A thermocouple can be a widely used sort of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications due to their relatively affordable and wide measurement ranges. Especially, thermocouples do well 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 created from two different metal alloys. The conductors are normally built in a cable having a heat-resistant sheath, often by having an integral shield conductor. At one end from the cable, the two conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple–the junction–is thermally connected to the object being measured. The other end–the cold junction, sometimes called reference junction–is connected to a measurement system. The goal, obviously, is to ascertain the temperature nearby the hot junction.
It ought to be noted how the “hot” junction, that is somewhat of your misnomer, may in reality be at a temperature lower than that of the reference junction if low temperatures are now being measured.
Since thermocouple voltage is really a purpose 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 appraise the reference junction temperature or control it to maintain it at a fixed, known temperature.
There exists a misconception of how thermocouples operate. The misconception is that the hot junction is definitely the method to obtain the output voltage. This can be wrong. The voltage is generated across the duration of the wire. Hence, if the entire wire length are at exactly the same temperature no voltage will be generated. If it were not true we connect a resistive load to a uniformly heated temperature controller inside an oven and employ additional heat in the resistor to create a perpetual motion machine in the first kind.
The erroneous model also claims that junction voltages are generated in the cold end in 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 will be the reference point for measuring the temperature difference across the size of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, rather than control, the reference junction temperature. This is certainly due to the fact that it must be usually less expensive to simply add a reference junction sensor to an existing measurement system rather than to add-on a whole-blown temperature controller.
Sensoray Smart A/D’s study the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating a unique channel to this particular function serves two purposes: no application channels are consumed from the reference junction sensor, as well as the dedicated channel is automatically pre-configured with this function without requiring host processor support. This special channel is designed for direct connection to the reference junction sensor that is certainly standard on many Sensoray termination boards.
Linearization In the “useable” temperature variety 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 extremely non-linear over their operating ranges. In order to obtain temperature data from a thermocouple, it really is necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is called “linearization.”
Several methods are generally accustomed to linearize thermocouples. With the low-cost end in the solution spectrum, one could restrict thermocouple operating range to ensure that the thermocouple is almost linear to inside the measurement resolution. With the opposite end in the spectrum, special thermocouple interface components (integrated circuits or modules) are offered to perform both linearization and reference junction compensation inside the analog domain. On the whole, neither of the methods is well-suited for cost-effective, multipoint data acquisition systems.
In addition to linearizing thermocouples in the analog domain, it is actually easy to perform such linearizations from the digital domain. This really is accomplished by way of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some instances a hybrid of the two methods.
The Linearization Process Sensoray’s Smart A/D’s hire a thermocouple measurement and linearization procedure that was designed to hold costs into a practical level without having to sacrifice performance.
First, both the thermocouple and reference junction sensor signals are digitized to acquire thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized in a higher rate in comparison to the reference junction because it is assumed the reference junction is pretty stable in comparison to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.