How temperature is detected and measured?

Living from the past to the present, our world has been directly affected by the changing of the temperature due to the shifting of seasons. With these, people had been adjusting its gears depending to the temperature they are into. Temperature is within almost every aspect of our lives, thus, temperature measurement is one of the most important measured parameters across all industries. Inside the temperature measurement world, there are these three basic and most used concept and techniques of measuring a temperature, these are: thermoelectric effect, thermal expansion, and radiation or optical temperature measurement.

Thermoelectric

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Reference junction block inside a Fluke CNX t3000 temperature meter. wikimedia

The first technique of temperature measurement is the thermoelectric effect (Peltier-Seebeck effect). This is derives from the individual discoveries of French physicist Jean Charles Athanase Peltier and Estonian-German physicist Thomas Johann Seebeck. The thermoelectric effect of temperature measurement is being driven by the thermoelectric devices, and these devices make very convenient temperature controllers. This was because of the principle being followed by the thermoelectric effect, where the temperature difference is directly converted to electric voltage and vice versa. Meaning, when a voltage is being applied to it, it creates a temperature difference and this effect can be best used to generate electricity, to measure temperature, to cool objects, and to heat or cook them. Moreover, the direction of heating and cooling being done by this technique in temperature measurement is being determined by the sign of the applied voltage.

A pair of base metals are connected together that forms a junction is called a thermocouple. It is paired specifically together to achieve the sensitivity needed to detect temperature change. If the reference junction part of the metal was immersed into a melting ice that has an exact measurement of 0 degree Celsius in this way, it would be easier to calculate the difference between the temperature of the measured junction and the reference junction with a certain amount of voltage.

Each materials has maximum and minimum threshold ion detecting temperature. It is prone to degradation over time, which greatly affect its performance. We need to place a protective suit to minimize degradation. With the thermocouple protection, the thermocouple becomes durable, and can be used for long. In contrast, thermopile is a thermocouple connected together in series in such a way that its reference junction has the same cold temperature while the hot junction is exposed. Measurement is done in the hot junction. It drastically increase the temperature sensitivity.

Furthermore, the Seebeck effect involves the conversion of temperature differences directly into electricity. It means this means that when there is a presence of temperature difference between two different metals or semiconductors, it creates a thermoelectric EMF in which causes a continuous flow of current in the conductors if they form a complete loop. Whereas the Peltier effect, it is an effect whereby heat is emitted or absorbed when an electric current passes across a junction between two materials. This means that it creates a temperature different by transferring het between two electric junctions. Though its main application is cooling, however, the Peltier effect can also be applied for heating or control of temperature.

Thermal Expansion

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Bimetallic Strip. wikimedia

One of the natural phenomenon that any solid, liquid and gas experience is that they will expand when exposed to heat and this phenomenon is called thermal expansion. Any matter that is exposed to heat will increased in length, volume, area, etc. We can then be able to measure temperature by the change of volume of a liquid. How would we be able to do such process?

We can use mercury, which is sensitive to temperature change. We placed in in a glass capillary tube. As the temperature rises, the fluid expands as to the change in temperature. The liquid expansion is directly proportional to change in temperature. The changes in the volume of the mercury is used to relate the temperature.

We can applied it with solid, specifically bimetallic strip. If the solid is exposed to a certain amount of heat, the metallic strip expands in length. We relate the change in length to the temperature. We take the difference between the initial and final length to get the value of the temperature. A bimetallic thermometer is an example of this method. It is usually connected to a thermostats. Two metal strip are bonded together. When the temperature increases, the metal bends towards the metal with low-temperature coefficient. In contrast, the metal bends towards the metal with high-temperature coefficient when the temperature decreases.

We can use gas to detect temperature change. Gas follows the same principle thermal expansion as to liquid and solid. By measuring the pressure change in the gas as it expands, pressure thermometer used this mechanism. We filled a stainless steel bulb with gas. Hence it is confined, the gas can not expand, and cause pressure to increase. When temperature rises, the pressure of the gas increases. A transducer is used to detect the pressure change and translate it to temperature values.

Radiation and Optical

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Infrared Thermometer. wikimedia

The last technique in temperature measurement is the radiation and optical temperature measurement. This temperature measurement of radiation and optical is also called as the radiation and optical pyrometers. Optical pyrometers allow easy measurement of the temperatures of objects that would normally be difficult to measure with contact devices. This means, the optical pyrometer is a non-contact type temperature measuring device. It works on the principle of matching the brightness of an object to the brightness of the filament which placed inside the pyrometer. The main application of this optical pyrometer is used for measuring the temperature of the furnaces, molten metals, and the other overheated material or liquids.

Moreover, optical pyrometer can also not be used for temperature measurement, but also can be used to see the heat that is measured. In radiation pyrometer, this type of pyrometer relies to the wavelengths in which being measured by the device known to be pure radiation wavelengths. This device is used in places where physical contact temperature sensors like: thermocouple, RTD or the resistance temperature detector, and the thermistors. One of the advantages of radiation pyrometer is that, this device can be used to measure very high temperature without direct contact with the hot source.

Conclusion

Temperature as a variable of measuring the heat of an object is one of the most common variable in our modern industrial era. Temperature is observed everywhere, from our body, up to the temperature of our environment. Thus, knowing the legit and the exact measurement of temperature is a must, through the use of the appropriate temperature measuring devices. With all of these temperature measurements techniques none of these is hundred percent accurate and must be maintain throughout the time because it is prone to miscalculation when there is a small change within the system. This different temperature measurements techniques made our modern day possible right now for an ever.

Reference

1. C. Johnsons, Process Control Instrumentation Technology
2. Dunn, W, Introduction to Instrumentation, Sensors, and Process Control
3. Prithwiraj Purkait, Electrical and Electronics Measurements and Instrumentation