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 the relatively low cost and wide measurement ranges. Specifically, thermocouples excel at measuring high thermocouple wire temperatures where 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 made of two different metallic alloys. The conductors are usually built into a cable having a heat-resistant sheath, typically with an integral shield conductor. At one stop of the cable, both conductors are electrically shorted collectively by crimping, welding, etc. This end of the thermocouple–the warm junction–is thermally attached to the object to be measured. Another end–the cold junction, sometimes called reference junction–is linked to a measurement system. The objective, of course, would be to determine the temperature close to the hot junction.
It should be noted that the “hot” junction, that is fairly of a misnomer, may actually be at a temperature less than that of the reference junction if minimal temperatures are being measured.
Reference Junction Compensation Thermocouples make an open-circuit voltage, known as the Seebeck voltage, that’s 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 is necessary to know both voltage and reference junction temperature in order to determine the heat range at the hot junction. Consequently, a thermocouple measurement method must either measure the reference junction temperature or handle it to maintain it at a set, known temperature.
You will find a misconception of how thermocouples work. The misconception can be that the hot junction is the way to obtain the output voltage. This is inappropriate. The voltage is generated across the length of the wire. Hence, if the complete wire length is at the same temperature no voltage would be generated. If this were not true we hook up a resistive load to a uniformly heated thermocouple inside an oven and use additional warmth from the resistor to make a perpetual motion machine of the initial kind.
The erroneous model in addition claims that junction voltages happen to be generated at the cold end between your special thermocouple cable and the copper circuit, therefore, a cold junction heat measurement is required. This concept is wrong. The cold -finish temperature is the reference stage for measuring the temperature distinction across the amount of the thermocouple circuit.
Most industrial thermocouple measurement techniques opt to measure, instead of control, the reference junction temp. That is due to the fact that it is almost always less expensive to simply add 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 dedicated analog input channel. Dedicating a particular channel to the function serves two purposes: no application channels are ingested 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 made for direct link with the reference junction sensor that is standard on several Sensoray termination boards.
Linearization Within the “useable” temp range of any thermocouple, there exists a proportional romance 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 running ranges. To be able to obtain temperature data from the thermocouple, it’s important to transfer the non-linear thermocouple voltage to temperatures units. This technique is called “linearization.”
Several methods are commonly utilized to linearize thermocouples. At the low-cost end of the solution spectrum, one can restrict thermocouple operating range in a way that the thermocouple ‘s almost linear to within the measurement image resolution. At the opposite end of the spectrum, particular thermocouple interface components (built-in circuits or modules) are available to perform both linearization and reference junction reimbursement in the analog domain. In general, neither of the methods is well-appropriate for cost-effective, multipoint data acquisition systems.