ranging from home appliances to industrial processes. For example, they are the most common temperature sensors in everyday appliances like kilns, pizza ovens, furnaces and stoves. On an industrial level they are used in electric power generation, mining, oil & gas, aircraft engines, spacecraft, rockets, automotive sensors, satellites, furnace monitoring and control, and in food & beverage processing.

Note: Although thermocouple sensors may not be as accurate as RTD sensors, they are accurate enough in many of the applications mentioned above.

Working Principle of a Thermocouple 

As previously stated, a thermocouple consists of two electrical conductors made of different materials (metals or metal alloys). The wires are joined together in one end referred to as the hot junction. The free ends of these two wires form two different connections, but with the same temperature. The two free-end connections are the thermocouple cold junctions. In thermocouple sensors, the hot-end junction is the point where temperature is measured. While the cold junction is considered as the reference point, which provides the reference temperature for measuring temperature at the hot junction.

A Simplified Working Principle of a Thermocouple

In the figure shown above, “Thermocouple material 1 and material 2” indicate the two different materials that make up the thermocouple wires. “T1” is the hot junction of the thermocouple, the point you use to measure the temperature. While the two “TCJ” represent the identical temperature of the two cold junctions-the reference point, which can be labeled as “T2”.
If there is a temperature difference between the hot end “T1” and the reference point “T2”, a thermal electromotive force is generated in the thermocouple. The level of the generated thermal voltage is only proportional to the materials which form the thermocouple, and the temperature difference between T1 and T2. So, the two endpoints don’t actually generate the thermoelectric voltage; it’s rather created by the temperature gradient along the two thermocouple wires, all the way between the hot and cold junctions. This is called the Seebeck effect.

The Seebeck effect was discovered by Thomas Johann Seebeck in the 1800s while trying to make electricity from heat energy. He experimented with a two-wire circuit made of Bismuth-Antimony and Bismuth-Copper wires. The circuit included two junctions (hot and cold). He observed that when the two junction points were at different temperatures, a sustained thermo-electric current was generated; converting thermal energy into electrical energy. This caused a small thermoelectric voltage across the two Thermo wires in their open ends.

He also found that the resulting thermoelectric voltage was dependent on the temperature difference and the materials of the two conductive wires. Thus, the functioning of a thermocouple sensor is based upon the principle of the Seebeck Effect; which states that a thermal electromotive force is generated in the thermocouple if there is a temperature difference between the hot junction and the open ends of the Thermo wires made of dissimilar materials.

What is Cold Junction Compensation? 

A thermocouple sensor uses the temperature difference between the cold junction and the hot junction to measure temperature. The hot junction is inserted in the heating process or appliance whose temperature needs to be measured. It is where the temperature is measured from. For example, if you need to measure the temperature in a pizza oven, then the hot junction point will be in the oven. As said, the temperature at the hot junction should be the temperature you need to measure. That’s why this junction is commonly known as the measured temperature or Tense
point.

Thermocouple Circuit with Hot and Cold Junctions

On the other hand, the cold/reference junction acts as the termination point outside the appliance/heating process, where the generated thermoelectric voltage is measured, as shown above. The cold junction is typically located in a temperature transmitter or a signal conditioner.

The thermoelectric voltage measured at the cold junction correlates to the temperature difference between the cold and hot junctions. Therefore, the temperature at the cold junction should be known so as to accurately determine the temperature at the hot junction. But considering just the temperature at the two junctions in a thermocouple measurement circuit is not always enough for correct readings. This is because the temperature reading from a thermocouple sensor is actually the temperature difference between the cold junction (Reference Temperature) and the hot junction (Measured Temperature). For this reason, variations in the reference temperature must be compensated with Cold Junction Compensation (CJC) measuring, to obtain accurate temperature readings of the thermocouple.

Cold Junction Compensation (CJC) is thus the process in which a thermoelectric voltage is subtracted or added to the output voltage of a thermocouple sensor so that the reference temperature at the cold junction can always seem to be 0 °C even when it’s not. In summary, CJC measuring compensates for the unavailable thermoelectric voltage due to the fact that the temperature of the cold junction point is not always at 0°C (32°F). When the temperature of the thermocouple cold junction is at 0°C, no thermoelectric voltage will be generated at that junction. Thus, one can use the standard thermoelectric voltage tables to determine the temperature at the thermocouples’ hot end. Cold Junction Compensation is the reason why thermocouple sensors are not limited to lab setup usage only, but they are also widely used in industrial processes.

Cold Junction Compensation Methods 

To calculate and compensate for the cold junction thermovoltage, you need to know the type of thermocouple being used and the cold junction temperature. A conventional way of making a cold junction at 0°C (32°F) temperature is by using an ice bath or a block with a fixed temperature. Hence, you can connect the two different thermocouple wires with copper wires in an ice bath, and there will be no thermovoltage generation in such a connection. This is a very accurate way of calibrating temperature measuring devices in laboratories. But it’s not very practical for thermocouple sensors used in industrial processes on a factory floor.

In most thermocouple sensors, compensation of the cold junction temperature is performed by temperature calibrators including thermocouple input cards in PLCs or in Distributed Control Systems (DCS), temperature transmitters, alarm trips, and other signal conditioners.

The temperature calibrator (being a PLC thermocouple input card or transmitter) can be measuring the cold junction temperature all the time, while automatically performing online CJC compensation. So, by factoring in the actual temperature of the cold junction, a temperature transmitter can improve the accuracy of the thermocouple readings. Also, since the measuring device knows the type of thermocouple being used (through a menu selection), it can carry out cold junction compensation automatically and continuously.

Automatic online compensation with a temperature measuring device is of course the easiest and most practical way of cold junction compensation, in normal temperature measurements and calibrations involving thermocouples. As you don’t have to worry about the cold junction temperature, you just need to plug the CJC jumper into the thermocouple device and it will take care of the cold junction.

Ideally, CJC measurements should be performed as close as possible to the measurement point because long thermocouple wires are susceptible to signal degradation and electrical noise. That’s why cold junctions are typically located in signal conditioners or in temperature transmitters.

Use of CJC Jumpers in Thermocouple PLC Modules 

As explained in the section above, to implement cold-junction compensation in thermocouples, the temperature of the cold junction must first be determined. This can be done using any type of temperature measuring device. Among the most common temperature measuring devices are RTDs, thermistors, and temperature-sensing Integrated Circuits (ICs). But the selected measuring device should match the requirements of a given application. Significant considerations in such a selection process could include temperature range, accuracy, linearity, and cost requirements.

Calibrated platinum RTDs are best suited for applications that require extreme accuracy. They offer excellent performance across the widest temperature range, but they’re the most expensive. Silicon temperature-sensing ICs and thermistors are cost-effective alternatives for RTDs, in applications not requiring extreme accuracy. The thermistors are capable of operating across a much wider temperature range compared to the silicon ICs. Even so, silicon ICs are often preferred due to their excellent linearity but they operate across a very narrow temperature range. The thermistor’s non-linearity can be readily corrected when used with high-performance microcontrollers and microprocessor-based controllers like PLCs.

In Programmable Logic Controllers (PLCs), a thermocouple sensor kit includes a cold junction temperature-measuring device like a thermistor or a silicon IC (such as a diode) and CJC jumpers. The CJC jumpers enable an operator to perform accurate cold junction compensation for the PLC thermocouple inputs. The assembly of the thermistor and CJC jumpers is critical in ensuring accurate input readings at each thermocouple channel of a PLC I/O module. It provides accurate correction of the cold junction temperature and precise thermocouple readings. Thus, if you remove the cold-junction compensating thermistor or the CJC jumpers, the thermocouple circuit will continue operating, but with reduced accuracy.

Examples of Thermocouple PLC Modules requiring CJC Jumpers 

CJC compensators are generally specific to the appliance or system they are made for, as they have different values of resistance at different cold-junction temperatures. Thus, be sure to check that the selected CJC compensators are compatible with the thermocouple modules in your PLC system. Otherwise, the CJC compensator/CJC jumper won’t work or it will miss-calibrate your system.

Examples of CJC compensators used in thermocouple PLC modules include:

Allen-Bradley 1756-CJC kit: This is a thermocouple sensor kit for ControlLogix PLCs. It includes two Cold Junction Compensation (CJC) jumpers and a thermistor. The kit provides accurate correction of the cold junction temperature and precise thermocouple readings. It’s compatible with the 1756-IT16 and 1756-IRT8I modules. The two ControlLogix I/O modules (1756-IRT81 and 1756-IT16) lack their own CJC sensors, and that’s why they require the 1756-CJC kit with CJC jumpers when switched to thermocouple mode. The jumpers are attached to the screw terminals of each of the two I/O modules.

To perform temperature correction of the cold junction in the thermocouple analog input module (1756-IT16), the 1756-CJC kit utilizes the cold junction compensation thermistor, which is temperature-dependent. For its operations and functionalities, this thermistor makes use of totalizer fill and power features. The optimal operating temperature of the 1756-CJC kit ranges between 0°C to 60 °C (32…140 °F).

Allen-Bradley 1794-CJC2 Kit: This is a Flex cold-junction compensation kit suitable for use with 1794 Flex I/O thermocouple modules or analog RTD input modules. Each 1794-CJC2 kit is available with two cold-junction compensators (two CJC jumpers). It can operate within a temperature range of -20°C to 70 °C (-4…158 °F). This cold-junction compensation kit is compatible with 1794-IRT8 and 1794-IT8 modules, as it supports their operation in thermocouple mode.

This entry was posted on April 25th, 2022 and is filed under Education, Technology, Uncategorized. Both comments and pings are currently closed.

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