A thermocouple is a commonly used type of sensor that’s used to measure temperature. Thermocouples are usually well-known in industrial control applications because of their relatively low cost and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures where some other common sensor types cannot functionality. Try operating a built-in circuit thermocouple wire (LM35, AD 590, etc.) at 800C.
Thermocouples are usually 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, generally with an essential shield conductor. At one conclusion of the cable, the two conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the scorching junction–is thermally attached to the object to be measured. Another end–the cold junction, occasionally called reference junction–is linked to a measurement system. The target, of course, would be to determine the temperature close to the hot junction.
It should be observed that the “hot” junction, that is fairly of a misnomer, may actually be at a temperature lower than that of the reference junction if reduced temperatures are being measured.
Reference Junction Compensation Thermocouples generate an open-circuit voltage, called the Seebeck voltage, that is proportional to the temperature distinction between your 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 so as to determine the temp at the hot junction. As a result, a thermocouple measurement system must either measure the reference junction temperature or management it to maintain it at a fixed, known temperature.
There exists a misconception of how thermocouples operate. The misconception is usually that the hot junction may be the way to obtain the output voltage. That is inappropriate. The voltage is generated across the amount 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 in a oven and use additional heat from the resistor to create a perpetual motion machine of the initial kind.
The erroneous model in addition claims that junction voltages are usually generated at the chilly end between your special thermocouple cable and the copper circuit, consequently, a cold junction heat measurement is required. This concept is wrong. The cold -stop temperature is the reference stage 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. This is due to the fact that it is almost always less costly to simply put in a reference junction sensor to an existing measurement system than to include on a full-blown temperature controller.
Sensoray Smart A/D’s measure the thermocouple reference junction temperature through a separate analog input channel. Dedicating a particular channel to this function serves two needs: no application channels are consumed by the reference junction sensor, and the dedicated channel will be automatically pre-configured for this function without requiring host processor support. This special channel is designed for direct connection to the reference junction sensor that’s standard on various Sensoray termination boards.
Linearization Within the “useable” heat range of any thermocouple, you will find a proportional romance between thermocouple voltage and temperature. This relationship, however, is by no means a linear relationship. Actually, most thermocouples are extremely non-linear over their working ranges. In order to obtain temperature data from the thermocouple, it’s important to change the non-linear thermocouple voltage to temperature units. This process is called “linearization.”
Several methods are commonly applied to linearize thermocouples. At the low-cost end of the solution spectrum, you can restrict thermocouple operating range in a way that the thermocouple is nearly linear to within the measurement quality. At the opposite end of the spectrum, particular thermocouple interface components (incorporated circuits or modules) are available to execute both linearization and reference junction settlement in the analog domain. Generally, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition techniques.