'The world is getting hotter every day.' This phrase started to appear from time to time in our daily lives. But we rarely ask ourselves what that means. What's hotter? How hot is hot? Is it hotter than hot or just hotter than cold? To our human senses, temperature is just a subjective assessment. For an objective and reproducible measurement, we need to quantify temperature values and for this we need a suitable measuring device. 
Figure 1: A representative image of temperature sensors
This is achieved through the use of temperature sensors.
Temperature and its measurement
Simply put, temperature is the degree of body heat, which is a measure of the heat content in the body. The problem of quantifying body heat content on a scale did not arise until the invention of the Steam Engine. Scientists' curiosity in understanding the behavior of water at different levels of thermal content gave rise to a formal and better structured study. One of the earliest references to “temperature” dates back to 1760, when Joseph Black declared that applying the same heat to different materials resulted in different temperatures. Years of rigorous scientific study have led to many theories ranging from the simple concept of “caloric,” which treated heat as a material substance that is exchanged between materials, to Carnot's description of heat as a form of energy (which launched the bases of the first law). of thermodynamics). However, none of them satisfactorily explained the concept of temperature. It was Maxwell's theory that offered a good reasoning for this. He defined the temperature of a body as its thermal property that provides information about the energy content of the system. It is the measure of the average kinetic energy (energy due to movement) of the molecules of the substance and means a thermal potential due to which heat flows from a higher temperature to a lower temperature.
The word 'temperature' itself is said to derive from the Latin word 'tempera', which means 'to moderate or soften'. Following Maxwell's line of thought, the speed of the molecules should be the basis for selecting the temperature value, with the absolute absence of heat being a state in which the molecules are totally static. However, this measurement is not possible in practice and therefore other manifestations of the effect of heat are used to measure temperature, for example the geometric expansion of materials. A brief history of temperature sensors with important milestones is shown in the figure below:
Figure 2: A table representing a brief history of temperature sensors with important milestones
Types: Contact Temperature Sensors
Types of temperature sensors:
There are basically two main classes of temperature sensors based on sensing distance:
1. Contact temperature sensor: The sensor is placed in physical contact with the object to be monitored. This method can be used with solids, liquids and gases. Sensors used for measurement can range from hair bulb thermometers and bimetallic sensors to sensors that use voltage signals or varying resistance values.
Expansion thermometers: These sensors use bimetallic strips that have different expansion rates at a given temperature. Thus, this difference in expansion can be translated into a change in temperature using a mechanical pointer. Although not very accurate, these devices offer the advantage of being portable. Low-cost applications such as time compensators in mechanical clocks, thermostats where a higher temperature can open the contact as in heating control or close it as in refrigerators make use of bimetallic strips to open and close mechanical switches which in turn control electrical switches such as circuit breakers.
Filled System Thermometers: These devices are filled with some substitute that expands or contracts due to temperature change. They may be full of mercury. However, as it is considered an environmental hazard, organic liquid types can be used. They do not require electrical energy to operate and are stable even after repeated use. However, they do not provide any type of read storage solution and cannot do point measurements either. They are used in the medical industry to measure body temperature.
Sensors based on voltage signals : Thermocouples are the main sensors in this category. The underlying principle is the Seebeck effect. When two different metals or alloys are placed together to form two junctions, a voltage is induced across the junctions when there is a difference in temperatures between the junctions. These sensors are capable of detecting very high temperatures (up to 1700 °C ), have a very simplistic design that makes them quite robust to shocks and vibrations and can have an almost immediate response to temperature changes. However, they provide localized temperature readings and need cold junction compensation to maintain the temperature gradient. Furthermore, they are highly non-linear devices when compared to other sensors and require extremely good algorithms on the part of the conditioning electronics and processors to compensate for the non-linearity. Thermocouples find application in extremely high temperature sensing applications, chemical reaction monitoring, metal cutting, gas chromatography, sensing temperatures inside internal combustion engines, etc., due to their wide temperature range and robustness; however, if high accuracy and linearity are desired, other temperature sensors must be used. Simple implementation ideas might look like the following:
Figure 3: A figure illustrating the architecture of sensors based on voltage signals
Sensors based on resistance values : The resistance of metals and semiconductors offered to current flow through them changes with temperature. This change can be monitored and mapped for various temperature values on a scale. Furthermore, when the temperature increases, the resistance value may increase or decrease. Substances with a positive temperature coefficient, such as most metals, experience a positive change in resistance with increasing temperature, while the resistance of most semiconductors decreases with increasing temperature due to their negative temperature coefficients. Based on temperature coefficients, Resistance Temperature Detectors (RTD) can be divided into two types:
· RTD Resistance Wire : Constructed primarily from materials with a positive coefficient of resistance, such as platinum, RTDs are resistive elements that exhibit predictable changes in resistance with temperature. The change in Resistance with temperature is given by the relationship:
Figure 4: An equation representing the change in resistance with temperature
Here, R. t and R ó are the resistances of the material at temperatures tet ó ó C and ? is the average temperature coefficient.
These devices can be in the form of thin film resistors or wirewound resistors. They offer a very wide linear range of temperature measurement (-200 to 650 o C) and are very stable with minimal deviation even with repeated operation year after year. A platinum resistance RTD has served as the National Bureau of Standards' primary interpolation instrument. The signal output is quite large compared to thermocouples and can use common copper wires for extension. Furthermore, they can be spread over a large area. Such sensors can be mounted on one arm of a balanced wheat stone bridge circuit as shown in the figure below and the entire circuit can be used to calculate and also control actuators for temperature maintenance using feedback. They provide the desired linear range of operation where thermocouples fall short. RTDs are used in applications such as cold junction compensation, calibration purposes, wheat stone bridge circuit and process control. Linearity simplifies the implementation of signal conditioning circuits and makes RTDs suitable for high-precision applications. RTDs measure absolute temperature in contrast to thermocouples and therefore may not be suitable for maintaining uniform temperature across the entire surface as thermocouples are used.
Fig. 5: A figure representing the RTD architecture
· Thermistors: Semiconductors offer a variety of phenomena and form the basis of electronics. Both positive temperature coefficient (PTC) and negative temperature coefficient (NTC) semiconductors are present and sensors based on them are differentiated as cold-wire PTC thermistors and hot-wire NTC thermistors. For PTC Thermistors, Ferroelectricity is the predominant phenomenon causing the positive coefficient in a short temperature range. The short operating temperature range of these materials makes them suitable for use as temperature limit switches. They have been used successfully in CRT monitors as timers on degaussing coils. They can be used as fuse replacements in the form of current limiting devices. If the current increases, more heat is generated which heats the thermistors. This increases the resistance which reduces the current and voltage available to the device, thus protecting it from increased currents. For NTC thermistors, the relationship between resistance and temperature is negative and exponential, which is very repeatable. In the range of use, this exponential curve can be seen as a very linear graph and can even provide more sensitivity than RTDs, which makes them more attractive in terms of measurement accuracy.
Figure 6: An exponential curve graph that offers more sensitivity than RTDs
Due to their low costs, they find wide use in automotive and consumer products industries such as refrigerant and oil temperature monitors, incubator temperature maintenance, low temperature thermometers, modern digital thermostats, battery temperature monitors, etc. . used is 3D printing where thermistors are used to maintain a constant temperature at the hot end of 3D printers for proper melting of plastic filaments.
Integrated Silicon Temperature Sensors : In addition to all these ratings, the integrated circuits are designed to provide ease of use in measuring temperatures in the desired range. For example, the LM35 IC from Texas Instruments is a precision temperature sensor IC that offers readings directly in the Celsius scale and the LM34 is another that offers readings in the Fahrenheit scale. These ICs provide voltage readings that are directly proportional to a given temperature multiplier and can therefore be read directly on a multimeter or fed directly to an ADC for further processing. They provide easy integration and interface with other circuit elements. Many semiconductor companies such as Analog Devices, Microchip, Smartek, ZMD and STMicroelectronics are involved in the design of temperature sensors and even provide signal processing circuits and digital I/O interfaces for microcontrollers. These temperature sensors are widely used in consumer products such as personal computers, office electronic equipment, cell phones, HVACs, and battery management solutions.
In addition to these important principles of temperature measurement, other methods have also been developed. Some of them are oscillating quartz temperature sensors, thermal noise thermometers, fiber optic thermometers, and temperature measurement systems.
Fig. 7: A figure representing integrated silicon temperature sensors
Non-contact temperature sensors
two. Non-contact temperature measurement: These sensors take temperature measurements without coming into physical contact with the object to be monitored. The most prevalent method in this class of measurement sensors is Pyrometry, which is an infrared measurement technology.
Figure 8: A figure illustrating non-contact temperature sensors
Pyrometry: It is the process of intercepting and measuring thermal radiation with a non-contact device. The radiations emanating from the body are focused onto a radiation receiver using a lens as shown in the figure above. The receiver can be any sensitive device such as thermocouple, photoresistor, photodiode, etc. The action of the transducer generates an electrical signal proportional to the amount of radiation that can be used to measure temperature. Different types of pyrometers are used, some of them being total radiation pyrometer, distribution pyrometer, spectral pyrometer, missing filament pyrometer, etc. These devices rarely replace contact sensors, as they only provide surface temperature values.
Thermal imaging cameras: Although similar in principle to pyrometers, these devices produce a thermal image of the object. These are mainly used in monitoring and controlling machines where localized heating may hamper normal operation.
Acoustic measurements: Such devices are based on the principle of variation in the speed of sound dispersion in different materials with temperature.
Absolute temperature = K v 2
Here v is the speed of sound. Furthermore, acoustic measurements may employ quartz resonators or non-resonant methods such as the Pulse-Echo principle of distance variation. They are used inside furnaces as incinerators.
Sensor Selection Criteria
Temperature sensor selection criteria
None of the temperature sensing devices are versatile enough to be used anywhere. If thermocouples are known for their wide operating temperature range, RTDs are unmatched in the linearity range and thermistors are very precise, while silicon sensors are easy to integrate into circuits. The use of a specific temperature sensor in some applications is governed by a series of parameters, the most important of which is the temperature itself. The temperature range for the application, the rate at which the temperature can change, etc. help decide the type of project. For example, for sensors with high operating temperatures, special connection cables would be required, while for sensors that need to deal with temperature shocks, the wire-wound type of construction is preferred.
The stability and accuracy of the sensor under prescribed operating conditions is another important factor to be considered when choosing the design. How sensitive the device is to measuring small changes and how prone it is to self-heating determines the device's reliability and performance. Sensor response time is generally determined by the size of the sensor. For example, the small dimensions of a film-type resistor-based sensor result in a minimum associated thermal capacity and therefore short response times (0.1 s in water and 3 to 6 s in air). In the same application area, the wire-type resistor would respond from 0.2 to 0.5s in water and from 4 to 25s in air. To help you choose the right temperature sensor for your application, a comparison table of the 4 popular sensors is drawn below for easy reference.
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Thermocouple
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RTD
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Thermistor
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Integrated Silicon
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Temperature range
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-270 – 1800ºC
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-250 – 900ºC
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-100 – 450ºC
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-55 – 150ºC
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Precision
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±0.5°C
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±0.01°C
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±0.1°C
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±1°C
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Linearity (minimum order of the polynomial, the smaller the better)
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4th order polynomial
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2 and order polynomial
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3 third order polynomial
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Linearization is not required. Within ±1°C
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Sensitivity
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? 10 μV/°C
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0.00385 ? / ? /°C (Pt)
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Several ? / ? /°C
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-2mV/°C
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Robustness
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The larger the wire gauge, the greater the strength.
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Quite susceptible to breakage due to vibration
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Hermetic thermistors housed in glass, not affected by shocks or vibrations
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As robust as an IC in plastic packaging like a DIP.
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Responsiveness (test conditions)
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T resolution <1s
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1s
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1s
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4s
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External excitation required
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None
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Current source
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voltage source
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supply voltage
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Exit
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Voltage
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Resistance
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Resistance
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Digital/Current/Voltage
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Cost
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$1 to $50
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$25 to $1000
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$2 to $10
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$1 to $10
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In addition to these considerations, the choice of contact or non-contact sensors is subject to several other environmental conditions. Although contact sensors can provide cost-effective measurements and are quite accurate, they require physical contact, which can lead to contamination, wear and heat dissipation, which changes the temperature being measured. Non-contact sensing, on the other hand, offers faster response and monitoring from a remote location, but cannot measure gas temperatures and has ambient temperature restrictions that can affect readings.
Almost everything in this world and universe remains in delicate balance. Life on Earth was founded because the temperature was ideal. Our body temperature needs to be regulated, otherwise enzymes can malfunction. If the temperature of the oceans increased a little, the Carbon Dioxide dissolved in them would return to the atmosphere, causing further warming. Air conditioning works because we can actually measure the temperature and take corrective action. Electronic circuits work perfectly in a specific temperature range. While temperature sensors may not be able to guarantee taste, they can definitely ensure that your meal is cooked well and that the wine tastes simply delicious. No wonder temperature is so important that it has been defined as one of the fundamental physical quantities in science. Therefore, the importance of temperature sensors cannot be undermined.
