A thermocouple can be a popular sort of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications because of their relatively low priced and wide measurement ranges. Especially, thermocouples excel at measuring high temperatures where other common sensor types cannot function. Try operating an integrated circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors manufactured from two different metal alloys. The conductors are generally built in a cable having a heat-resistant sheath, often having an integral shield conductor. At one end from the cable, both the conductors are electrically shorted together by crimping, welding, etc. This end of your thermocouple–the junction–is thermally connected to the object being measured. One other end–the cold junction, sometimes called reference junction–is linked to a measurement system. The goal, needless to say, is to determine the temperature near to the hot junction.
It should be noted the “hot” junction, which can be somewhat of a misnomer, may in reality attend a temperature lower than that of the reference junction if low temperatures are measured.
Since thermocouple voltage can be a function of the temperature difference between junctions, it can be required to know both voltage and reference junction temperature so that you can determine the temperature in the hot junction. Consequently, a thermocouple measurement system must either look at the reference junction temperature or control it to keep it in a fixed, known temperature.
You will find a misconception of methods thermocouples operate. The misconception would be that the hot junction will be the source of the output voltage. This can be wrong. The voltage is generated across the duration of the wire. Hence, in case the entire wire length reaches the same temperature no voltage will be generated. If the were not true we connect a resistive load to a uniformly heated thermocouple controller inside an oven and utilize additional heat in the resistor to generate a perpetual motion machine of your first kind.
The erroneous model also claims that junction voltages are generated in the cold end involving the special thermocouple wire along with the copper circuit, hence, a cold junction temperature measurement is required. This concept is wrong. The cold -end temperature is the reference point for measuring the temperature difference across the duration of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, as an alternative to control, the reference junction temperature. This is certainly simply because that it is more often than not less costly just to add a reference junction sensor for an existing measurement system rather than add on a full-blown temperature controller.
Sensoray Smart A/D’s study the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a unique channel to this function serves two purposes: no application channels are consumed through the reference junction sensor, along with the dedicated channel is automatically pre-configured for this function without requiring host processor support. This special channel is made for direct link to the reference junction sensor that may be standard on many Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, there exists a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. In fact, most thermocouples are exceedingly non-linear over their operating ranges. In order to obtain temperature data from the thermocouple, it is needed to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is known as “linearization.”
Several methods are typically accustomed to linearize thermocouples. With the low-cost end in the solution spectrum, one could restrict thermocouple operating range such that the thermocouple is nearly linear to within the measurement resolution. In the opposite end of the spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation from the analog domain. Generally, neither of these methods is well-designed for cost-effective, multipoint data acquisition systems.
Along with linearizing thermocouples inside the analog domain, it can be possible to perform such linearizations inside the digital domain. This is certainly accomplished by using either piecewise linear approximations (using look-up tables) or arithmetic approximations, or sometimes a hybrid of the two methods.
The Linearization Process Sensoray’s Smart A/D’s use a thermocouple measurement and linearization procedure that is made to hold costs into a practical level without sacrificing performance.
First, the two thermocouple and reference junction sensor signals are digitized to get thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized in a higher rate compared to reference junction since it is assumed that the reference junction is fairly stable when compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.