A temperature sensor is a device that measures temperature through electrical signals. This electronic device converts the information obtained into electronic data that can be monitored and recorded.
Temperature control is one of the most important measurements on a day-to-day basis.
For what are temperature sensors?
Temperature sensing is in everyday life. They are used in automobiles, medical devices, computers, kitchen appliances, and other types of machinery.
The temperature sensor is a fundamental element when it is required to work at a specific temperature within any manufacturing process, such as foundries, steam engines, food product processing, etc.
It is vital to have precisely controlled temperature and adequate response capacity to work in this type of process.
Temperature is the most common type of physical measurement in industrial applications. In many cases, the success of many processes depends on the temperature. Consequently, many applications that are not so obvious that use temperature sensors:
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The thermostat of the heating system of a house.
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Thermal control systems to prevent a car engine from overheating.
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Control of the overheating of a photovoltaic panel.
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Generation of electricity in thermal power plants and solar plants.
Types of temperature sensors
There are many different types of temperature sensors.
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Contact temperature sensors. These temperature sensors measure surface temperatures being in contact with the sensor.
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Non-contact temperature sensors. These sensors indirectly measure the temperature of an object.
Non-contact sensors
This type of temperature sensor is in direct contact with the object to be measured. These are usually infrared (IR) sensors.
These sensors detect the thermal radiation emitted by an object or a heat source at a distance and send a signal to an electronic circuit to determine the element’s temperature.
These temperature sensors are especially useful in dangerous environments where it is not possible to get close to the object to be measured. They are also used to measure moving objects or extremely high surface temperatures.
Contact temperature sensors
Contact type sensors include thermocouples and thermistors.
Thermocouple sensor
The thermocouple is a sensor made up of conductors. Each of these two conductors is made of dissimilar metals, and they are connected at two points. If this junction is exposed to a heat source, it generates an output voltage due to the thermoelectric effect that reflects the temperature change. The temperature range they can measure is between -200 and 1750 degrees Celsius.
Thermistors
In this type of contact temperature sensor, resistance decreases as temperature increases. The measured temperatures are reflected very quickly and with great precision. Thus, they require specific mathematical calculations due to their nature.
There are two main types of thermistors:
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Negative temperature coefficient (NTC thermistor).
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Positive temperature coefficient (PTC thermistor).
Thermistors can measure between 0.05 and 1.5 degrees Celsius, which makes them more accurate sensors than thermocouples. This is because they are made of ceramic or polymers.
The first NTC thermistor was discovered in 1833 by Michael Faraday, who reported on the semiconductor behavior of silver sulfide. Faraday noticed that the resistance of the silver sulfide decreased rapidly as the temperature increased.
The operation is based on the variation of the resistance of the semiconductor due to the change of the ambient temperature. It creates a variation in the concentration of carriers.
For NTC thermistors, as the temperature increases, the concentration of carriers will also increase so that the resistance will be lower. Hence the coefficient is negative.
For PTC thermistors, in the case of a highly doped semiconductor, it will acquire metallic properties, taking a positive coefficient in a limited temperature range. Thermistors are usually made from semiconductor oxides, such as ferric oxide, nickel oxide, or cobalt oxide. They have low thermal sensitivity.
Resistance detectors (RTDs)
They are also known as resistance thermometers and are the metallic counterpart of thermistors. Compared to thermistors, they are more expensive but more accurate.
These detectors measure temperature by the resistance of the RTD element with temperature.
Different metals such as copper, nickel, or platinum can be used (which is the most expensive and accurate)
What are thermometers, and what types can they be?
A thermometer is a contact temperature sensor. Depending on its operation, there are three types of thermometers:
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Glass thermometer: like the glass tube and ethanol or mercury inside. Currently, ethanol is used more frequently due to the danger of mercury in case of breakage. Mercury or ethanol expands or contracts linearly depending on the temperature it measures. Gabriel Fahrenheit invented the mercury thermometer in 1714.
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Bimetallic Thermometer - This type of thermometer consists of a connected pressure gauge and stem. The sensor’s tip has a spring attached to a rod, which leads to the meter’s needle. When the search coil is in contact with a heat source, it creates a movement in the coil and moves the needle in the meter. It is mainly used as a temperature sensor in the thermohygrograph.
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Gas and liquid thermometer: There is a bulb filled with gas or liquid. When heated, the element inside the bulb heats up and expands. The attached rod moves the needle indicating the measured temperature. They can be at constant pressure or constant volume. These types of thermometers are very accurate and are generally used for the calibration of other thermometers.
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Digital Thermometer: This digital thermometer uses functions as a resistance temperature detector (RTD) or a thermocouple. The temperature is displayed as a digital reading.
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Pyrometers: thermometers for high temperatures are used in foundries, glass factories, kilns for ceramic firing, etc. There are several types according to their operating principle:
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Optical pyrometer: they are based on Wien's law of distribution of thermal radiation, according to which the color of the radiation varies with temperature. The color of the radiation from the surface to be measured is compared with the color emitted by a filament adjusted with a calibrated rheostat. They measure elevated temperatures, from 700 ° C to 3,200 ° C, at which enough energy is radiated in the visible spectrum to allow optical measurement.
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The total radiation pyrometer is based on the Stefan-Boltzmann law. Thus, the intensity of energy emitted by a black body is proportional to the fourth power of its absolute temperature.
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Infrared pyrometer: they capture infrared radiation, filtered by a lens, using a photoresistive sensor, giving rise to an electric current from which an electronic circuit calculates the temperature. It is possible to measure from temperatures below 0 ° C to values above 2,000 ° C.
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Photoelectric pyrometer is based on the photoelectric effect, by which electrons are released from crystalline semiconductors when thermal radiation hits them.
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Resistance thermometer: consists of a metal wire (such as platinum ) whose electrical resistance changes when the temperature varies.
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Thermocouple: A thermocouple or thermocouple is a device used to measure temperatures based on the electromotive force generated by heating the solder of two different metals.
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Thermistor: it is a device that varies its electrical resistance as a function of temperature. Some thermometers make use of integrated circuits that contain a thermistor, such as the LM35.
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Digital thermometers: these are those that, using transducer devices such as those mentioned, then use electronic circuits to convert the minor voltage variations obtained into numbers, finally showing the temperature on display. One of their main advantages is that they do not pollute the environment by not using mercury
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Clinical thermometers: are those used to measure body temperature. There are traditional mercury and digital ones, the latter having some additional advantages such as easy reading, quick response, memory, and in some models vibrating alarm.