The thermocouple is an active sensor employed for the measurement of temperature. All engineers are familiar with this unique sensor but few of them know its advantages and disadvantages.
Advantages of thermocouple
The benefits of the thermocouple are listed below.
Simple working principle
A thermocouple is created when two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature.
Short response time
One of the greatest advantages of thermocouples is its small point of contact that delivers generally fast response times. It can respond to rapidly changing temperatures within a few hundred milliseconds.
The thermocouple is characterized by the simplest construction among all electrical sensors for temperature measurement, and thus the lowest unit price.
Wide temperature ranges
Thermocouple has a wide operating temperature range. It can be used at very high temperatures. According to EN/ANSI standards and most common industrial applications, thermocouple sensors are used for temperature measurement and control in the range from -200°C to 2500°C.
The thermocouple has a simple but rugged construction. The thermocouple can be used in demanding environments. Because it is durable to vibrations and shocks.
The thermocouple is a self-powered active device, so it does not require a current or voltage source.
It has a small junction. The good dynamic properties of thermocouples are directly related to their small size. Thus, it can be installed easily.
Variety in types
You can choose between different types of thermocouples named by capital letters that show their compositions. The most common thermocouple types of thermocouples include B, E, K, N, R, S, and T.
All thermocouple types are color-coded and the red wire is always the negative lead (opposite the convention used for DC power where Red typically denotes positive).
Can be grounded
The junction of the thermocouple can be grounded and brought into direct contact with the surrounding case metal which drives a faster response time.
It is not prone to self-heating and is intrinsically safe. Because it does not require any excitation power.
Wide application areas
The thermocouple can be used in wide applications such as electric arc furnaces, fog machines, gas turbines, diesel engines, industrial ovens, gas control systems, and aerospace systems.
Thermocouple has a long history. The first basis of the thermocouple was discovered by Thomas Johann Seebeck in 1821. Thirty years later the exact correlations were found by William Thomson.
Disadvantages of thermocouple
The drawbacks of the thermocouple are listed below.
Thermocouple has serious nonlinearity problems in the larger range of measuring temperature, and result in a big error in the measuring results. The types, specifications, and structures of thermocouple variety, almost all have the serious problem of non-linear, and there is a non-linear relationship between its output and the measured temperature.
Another limitation is the accuracy. System errors of less than 1Â°C can be difficult to achieve. The emf or voltage produced by a thermocouple is very small. As a result, errant current flow through the thermocouple can produce an IR drop that can negatively affect the thermoelectric voltage being measured across the thermocouple. Thus, measurement equipment must have a very high input impedance so as not to introduce excess current flow that can affect the measured voltage.
Interference can cause errors
Due to the small voltage output from the thermocouple, electrical interference can cause errors when the thermocouple is used in electrically noisy environments. (e.g., near electric lights or radio equipment)
Thermocouples have often been used for research purposes, but are becoming less common for air temperature measurement because of the requirement for accurate measurement of reference temperature.
The thermocouple calibration must be performed while in use by comparing it with a nearby thermocouple comparison. If the thermocouple is removed and placed in a calibration bath, it does not exactly reproduce the output integrated over the length.
It requires extra protection from corrosion. The wires can be very delicate and may break easily. Special care must be taken to reduce the strain imposed on the thermocouple wires.
The stray voltage pick-up is possible. The low thermoelectric voltages, high conductor impedances, and high impedance inputs of the measuring equipment make long thermocouple wires an easy pickup for errant signals from nearby equipment and power lines. This usually means that additional filtering in the form of low-pass filtering may be required, in particular for removing power-line noise. Most modern instruments already include this filtering.
The cold junction and lead compensation are essential. The interface circuitry must provide cold junction compensation and the location of the cold junction circuit becomes critical to obtaining an accurate measurement.
Potential measurement error is often a result of poor connections which drive unintended thermoelectric voltage contributions to our measurement voltage. If you need to increase the length of thermocouple wires, you must use the correct type of extension cable for the thermocouple. Substitution of any other type will add errant thermocouple junctions to our measurement system. If terminals are used to connect the wires, then you must additionally select connectors made up of the same material type, unless you can ensure that the connections are kept at the same temperature. You also need to observe the proper polarity when making connections.