A thermocouple is a commonly used type of sensor that’s used to measure temperature. Thermocouples happen to be favorite in industrial control applications because of their relatively low priced and wide measurement ranges. Specifically, thermocouples excel at measuring high temperatures where various other common sensor types cannot feature. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples will be fabricated from two electrical conductors manufactured from two different metallic alloys. The conductors are typically built into a cable connection having a heat-resistant sheath, often with an integral shield conductor. At one finish of the cable, both conductors are electrically shorted together with each other by crimping, welding, etc. This end of the thermocouple–the scorching junction–is thermally attached to the object to be measured. The other end–the cold junction, in some cases called reference junction–is connected to a measurement system. The target, of course, would be to determine the temperature close to the hot junction.
It should be mentioned that the “hot” junction, that is fairly 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 produce an open-circuit voltage, referred to as the Seebeck voltage, that’s proportional to the temperature variation 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 learn both voltage and reference junction temperatures in order to determine the temp at the hot junction. Consequently, a thermocouple measurement method must either gauge the reference junction temperature or management it to keep it at a fixed, known temperature.
You will find a misconception of how thermocouples work. The misconception can be that the hot junction may be the source of the output voltage. This is inappropriate. The voltage is generated across the amount 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 connect a resistive load to a uniformly heated thermocouple in a oven and use additional heating from the resistor to make a perpetual motion machine of the initial kind.
The erroneous model also claims that junction voltages happen to be generated at the frigid end between the special thermocouple cable and the copper circuit, therefore, 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, rather than control, the reference junction temp. This 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 through a separate analog input channel. Dedicating a special channel to this function serves two requirements: no application stations are taken 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 link with the reference junction sensor that is standard on numerous Sensoray termination boards.
Linearization Within the “useable” heat range range of any thermocouple, there exists a proportional romantic relationship between thermocouple voltage and temp. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are really non-linear over their running ranges. To be able to obtain temperature data from the thermocouple, it is necessary to switch the non-linear thermocouple voltage to heat units. This process is called “linearization.”
Several methods are commonly utilized to linearize thermocouples. At the low-cost end of the answer spectrum, one can restrict thermocouple operating range in a way that the thermocouple is nearly linear to within the measurement quality. At the opposite end of the spectrum, special thermocouple interface components (included circuits or modules) are available to perform both linearization and reference junction compensation in the analog domain. In general, neither of these methods is well-suited for cost-effective, multipoint data acquisition devices.