A thermocouple can be a widely used kind of sensor which is used to measure temperature. Thermocouples are popular in industrial control applications due to their relatively affordable and wide measurement ranges. Specifically, thermocouples do well at measuring high temperatures where other common sensor types cannot function. Try operating an incorporated 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 into a cable having a heat-resistant sheath, often by having an integral shield conductor. At one end from the cable, both conductors are electrically shorted together by crimping, welding, etc. This end from the thermocouple–the hot junction–is thermally coupled to the object being measured. The other end–the cold junction, sometimes called reference junction–is linked to a measurement system. The goal, of course, is to ascertain the temperature near to the hot junction.
It ought to be noted that this “hot” junction, which is somewhat of the misnomer, may the truth is attend a temperature lower compared to the reference junction if low temperatures are being measured.
Since thermocouple voltage is actually a function of the temperature distinction between junctions, it is required to know both voltage and reference junction temperature in order to determine the temperature with the hot junction. Consequently, a thermocouple measurement system must either look at the reference junction temperature or control it to keep it at the fixed, known temperature.
There exists a misconception of methods thermocouples operate. The misconception is the fact that hot junction may be the source of the output voltage. This really is wrong. The voltage is generated across the duration of the wire. Hence, if the entire wire length is at the same temperature no voltage will be generated. If the were not true we connect a resistive load to a uniformly heated thermocouple inside an oven and utilize additional heat in the resistor to make a perpetual motion machine of your first kind.
The erroneous model also claims that junction voltages are generated on the cold end between the special thermocouple wire along with the copper circuit, hence, a cold junction temperature measurement is essential. This idea is wrong. The cold -end temperature may be the reference point for measuring the temperature difference across the duration of the thermocouple circuit.
Most industrial thermocouple measurement systems prefer to measure, as opposed to control, the reference junction temperature. This can be simply because that it must be usually cheaper to merely include a reference junction sensor for an existing measurement system than to add-on a whole-blown temperature controller.
Sensoray Smart A/D’s appraise 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 consumed by the reference junction sensor, and the dedicated channel is automatically pre-configured just for this function without requiring host processor support. This special channel is for direct connection to the reference junction sensor that is standard on many Sensoray termination boards.
Linearization Throughout the “useable” temperature array of any thermocouple, there exists a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. In reality, most thermocouples are exceedingly non-linear over their operating ranges. To be able to obtain temperature data from a thermocouple, it can be necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is referred to as “linearization.”
Several methods are generally accustomed to linearize thermocouples. On the low-cost end of the solution spectrum, one can restrict thermocouple operating range in a way that the thermocouple is nearly linear to inside the measurement resolution. At the opposite end of the spectrum, special thermocouple interface components (integrated circuits or modules) are for sale to perform both linearization and reference junction compensation in the analog domain. On the whole, neither of the methods is well-designed for cost-effective, multipoint data acquisition systems.
Together with linearizing thermocouples in the analog domain, it really is easy to perform such linearizations in the digital domain. This is accomplished by means of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or sometimes a hybrid of such two methods.
The Linearization Process Sensoray’s Smart A/D’s employ a thermocouple measurement and linearization process that was created to hold costs into a practical level without sacrificing performance.
First, both thermocouple and reference junction sensor signals are digitized to acquire thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized at a higher rate in comparison to the reference junction since it is assumed the reference junction is relatively stable in comparison to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.