A thermocouple is a frequently used form of sensor that is utilized to measure temperature. Thermocouples are popular in industrial control applications because of the relatively affordable and wide measurement ranges. Particularly, 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 created 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 in the cable, both the conductors are electrically shorted together by crimping, welding, etc. This end in the thermocouple–the junction–is thermally linked to the object to be measured. Another end–the cold junction, sometimes called reference junction–is attached to a measurement system. The goal, naturally, is to ascertain the temperature close to the hot junction.
It needs to be noted how the “hot” junction, that is somewhat of a misnomer, may the truth is attend a temperature lower compared to the reference junction if low temperatures are measured.
Since thermocouple voltage is a function of the temperature difference between junctions, it is actually needed to know both voltage and reference junction temperature to be able to determine the temperature on the hot junction. Consequently, a thermocouple measurement system must either appraise the reference junction temperature or control it to keep up it with a fixed, known temperature.
There exists a misconception of how thermocouples operate. The misconception is the fact that hot junction is the supply of the output voltage. This is wrong. The voltage is generated across the length of the wire. Hence, in case the entire wire length are at the same temperature no voltage can be generated. If this were not true we connect a resistive load to your uniformly heated cartridge heater with thermocouple inside an oven and make use of additional heat in the resistor to generate a perpetual motion machine from the first kind.
The erroneous model also claims that junction voltages are generated on the cold end involving the special thermocouple wire and the copper circuit, hence, a cold junction temperature measurement is necessary. This idea is wrong. The cold -end temperature will be the reference point for measuring the temperature difference across the duration of the thermocouple circuit.
Most industrial thermocouple measurement systems choose to measure, instead of control, the reference junction temperature. This is certainly mainly because that it is typically cheaper to easily put in a reference junction sensor to an existing measurement system rather than to add on a complete-blown temperature controller.
Sensoray Smart A/D’s study the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a special channel to the function serves two purposes: no application channels are consumed with the reference junction sensor, and the dedicated channel is automatically pre-configured with this function without requiring host processor support. This special channel is designed for direct link with the reference junction sensor which is standard on many Sensoray termination boards.
Linearization In the “useable” temperature variety of any thermocouple, you will discover a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. The truth is, most thermocouples are incredibly non-linear over their operating ranges. So that you can obtain temperature data from the thermocouple, it can be necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is known as “linearization.”
Several methods are normally used to linearize thermocouples. With the low-cost end of the solution spectrum, anybody can restrict thermocouple operating range in a way that the thermocouple is nearly linear to within the measurement resolution. On the opposite end in the spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation inside the analog domain. Generally speaking, neither of these methods is well-designed for inexpensive, multipoint data acquisition systems.
As well as linearizing thermocouples inside the analog domain, it is actually possible to perform such linearizations within the digital domain. This really is accomplished by using either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some cases a hybrid of those two methods.
The Linearization Process Sensoray’s Smart A/D’s hire a thermocouple measurement and linearization method that was designed to hold costs to a practical level without having to sacrifice performance.
First, both the thermocouple and reference junction sensor signals are digitized to have thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized at the higher rate than the reference junction as it is assumed that this reference junction is fairly stable compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.