A thermocouple is a commonly used type of sensor that’s used to measure temperature. Thermocouples are usually well-known in industrial control applications because of their relatively low cost and wide measurement ranges. In particular, thermocouples master measuring high temperatures where additional common sensor types cannot purpose. Try operating a built-in circuit thermocouple sensor (LM35, AD 590, etc.) at 800C.
Thermocouples happen to be fabricated from two electric conductors made of two different metallic alloys. The conductors are typically built into a cable connection having a heat-resistant sheath, usually with an essential shield conductor. At one conclusion of the cable, both conductors are electrically shorted along by crimping, welding, etc. This end of the thermocouple–the popular junction–is thermally attached to the object to be measured. The other end–the cold junction, oftentimes called reference junction–is connected to a measurement system. The target, of course, is to determine the temperature near the hot junction.
It should be mentioned that the “hot” junction, which 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, called the Seebeck voltage, that’s proportional to the temperature difference 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 temperature in order to determine the temperatures at the hot junction. Therefore, a thermocouple measurement technique must either measure the reference junction temperature or management it to maintain it at a fixed, known temperature.
There is a misconception of how thermocouples function. The misconception can be that the hot junction may be the source of the output voltage. That is incorrect. The voltage is generated across the amount of the wire. Hence, if the entire wire length is at exactly the same temperature no voltage will be generated. If this weren’t true we link a resistive load to a uniformly heated thermocouple inside an oven and use additional temperature from the resistor to make a perpetual motion machine of the initial kind.
The erroneous model likewise claims that junction voltages are usually generated at the wintry end between your special thermocouple cable and the copper circuit, hence, a cold junction heat measurement is required. This idea is wrong. The cold -finish temperature is the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, instead of control, the reference junction temperatures. 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 gauge the thermocouple reference junction temperature through a separate analog input channel. Dedicating a special channel to this function serves two functions: no application channels are consumed by the reference junction sensor, and the dedicated channel is automatically pre-configured for this reason without requiring host processor help. This special channel is designed for direct link with the reference junction sensor that is standard on several Sensoray termination boards.
Linearization Within the “useable” heat range range of any thermocouple, there exists a proportional romantic relationship between thermocouple voltage and temperatures. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are extremely non-linear over their working ranges. In order to obtain temperature data from a thermocouple, it is necessary to transform 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 perfect solution is spectrum, one can restrict thermocouple operating range such that the thermocouple is nearly linear to within the measurement quality. At the contrary end of the spectrum, unique thermocouple interface components (built-in circuits or modules) can be found to execute both linearization and reference junction payment in the analog domain. In general, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition methods.