A thermocouple is a popular type of sensor that’s used to measure temperature. Thermocouples will be popular in industrial control applications because of their relatively low priced and wide measurement ranges. In particular, thermocouples excel at measuring high temperatures thermocouple types where various other common sensor types cannot performance. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are usually 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 end of the cable, both conductors are electrically shorted along by crimping, welding, etc. This end of the thermocouple–the warm junction–is thermally attached to the object to be measured. The other end–the cold junction, in some cases called reference junction–is linked to a measurement system. The target, of course, would be to determine the temperature near the hot junction.
It should be mentioned that the “hot” junction, that is considerably 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, referred to as 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 distinction between junctions, it is necessary to learn both voltage and reference junction heat as a way to determine the temperature at the hot junction. Consequently, a thermocouple measurement method must either gauge the reference junction temperature or management it to maintain it at a set, known temperature.
There exists a misconception of how thermocouples function. The misconception will be that the hot junction is the source of the output voltage. This is incorrect. The voltage is generated across the length 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 hook up a resistive load to a uniformly heated thermocouple inside an oven and use additional heating from the resistor to produce a perpetual motion machine of the initial kind.
The erroneous model in addition claims that junction voltages happen to be generated at the cool end between the special thermocouple wire and the copper circuit, hence, a cold junction heat range measurement is required. This idea is wrong. The cold -ending temperature is the reference point for measuring the temperature variation across the length of the thermocouple circuit.
Most industrial thermocouple measurement devices opt to measure, instead of control, the reference junction heat. 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 gauge the thermocouple reference junction temperature through a separate analog input channel. Dedicating a particular channel to the function serves two requirements: 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 assistance. This special channel is made for direct connection to the reference junction sensor that’s standard on several Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, there exists a proportional partnership between thermocouple voltage and heat. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are really non-linear over their running ranges. In order to obtain temperature data from a thermocouple, it is necessary to switch the non-linear thermocouple voltage to temperatures units. This technique is called “linearization.”
Several methods are commonly used 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 image resolution. At the contrary end of the spectrum, exceptional thermocouple interface components (integrated 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.