A thermocouple is a popular type of sensor that is used to measure temperature. Thermocouples will be common in industrial control applications because of the relatively low cost and wide measurement ranges. Specifically, thermocouples master measuring high temperatures where additional common sensor types cannot performance. Try operating a built-in circuit (LM35, AD 590, etc.) thermocouple types 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 having a heat-resistant sheath, often with an integral shield conductor. At one stop of the cable, the two conductors are electrically shorted along 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, often called reference junction–is connected to a measurement system. The objective, of course, is to determine the temperature close to the hot junction.
It should be observed that the “hot” junction, which is considerably of a misnomer, may in fact be at a temperature lower than that of the reference junction if low 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 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 know both voltage and reference junction temperature so that you can determine the heat at the hot junction. Therefore, a thermocouple measurement method must either gauge the reference junction temperature or management it to keep it at a fixed, known temperature.
There is a misconception of how thermocouples function. The misconception can be that the hot junction may be the way to obtain the output voltage. That is wrong. The voltage is generated over the amount of the wire. Hence, if the complete wire length is at exactly the same temperature no voltage would be generated. If this weren’t true we hook up a resistive load to a uniformly heated thermocouple in a oven and use additional high temperature from the resistor to create a perpetual motion machine of the first kind.
The erroneous model in addition claims that junction voltages happen to be generated at the wintry end between your special thermocouple wire and the copper circuit, hence, a cold junction temperatures measurement is required. This concept is wrong. The cold -end temperature is the reference level for measuring the temperature distinction across the length of the thermocouple circuit.
Most industrial thermocouple measurement techniques opt to measure, rather than control, the reference junction temp. That is due to the fact that it is almost always less expensive to simply put in a reference junction sensor to a preexisting measurement system than to include 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 special channel to this function serves two functions: no application stations are consumed by the reference junction sensor, and the dedicated channel is certainly automatically pre-configured for this function without requiring host processor support. This special channel is made for direct connection to the reference junction sensor that is standard on many Sensoray termination boards.
Linearization Within the “useable” heat range of any thermocouple, there is a proportional partnership between thermocouple voltage and temperatures. This relationship, however, is in no way a linear relationship. In fact, most thermocouples are extremely non-linear over their functioning ranges. To be able to obtain temperature data from a thermocouple, it is necessary to transform the non-linear thermocouple voltage to temperatures units. This technique is called “linearization.”
Several methods are commonly employed to linearize thermocouples. At the low-cost end of the answer spectrum, one can restrict thermocouple operating range such that the thermocouple ‘s almost linear to within the measurement quality. At the opposite end of the spectrum, special thermocouple interface components (built-in circuits or modules) can be found 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 methods.