A thermocouple is a commonly used type of sensor that’s used to measure temperature. Thermocouples are favorite in industrial control applications because of their relatively low cost and wide measurement ranges. Specifically, thermocouples excel at measuring thermocouple temperature range high temperatures where additional common sensor types cannot performance. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples happen to be fabricated from two electrical conductors manufactured from two different steel alloys. The conductors are typically built into a cable having a heat-resistant sheath, often 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 warm junction–is thermally attached to the thing to be measured. Another end–the cold junction, occasionally called reference junction–is connected to a measurement system. The target, of course, is to determine the temperature close to the hot junction.
It should be observed that the “hot” junction, that is considerably of a misnomer, may actually be at a temperature lower than that of the reference junction if low temperatures are being measured.
Reference Junction Compensation Thermocouples make an open-circuit voltage, known as the Seebeck voltage, that’s proportional to the temperature distinction between the hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is really a function of the temperature difference between junctions, it is necessary to know both voltage and reference junction temp so as to determine the temp at the hot junction. As a result, a thermocouple measurement technique must either measure the reference junction temperature or control it to maintain it at a set, known temperature.
You will find a misconception of how thermocouples work. The misconception is that the hot junction may be the way to obtain the output voltage. This is incorrect. The voltage is generated over the length of the wire. Hence, if the entire wire length is at exactly the same temperature no voltage would be generated. If this were not true we hook up a resistive load to a uniformly heated thermocouple in a oven and use additional temperature from the resistor to create 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 your special thermocouple cable and the copper circuit, consequently, a cold junction temperatures measurement is required. This idea is wrong. The cold -conclusion temperature is the reference level for measuring the temperature variation across the length of the thermocouple circuit.
Most industrial thermocouple measurement methods opt to measure, rather than control, the reference junction heat. This 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 through a dedicated analog input channel. Dedicating a special channel to this function serves two requirements: no application channels are consumed by the reference junction sensor, and the dedicated channel is usually automatically pre-configured for this function without requiring host processor support. This special channel is designed for direct link with the reference junction sensor that is standard on numerous Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, you will find a proportional romantic relationship between thermocouple voltage and temperatures. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are really non-linear over their operating ranges. So that you can obtain temperature data from the thermocouple, it’s important to convert the non-linear thermocouple voltage to temp units. This process is called “linearization.”
Several methods are commonly employed to linearize thermocouples. At the low-cost end of the solution spectrum, one can restrict thermocouple operating range such 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 perform both linearization and reference junction payment in the analog domain. Generally, neither of these methods is well-suited for cost-effective, multipoint data acquisition devices.