A thermocouple is really a popular kind of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications because of their relatively inexpensive and wide measurement ranges. In particular, 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 made from two different metal alloys. The conductors are generally included in a cable using a heat-resistant sheath, often by having an integral shield conductor. At one end in the cable, both conductors are electrically shorted together by crimping, welding, etc. This end in the thermocouple–the hot junction–is thermally linked to the object to get measured. The other end–the cold junction, sometimes called reference junction–is attached to a measurement system. The goal, needless to say, is to discover the temperature close to the hot junction.
It must be noted that the “hot” junction, that is somewhat of any misnomer, may the truth is attend a temperature lower than that of the reference junction if low temperatures are now being measured.
Since thermocouple voltage can be a purpose of the temperature distinction between junctions, it can be required to know both voltage and reference junction temperature as a way to determine the temperature in the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to keep it with a fixed, known temperature.
You will discover a misconception of methods thermocouples operate. The misconception would be that the hot junction is definitely the method to obtain the output voltage. This is wrong. The voltage is generated across the length of the wire. Hence, if the entire wire length reaches a similar temperature no voltage could be generated. If it were not true we connect a resistive load to some uniformly heated thermocouplers inside an oven and utilize additional heat from the resistor to generate a perpetual motion machine of the first kind.
The erroneous model also claims that junction voltages are generated with the cold end in between the special thermocouple wire and also the copper circuit, hence, a cold junction temperature measurement is required. 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 opt to measure, as an alternative to control, the reference junction temperature. This can be simply because that it must be usually more affordable just to put in a reference junction sensor with an existing measurement system rather than 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 this particular function serves two purposes: no application channels are consumed from the reference junction sensor, and the dedicated channel is automatically pre-configured for this particular function without requiring host processor support. This special channel is for direct connection to the reference junction sensor which is standard on many Sensoray termination boards.
Linearization Within the “useable” temperature range of any thermocouple, you will discover a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are incredibly non-linear over their operating ranges. As a way to obtain temperature data from a thermocouple, it really is required to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is referred to as “linearization.”
Several methods are commonly employed to linearize thermocouples. In the low-cost end from the solution spectrum, you can restrict thermocouple operating range such that the thermocouple is almost linear to throughout the measurement resolution. On the opposite end of your spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation in the analog domain. On the whole, neither of such methods is well-designed for cost-effective, multipoint data acquisition systems.
In addition to linearizing thermocouples within the analog domain, it really is easy to perform such linearizations within the digital domain. This really is accomplished through either piecewise linear approximations (using look-up tables) or arithmetic approximations, or in some cases a hybrid of such two methods.
The Linearization Process Sensoray’s Smart A/D’s use a thermocouple measurement and linearization process that was created to hold costs to some 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 compared to reference junction since it is assumed how the reference junction is fairly stable when compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.