Thermocouple Concepts: Cold Junction Compensation(CJC); Thermocouple Loop Resistance; Thermocouple Degradation ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Thermocouple Concepts: Cold Junction Compensation(CJC); Thermocouple Loop Resistance; Thermocouple Degradation

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Thermocouple Cold Junction
A thermocouple has two junctions. The difference in temperature of these junctions is what is used to measure temperature. One is called the hot junction which is inserted in the process whose temperature is required to be measured while the cold junction also known as the reference junction is the termination point outside of the process where the temperature is known and where the voltage is being measured. Typically the cold junction is located in a transmitter or signal conditioner.

Thermocouple Cold Junction Compensation (CJC):
The voltage measured at the cold junction correlates to the temperature difference between the hot and cold junctions; therefore, the temperature at the cold junction must be known for the hot junction temperature to be accurately determined. This process is known as cold junction compensation.
Cold junction compensation (CJC) is performed by either the temperature transmitter, Thermocouple input cards for a DCS or PLC, alarm trips, or other signal conditioner. Ideally the CJC measurement is performed as close to the measurement point as possible because long Thermocouple wires are susceptible to electrical noise and signal degradation.  Cold junction compensation can also be described as the adjustment made by a temperature transmitter to improve accuracy by factoring in the actual cold junction temperature of a thermocouple. The CJC depends on a reference temperature device.

Thermocouple Loop Resistance
The total resistance of the control loop of a thermocouple is called the loop resistance. To ensure that we don’t have excessive loop resistance, it is common practice to keep the thermocouple wire length at 100 feet maximum and to use bigger gauge thermocouple wire(20 AWG or bigger) ensuring that in most practical applications, the total loop resistance does not exceed 100Ω. Also the thermocouple wires must be used in an area that is free from electromagnetic interference. Electromagnetic interference will introduce electrical noise into the temperature measurement setup of the thermocouple introducing errors in the process.

Thermocouple Degradation
In thermocouple applications, attempt is made to predict the failure of a thermocouple before it causes the tripping of a very crucial and sensitive industrial process. This feature is called thermocouple degradation. The thermocouple degradation feature continually monitors the resistance of the thermocouple loop. If the resistance goes above a certain designated trigger level, an alert is sent suggesting sensor replacement. The degrading thermocouple can be caused by wire thinning, sensor breakdown, moisture intrusion or corrosion and can be an indica¬tion of an eventual sensor failure. Identifying this degraded condition prior to a complete Thermocouple failure could prevent an unscheduled process trip and save an expensive unscheduled shutdown.

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