A thermocouple is a frequently used kind of sensor which is used to measure temperature. Thermocouples are popular in industrial control applications because of the relatively affordable and wide measurement ranges. Specifically, thermocouples do well at measuring high temperatures where other common sensor types cannot function. Try operating a built-in 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 developing a heat-resistant sheath, often by having an integral shield conductor. At one end of the cable, the two conductors are electrically shorted together by crimping, welding, etc. This end in the thermocouple–the new junction–is thermally attached to the object to get measured. The other end–the cold junction, sometimes called reference junction–is linked to a measurement system. The goal, obviously, is to discover the temperature near to the hot junction.
It must be noted that this “hot” junction, that is somewhat of a misnomer, may in reality be at a temperature lower compared to the reference junction if low temperatures are increasingly being measured.
Since thermocouple voltage can be a function of the temperature difference between junctions, it is actually necessary to know both voltage and reference junction temperature as a way to determine the temperature on the hot junction. Consequently, a thermocouple measurement system must either study the reference junction temperature or control it to preserve it in a fixed, known temperature.
You will discover a misconception of methods thermocouples operate. The misconception would be that the hot junction is definitely the way to obtain the output voltage. This can be wrong. The voltage is generated across the size of the wire. Hence, in the event the entire wire length is at the identical temperature no voltage would be generated. If it were not true we connect a resistive load to some uniformly heated thermocoupler inside an oven and employ additional heat through the resistor to produce a perpetual motion machine of your first kind.
The erroneous model also claims that junction voltages are generated at the cold end between the special thermocouple wire as well as the copper circuit, hence, a cold junction temperature measurement is required. This concept 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 decide to measure, as opposed to control, the reference junction temperature. This can be because of the fact that it must be more often than not less costly to merely give a reference junction sensor for an existing measurement system rather than to add-on a whole-blown temperature controller.
Sensoray Smart A/D’s look at the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating a special channel to the function serves two purposes: no application channels are consumed through the reference junction sensor, along with the dedicated channel is automatically pre-configured with this function without requiring host processor support. This special channel is ideal for direct link to the reference junction sensor that may be standard on many Sensoray termination boards.
Linearization Inside the “useable” temperature array of any thermocouple, there is a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. In fact, most thermocouples are extremely non-linear over their operating ranges. To be able to obtain temperature data coming from a thermocouple, it can be necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is called “linearization.”
Several methods are commonly employed to linearize thermocouples. On the low-cost end of the solution spectrum, one can restrict thermocouple operating range to ensure that the thermocouple is nearly linear to in the measurement resolution. With the opposite end of the spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation inside the analog domain. In general, neither of these methods is well-designed for cost-effective, multipoint data acquisition systems.
Along with linearizing thermocouples from the analog domain, it can be possible to perform such linearizations in the digital domain. This can be accomplished by way of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or occasionally a hybrid of such two methods.
The Linearization Process Sensoray’s Smart A/D’s hire a thermocouple measurement and linearization process that is made to hold costs to a practical level without sacrificing performance.
First, the thermocouple and reference junction sensor signals are digitized to acquire thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized at the higher rate than the reference junction since it is assumed that the reference junction is pretty stable when compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.