A thermocouple is a popular type of sensor that’s used to measure temperature. Thermocouples will be well-known in industrial control applications because of the relatively low cost and wide measurement ranges. Specifically, thermocouples excel at measuring 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 electric conductors made of two different steel alloys. The conductors are typically built into a wire having a heat-resistant sheath, usually with an essential shield conductor. At one end of the cable, both conductors are electrically shorted together with each other by crimping, welding, etc. This end of the thermocouple–the popular junction–is thermally attached to the thing to be measured. Another end–the cold junction, oftentimes called reference junction–is linked to a measurement system. The objective, of course, would be to determine the temperature near the hot junction.
It should be mentioned 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 minimal temperatures are being measured.
Reference Junction Compensation Thermocouples produce an open-circuit voltage, referred to as the Seebeck voltage, that is proportional to the temperature difference 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 learn both voltage and reference junction temp to be able to determine the temperatures at the hot junction. Consequently, a thermocouple measurement system must either gauge the reference junction temperature or command it to keep it at a fixed, known temperature.
There exists a misconception of how thermocouples function. 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 length 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 connect a resistive load to a uniformly heated thermocouple in a oven and use additional heat from the resistor to create a perpetual motion machine of the initial kind.
The erroneous model also claims that junction voltages are usually generated at the cool end between your special thermocouple wire and the copper circuit, consequently, a cold junction heat range measurement is required. This idea is wrong. The cold -ending temperature is the reference stage for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement devices opt to measure, instead of control, the reference junction heat. That 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 by means of a separate analog input channel. Dedicating a particular channel to this function serves two purposes: no application channels are taken by the reference junction sensor, and the dedicated channel is usually automatically pre-configured for this function without requiring host processor help. This special channel is designed for direct connection to the reference junction sensor that is standard on countless Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, there exists a proportional marriage 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 working ranges. In order to obtain temperature data from the thermocouple, it’s important to turn 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 resolution. At the contrary end of the spectrum, exclusive thermocouple interface components (included circuits or modules) are available to execute both linearization and reference junction compensation in the analog domain. Generally, neither of the methods is well-suited for cost-effective, multipoint data acquisition methods.