A thermocouple is a commonly used thermocouple temperature range type of sensor that is used to measure temperature. Thermocouples happen to be popular in industrial control applications because of their relatively low priced and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures where various other common sensor types cannot feature. Try operating a circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are usually fabricated from two electric conductors made of two different steel alloys. The conductors are typically built into a wire having a heat-resistant sheath, typically with an integral shield conductor. At one conclusion of the cable, the two conductors are electrically shorted mutually by crimping, welding, etc. This end of the thermocouple–the popular junction–is thermally attached to the thing to be measured. The other end–the cold junction, often called reference junction–is connected to a measurement system. The target, of course, would be to determine the temperature near the hot junction.
It should be observed that the “hot” junction, that is fairly of a misnomer, may in fact be at a temperature lower than that of the reference junction if very low temperatures are being measured.
Reference Junction Compensation Thermocouples create an open-circuit voltage, known 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 distinction between junctions, it is necessary to know both voltage and reference junction temperature as a way to determine the temperature at the hot junction. As a result, a thermocouple measurement program must either measure the reference junction temperature or control it to maintain it at a fixed, known temperature.
There is a misconception of how thermocouples function. The misconception is certainly that the hot junction is the way to obtain the output voltage. That is wrong. The voltage is generated over the length of the wire. Hence, if the entire wire length is at exactly 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 heat from the resistor to create a perpetual motion machine of the first kind.
The erroneous model in addition claims that junction voltages are generated at the chilly end between your special thermocouple wire and the copper circuit, therefore, 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 heat. That is due to the fact that it’s almost always less costly to simply add a reference junction sensor to an existing 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 special channel to the function serves two purposes: no application stations are taken by the reference junction sensor, and the dedicated channel is usually automatically pre-configured for this reason without requiring host processor assistance. This special channel is made for direct link with the reference junction sensor that’s standard on many Sensoray termination boards.
Linearization Within the “useable” heat range of any thermocouple, you will find a proportional connection between thermocouple voltage and heat range. This relationship, however, is by no means a linear relationship. Actually, most thermocouples are really non-linear over their functioning ranges. As a way to obtain temperature data from a thermocouple, it’s important to convert the non-linear thermocouple voltage to temp units. This process is called “linearization.”
Several methods are commonly used 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 quality. At the contrary end of the spectrum, unique thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation in the analog domain. Generally, neither of these methods is well-appropriate for cost-effective, multipoint data acquisition systems.