A thermocouple can be a widely used kind of sensor that is utilized 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 a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors made of two different metal alloys. The conductors are generally built into a cable possessing a heat-resistant sheath, often by having an integral shield conductor. At one end in the cable, the two conductors are electrically shorted together by crimping, welding, etc. This end of your thermocouple–the junction–is thermally coupled to the object to become measured. The other end–the cold junction, sometimes called reference junction–is associated with a measurement system. The goal, obviously, is to ascertain the temperature nearby the hot junction.
It must be noted the “hot” junction, that is somewhat of any misnomer, may actually be at a temperature lower than that of the reference junction if low temperatures are increasingly being measured.
Since thermocouple voltage is really a purpose of the temperature difference between junctions, it is necessary to know both voltage and reference junction temperature as a way to determine the temperature at the hot junction. Consequently, a thermocouple measurement system must either appraise the reference junction temperature or control it to preserve it at the fixed, known temperature.
There exists a misconception of methods thermocouples operate. The misconception is the hot junction is definitely the method to obtain the output voltage. This is certainly wrong. The voltage is generated across the size of the wire. Hence, in case the entire wire length are at a similar temperature no voltage would be generated. If this were not true we connect a resistive load to some uniformly heated thermocouple temperature sensor inside an oven and use additional heat through the resistor to generate a perpetual motion machine of your first kind.
The erroneous model also claims that junction voltages are generated with the cold end between the special thermocouple wire along with the copper circuit, hence, a cold junction temperature measurement is needed. This idea is wrong. The cold -end temperature will be the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems decide to measure, as an alternative to control, the reference junction temperature. This is certainly mainly because that it must be typically cheaper to simply add a reference junction sensor with an existing measurement system rather than to add on a complete-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 this function serves two purposes: no application channels are consumed by 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 Throughout 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. Actually, most thermocouples are extremely non-linear over their operating ranges. To be able to obtain temperature data from the thermocouple, it is actually 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 of the solution spectrum, one could restrict thermocouple operating range such that the thermocouple is almost linear to inside the measurement resolution. At the opposite end of your spectrum, special thermocouple interface components (integrated circuits or modules) are offered to perform both linearization and reference junction compensation in 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 can be possible to perform such linearizations from the digital domain. This really is accomplished by using either piecewise linear approximations (using look-up tables) or arithmetic approximations, or occasionally a hybrid of the two methods.
The Linearization Process Sensoray’s Smart A/D’s use a thermocouple measurement and linearization process that is designed to hold costs to your practical level without sacrificing performance.
First, both the thermocouple and reference junction sensor signals are digitized to obtain thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized at a higher rate than the reference junction because it is assumed that 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.