A thermocouple is actually a widely used sort of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications due to their relatively low priced and wide measurement ranges. In particular, thermocouples excel 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 made from two different metal alloys. The conductors are typically included in a cable possessing a heat-resistant sheath, often with the integral shield conductor. At one end from the cable, both the conductors are electrically shorted together by crimping, welding, etc. This end in the thermocouple–the hot junction–is thermally attached to the object to be measured. Other end–the cold junction, sometimes called reference junction–is linked to a measurement system. The objective, needless to say, is to determine the temperature near to the hot junction.
It should be noted that this “hot” junction, which can be somewhat of a misnomer, may in fact be at a temperature lower than that of the reference junction if low temperatures are being measured.
Since thermocouple voltage is really a purpose of the temperature distinction between junctions, it really is essential to know both voltage and reference junction temperature so that you can determine the temperature with the hot junction. Consequently, a thermocouple measurement system must either appraise the reference junction temperature or control it to preserve it in a fixed, known temperature.
There is a misconception of how thermocouples operate. The misconception would be that the hot junction will be the way to obtain the output voltage. This really is wrong. The voltage is generated across the size of the wire. Hence, when the entire wire length is in the same temperature no voltage can be generated. If it were not true we connect a resistive load to a uniformly heated k type temperature sensor inside an oven and employ additional heat through the resistor to produce a perpetual motion machine of the first kind.
The erroneous model also claims that junction voltages are generated with the cold end between the special thermocouple wire as well as the copper circuit, hence, a cold junction temperature measurement is essential. This concept is wrong. The cold -end temperature is definitely the reference point for measuring the temperature difference across the size of the thermocouple circuit.
Most industrial thermocouple measurement systems prefer to measure, as an alternative to control, the reference junction temperature. This really is simply because that it is almost always less costly to merely put in a reference junction sensor to an existing measurement system than to add on an entire-blown temperature controller.
Sensoray Smart A/D’s study the thermocouple reference junction temperature through a dedicated analog input channel. Dedicating an exclusive channel to this function serves two purposes: no application channels are consumed from the reference junction sensor, as well as the dedicated channel is automatically pre-configured for this function without requiring host processor support. This special channel is made for direct connection to the reference junction sensor that may be standard on many Sensoray termination boards.
Linearization Inside the “useable” temperature array 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. To be able to obtain temperature data coming from a thermocouple, it is necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is named “linearization.”
Several methods are normally employed to linearize thermocouples. At the low-cost end in the solution spectrum, one can restrict thermocouple operating range to ensure that the thermocouple is almost linear to inside the measurement resolution. At the opposite end from the spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation inside the analog domain. On the whole, neither of those methods is well-suitable for cost-effective, multipoint data acquisition systems.
Along with linearizing thermocouples within the analog domain, it is actually possible to perform such linearizations inside the digital domain. This can be accomplished by way of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some instances 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 into a practical level without having to sacrifice performance.
First, the two thermocouple and reference junction sensor signals are digitized to acquire thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized with a higher rate than the reference junction because it is assumed that this reference junction is pretty stable in comparison to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.