A thermocouple is a commonly used type of sensor that’s used to measure temperature. Thermocouples are popular in industrial control applications because of the relatively low cost and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures where other common sensor types cannot function. Try operating an integrated circuit (LM35, AD 590, etc.) thermocouple wire at 800C.
Thermocouples happen to be fabricated from two electric conductors manufactured from two different steel alloys. The conductors are usually built into a cable connection having a heat-resistant sheath, normally with an essential shield conductor. At one finish of the cable, both conductors are electrically shorted together 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, quite often called reference junction–is connected to a measurement system. The objective, of course, would be to determine the temperature close to the hot junction.
It should be observed that the “hot” junction, which is fairly of a misnomer, may in fact be at a temperature less than that of the reference junction if very low temperatures are being measured.
Reference Junction Compensation Thermocouples crank out an open-circuit voltage, referred to as the Seebeck voltage, that’s proportional to the temperature difference between your hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is really a function of the temperature distinction between junctions, it is necessary to learn both voltage and reference junction heat in order to determine the temperature at the hot junction. Consequently, a thermocouple measurement method must either measure the reference junction temperature or management it to keep up it at a set, known temperature.
You will find a misconception of how thermocouples work. The misconception is usually that the hot junction is the way to obtain the output voltage. This is wrong. The voltage is generated across the length of the wire. Hence, if the entire wire length is at the same temperature no voltage will be generated. If this were not true we hook up a resistive load to a uniformly heated thermocouple inside an oven and use additional heating from the resistor to generate a perpetual motion machine of the first kind.
The erroneous model in addition claims that junction voltages are usually generated at the chilly end between the special thermocouple cable and the copper circuit, hence, a cold junction heat range measurement is required. This idea is wrong. The cold -ending temperature is the reference level for measuring the temperature distinction across the length of the thermocouple circuit.
Most industrial thermocouple measurement methods opt to measure, rather than control, the reference junction temperature. That is due to the fact that it’s almost always less costly to simply put in a reference junction sensor to a preexisting measurement system than to add on a full-blown temperature controller.
Sensoray Smart A/D’s measure the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a particular channel to the function serves two functions: no application channels are ingested by the reference junction sensor, and the dedicated channel is certainly automatically pre-configured for this function without requiring host processor assistance. This special channel is designed for direct connection to the reference junction sensor that’s standard on several Sensoray termination boards.
Linearization Within the “useable” heat range of any thermocouple, there exists a proportional marriage between thermocouple voltage and temp. This relationship, however, is by no means a linear relationship. Actually, most thermocouples are extremely non-linear over their operating ranges. To be able to obtain temperature data from the thermocouple, it’s important to switch the non-linear thermocouple voltage to heat units. This technique is called “linearization.”
Several methods are commonly utilized to linearize thermocouples. At the low-cost end of the answer spectrum, you can restrict thermocouple operating range in a way that the thermocouple ‘s almost linear to within the measurement resolution. At the opposite end of the spectrum, exclusive thermocouple interface components (built-in circuits or modules) can be found to execute both linearization and reference junction settlement in the analog domain. In general, neither of the methods is well-suited for cost-effective, multipoint data acquisition systems.