A thermocouple is a popular type of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications because of the relatively low priced and wide measurement ranges. Specifically, thermocouples master measuring high temperatures where different common sensor types cannot feature. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples happen to be fabricated thermocouple manufacturers from two electric conductors made of two different metal alloys. The conductors are typically built into a wire having a heat-resistant sheath, often with an essential shield conductor. At one conclusion of the cable, the two conductors are electrically shorted collectively 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, often called reference junction–is linked to a measurement system. The objective, of course, is to determine the temperature near the hot junction.
It should be noted that the “hot” junction, that is somewhat 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 create an open-circuit voltage, named the Seebeck voltage, that’s proportional to the temperature variation between your hot and reference junctions :
Vs = V(Thot-Tref)
Since thermocouple voltage is really a function of the temperature distinction between junctions, it is necessary to know both voltage and reference junction temperatures to be able to determine the temperatures at the hot junction. Consequently, a thermocouple measurement technique must either measure the reference junction temperature or command it to keep it at a set, known temperature.
There exists a misconception of how thermocouples operate. The misconception is definitely that the hot junction may be the source of the output voltage. This is incorrect. 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 weren’t true we hook up a resistive load to a uniformly heated thermocouple inside an oven and use additional high temperature from the resistor to make a perpetual motion machine of the first kind.
The erroneous model likewise claims that junction voltages will be generated at the chilly end between your special thermocouple wire and the copper circuit, consequently, a cold junction heat range measurement is required. This concept 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 techniques opt to measure, instead of control, the reference junction temperature. This is due to the fact that it’s almost always less costly to simply put in 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 by means of a dedicated analog input channel. Dedicating a special channel to the function serves two uses: 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 assistance. This special channel is made for direct connection to the reference junction sensor that’s standard on various Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, there is a proportional relationship between thermocouple voltage and heat. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are extremely non-linear over their functioning ranges. So that you can obtain temperature data from the thermocouple, it is necessary to transfer the non-linear thermocouple voltage to heat range units. This process is called “linearization.”
Several methods are commonly employed to linearize thermocouples. At the low-cost end of the perfect solution is spectrum, you can restrict thermocouple operating range in a way that the thermocouple ‘s almost linear to within the measurement quality. At the opposite end of the spectrum, special thermocouple interface components (included circuits or modules) can be found 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.