A thermocouple is a popular type of sensor that’s used to measure temperature. Thermocouples are common in industrial control applications because of thermocouple the relatively low priced and wide measurement ranges. In particular, thermocouples master measuring high temperatures where different common sensor types cannot functionality. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples will be fabricated from two electric conductors manufactured from two different metallic alloys. The conductors are typically built into a cable connection having a heat-resistant sheath, usually with an integral shield conductor. At one stop of the cable, the two conductors are electrically shorted mutually by crimping, welding, etc. This end of the thermocouple–the sizzling junction–is thermally attached to the object to be measured. Another end–the cold junction, oftentimes called reference junction–is linked to a measurement system. The objective, of course, would be to determine the temperature near the hot junction.
It should be mentioned that the “hot” junction, which is fairly of a misnomer, may in fact be at a temperature lower than that of the reference junction if reduced temperatures are being measured.
Reference Junction Compensation Thermocouples produce an open-circuit voltage, referred to as the Seebeck voltage, that is proportional to the temperature variation between the hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is a function of the temperature variation between junctions, it’s important to learn both voltage and reference junction temperatures as a way to determine the heat range at the hot junction. As a result, a thermocouple measurement program must either measure the reference junction temperature or command it to maintain it at a fixed, known temperature.
There is a misconception of how thermocouples work. The misconception can be that the hot junction may be the way to obtain the output voltage. That is incorrect. The voltage is generated across the length of the wire. Hence, if the complete wire length is at the same temperature no voltage will be generated. If this weren’t true we connect a resistive load to a uniformly heated thermocouple inside an oven and use additional temperature from the resistor to produce a perpetual motion machine of the initial kind.
The erroneous model in addition claims that junction voltages happen to be generated at the chilly end between your special thermocouple wire and the copper circuit, consequently, a cold junction temp measurement is required. This idea is wrong. The cold -end temperature is the reference level for measuring the temperature variation across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, instead of control, the reference junction temperatures. That is due to the fact that it’s almost always less costly to simply add a reference junction sensor to a preexisting measurement system than to add on a full-blown temperature controller.
Sensoray Smart A/D’s gauge the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating a particular channel to this function serves two requirements: no application stations are consumed by the reference junction sensor, and the dedicated channel can be automatically pre-configured for this function without requiring host processor help. This special channel is designed for direct connection to the reference junction sensor that is standard on numerous Sensoray termination boards.
Linearization Within the “useable” heat range of any thermocouple, there exists a proportional marriage between thermocouple voltage and temperature. This relationship, however, is by no means a linear relationship. In fact, most thermocouples are extremely non-linear over their working ranges. As a way to obtain temperature data from the thermocouple, it’s important to convert the non-linear thermocouple voltage to temperature units. This process is called “linearization.”
Several methods are commonly employed to linearize thermocouples. At the low-cost end of the answer spectrum, you can restrict thermocouple operating range such that the thermocouple ‘s almost linear to within the measurement quality. At the opposite end of the spectrum, unique thermocouple interface components (built-in circuits or modules) can be found to execute both linearization and reference junction reimbursement in the analog domain. Generally, neither of the methods is well-suited for cost-effective, multipoint data acquisition systems.