Temperature Sensors: Types, Uses, Benefits, Design (2023)

Introduction

This article will take an in-depth look at temperaturesensors.

The article will bring more detail on topics such as:

• Principle of Temperature Sensors
• Classes and Types of Temperature Sensors
• Applications and Benefits of Temperature Sensors
• And Much More…

Chapter 1: Principle of Temperature Sensors

This chapter will discuss what temperature sensors are, theiruses, their components, and how they function.

What are Temperature Sensors?

Temperature sensors are devices that detect and measure coldnessand heat and convert it into an electrical signal. Temperaturesensors are utilized in our daily lives, be it in the form ofdomestic water heaters, thermometers, refrigerators, ormicrowaves. There is a wide range of applications of temperaturesensors, including the geotechnical monitoring field.

A temperature sensor can also be defined as a simple instrumentthat measures the degree of coldness or hotness and thenconverts it into a readable unit. There are specializedtemperature sensors used to measure the temperature of theboreholes, soil, huge concrete dams, or buildings.

What do Temperature Sensors do?

Temperature sensors are devices designed for measuring thedegree of coolness and hotness in an object. The voltage acrossthe diode determines the working of a temperature meter. Thechange of temperature varies directly proportional to thediode’s resistance.

The cooler the temperature, the lesser the resistance will beand vice-versa. A measurement of the resistance across the diodeis done, and the measurement is converted into units oftemperature that are readable and displayed in numeric form overreadout units. In the field of geotechnical monitoring, thesetemperature sensors are utilized in the measuring of internaltemperatures of structures such as dams, bridges, power plants.

Uses of Temperature Sensors

There are many different types of temperature sensors, but themost common way that is used in their categorization is based onthe mode of connection that includes contact and non-contacttemperature sensors. Examples of contact sensors includethermistors and thermocouples because their contact with theobjects they measure is direct, whereas non-contact typetemperature sensors measure the heat source’s radiation.

Such temperature meters are mostly used in hazardousenvironments such as thermal power plants or nuclear powerplants. Temperature sensors are used to measure the hydrationheat in mass concrete structures, in the field of geotechnicalmonitoring. They can also be utilized to monitor the migrationof seepage or groundwater.

One area where they are commonly used is in curing concretesince the concrete has to be relatively warm to properly set andcure. The variations of seasons causes expansion or contractionof structures, bringing an overall change to their volume.

How Temperature Sensors Work

The working principle of a temperature sensor is the voltageacross the terminals of the diode. If there is an increase inthe voltage, the temperature also increases. This is followed bya drop in the voltage between the terminals of the transistor ofbase and emitter in a diode. There are also temperature sensorsthat work on the principle of stress change caused by changes intemperature.

In a vibrating wire temperature meter, dissimilar metals havedifferent linear coefficients of expansion. It mainly consistsof a magnetic stretched wire of high tensile strength with twoends fixed to any dissimilar metal so that any temperaturechange will directly affect the tension in the wire and itsnatural vibration frequency.

The dissimilar metal can be made from aluminum since it has alarger linear expansion coefficient than steel. When theconversion of the temperature signal into frequency occurs, thevery same read-out unit that is used for other vibrating wiresensors can also be utilized in the monitoring of temperaturealso.

The specially built vibrating wire sensor is the one that sensesthe temperature change and then the temperature change isconverted into an electrical signal which is then transmitted tothe read out the unit as a frequency.

Temperature Sensor Components

There are three types of components in temperature sensors.There are essential components of a temperature sensor includingthermocouple or extension cables and wires, as well as thesensing elements. The following are examples of components whichcomplete the sensor: insulating beads, connectors, connectingheads, and protecting tubes. There are also associatedcomponents that are necessary in the use of sensors likeconverters and controllers.

Temperature Sensors in Control and Compensation Circuits

The detection circuit must offer an output in a usable format inorder to use a temperature sensor in a control or compensationcircuit. For analog circuits, usually, the output is resistance.The measurement must be converted to a digital format forprocessing by an MCU for digital control and compensation.Commonly, this is achieved by reading the measurement as avoltage by means of an analog-to-digital converter.

Semiconductor-Based Sensors

Semiconductor based sensors have a digital interface that makescommunicating the temperature clearer to an MCU that is able toaccess thermocouples due to their voltage. RTDs and thermistorshave the flexibility to provide voltage or resistance with ease.This gives engineers choices as to the methods of connecting thedetector to the control or compensation system.

RTDs and thermistors output a variable resistance, which makesintegrating them into an analog control or compensation circuitstraightforward. If there is a requirement of a voltage output,the resistance can be converted easily to voltage by means ofthree additional resistors in a Wheatstone bridge configurationas shown in the figure below.

For non-linear sensors like thermocouples and thermistors, theoutput needs to go through a straightforward linearization. Thiscan be implemented in a circuit that is simple for analogcontrol and compensation. For control and compensation that isdigitally based, the CPU can measure the adjusted temperatureusing a simple lookup table that reflects thetemperature/resistance chart that is incorporated in the specsheet of the sensor.

Engineers have many choices in designing a temperature detectioncircuit to prevent overheating or implement temperature controland/or compensation functionality. For extreme temperatures,thermocouples are mostly the ideal option. When there is arequirement for the greatest accuracy, platinum RTDs providehigh precision. For PCB-based applications where installing anexternal sensor internally can be difficult, semiconductor-basedsensors make the sensing of ambient temperature near sensitiveelectronics possible.

Temperature Sensor Elements

RTD elements are utilized in the manufacturing of temperaturesensors. A resistance element is a component that sensestemperature at the heart of a resistance thermometer or an RTD.They can’t be used in their bare form typically, but they may bebuilt into a probe or assembly that enables them to withstanddifferent conditions of their application. Each has a resistancevalue that is pre-specified at a known temperature which changesin a predictable fashion. In this way, by measuring theelement’s resistance, that element’s temperature can bedetermined from calculations, tables, or instrumentation.

Thin Film Temperature Elements

These elements are mass produced by automated equipment, whichdeposits a layer of platinum onto the ceramic substrate andutilizes photolithography in the etching of an electricalcurrent path that corresponds to the value required in ohms. Theelements have a smaller size than that of traditional wire woundelements and as a result have a response time that is fast andare suitable in more applications, while reducing the costs ofthe user at the same time.

Ceramic Temperature Elements

Ceramic elements can be used to make temperature sensors. Theseelements are wound on either ceramic or glass former, or a helixof platinum wire can be semi-supported within a ceramic tube’sbores. This semi-supported type is capable of providing thewidest temperature range of operation and, typically, the beststability. Although there are other usable metals, platinum isthe most prevalent and widely used type with either ceramic orglass insulators.

The use of metals, aside from platinum, can lead to linearity atlow temperatures and drift, which throws off temperatureprocessing. The error corrections and adjustments necessary withother metals is why platinum is preferred.

Thermometrics can make an addition of lead wires to the elementsof a temperature sensor as per specifications. The commonadditions are 20 AWG, Teflon coated nickel wire.

Glass Temperature Elements

Glass (quartz) encased elements that are wire wound areavailable to meet the requirements of a user.

Chapter 2: Classes and Types of Temperature Sensors

This chapter will discuss the different classes and types oftemperature sensors as applied in control and compensationcircuits and temperature sensor elements.

Classes of Temperature Sensors

Temperature sensors are found in different types, sizes, andshapes. There are two main temperature sensors classes: contacttemperature sensors and non-contact temperature sensors.

Contact Temperature Sensors

A few temperature meters are capable of measuring the degree ofhotness or coldness in an object by being in direct contact withthe object. These types of temperature sensors fall under theclass of contact-type sensors. They can be utilized in thedetection of liquids, solids, or gasses over a broad range oftemperatures.

Non-Contact Temperature Sensors

These types of temperature meters do not measure the temperatureof an object while in direct contact; rather, they measure thedegree of hotness or coldness from the radiation that is emittedby the heat source.

Types of Temperature Sensors

The contact and non-contact temperature sensors are dividedfurther into the following mentioned types of temperaturesensors.

The Resistance Temperature Detector (RTD)

This is known as the resistance thermometer and uses theresistance of the RTD element with temperature to measure thetemperature. Different types of materials can be used to makethe metal.

The materials include nickel, platinum, and copper. However,platinum is the most accurate and therefore the most expensive.

Thermocouple Sensor

Thermocouple sensors have two wires made of different metalsconnected at two points. The voltage between the two wiresreflects the change in temperature.

Although their accuracy may be slightly reduced to a degree thatis lower than an RTD, they have a temperature range between -328°F to 3182 °F (-200 °C to 1750 °C) and aregenerally more cost-effective.

Thermocouples are separated into types, with each type beingsuitable for specific temperature conditions. The variousclasses of thermocouples are constructed to meet the needs of aspecific application.

Uses for Thermocouple Types:

• E: suitable for vacuum, inert, mildly oxidizing, or reducingconditions
• J: used where there is limited oxygen
• K: requires metal or ceramic protection
• N: resists oxidation from sulfur
• T: used in oxidizing or reducing environments
• S, R, and B: must be protected with a form of tubing and usedfor high temperature applications
• C (tungsten/rhenium): very common; requires protectivesheathing and used for high temperature applications
• A: a variant of type C and has limited use

Type E Thermocouples - Type E thermocouples have chromel, anickel and chromium alloy, and constantan with a temperaturerange of -330o F to 1600oF (0oC to 870oC) and excellent EMFversus temperature values. They can be used in sub-zerotemperatures and are used in inert environments but must beprotected in sulfurous environments.

Type J Thermocouples - Type J thermocouples, like type Kthermocouples, are a general-purpose thermocouple made of ironand constantan, with the iron leg being positive and theconstantan leg being negative. They can be used exposed orunexposed with a protective tube being recommended. Type Jthermocouples are used in vacuum, inert, and reducingenvironments. As with type K thermocouples, type J thermocoupleshave to be carefully calibrated and do not react well to noise.

Type K Thermocouples - Type K thermocouples are made ofChromel®–Alumel® with small percentages of manganese andsilicone. They are a general purpose thermocouple with atemperature range of -328 °F up to 2462 °F (-200 °Cup to 1350 °C). Type K thermocouples need to be carefullycalibrated and have small output signals. They are used in anassortment of environments including water, mild chemicals,gasses, and dry conditions. Common industries that use type Kthermocouples are hospitals and food preparation. Regardless oftheir wide temperature range, type K thermocouples are mostlyused for temperatures over 1004 °F (540 °C).

Type N Thermocouples - Type N thermocouples have nicrosil, anickel chromium alloy, and nisil, a nickel, silicon, andmagnesium alloy. Their temperature range is from 32°F to2300°F (650°C to 1260°C). Type N thermocouples areresistant to green rot and hysteresis and are used in refineriesand the petrochemical industry.

Type R Thermocouples - Type R thermocouples are made of platinumand rhodium and are usable for temperatures up to 2700 °F(1480 °C). They have to be protected by a gas-tight ceramictube and a secondary outer tube. Type R thermocouples haveimproved stability, an increased temperature range over Type Sthermocouples, and are often used in place of Type Sthermocouples. Applications for Type R are heat treating,control sensors, semiconductor industry, glass manufacturing,and ferrous and non-ferrous metals.

Type S Thermocouple - Type S thermocouples are used in hightemperature applications in the BioTech and Pharmaceuticalindustries. They are also used for low temperature applicationsdue to their accuracy and stability. Type S thermocouples have atemperature range of -58°F to 2700°F (980°C to1450°C).

Type T Thermocouples - Type T thermocouples are made of copperand constantan with a temperature range of -330°F to700°F (-200°C to 370°C). They are used in inertatmospheres and are resistant to decomposition even if there ismoisture present. Type T thermocouples are used in foodproduction and cryogenics.

Type B Thermocouple - Type B thermocouples are used for hightemperature applications and have the highest temperature limitof the thermocouples with exceptional accuracy and stability.Their alloy combinations are platinum (6% Rhodium) and platinum(30% rhodium) with a temperature range of 2500°F to3100°F (1370°C to 1700°C).

Type C Thermocouples - Type C thermocouples have tungsten andrhenium legs and are used with applications that requiretemperatures up to 4200°F (2315°C). They are used inhydrogen, inert, or vacuum atmospheres to prevent failure fromoxidation. Type C thermocouples have protective sheaths made ofmolybdenum, tantalum, and inconel with insulators of alumina,hafina, and magnesium oxide.

Thermistor Sensor

This type of temperature sensor displays a change that isprecise, predictable, and large in the alteration of differenttemperatures. With a change this large, it means that thereflection of the temperatures occurs rapidly and accurately.

Thermometer Temperature Sensor

This type of temperature sensor is the standard temperaturesensor, particularly the mercury-filled glass tube. However,thermometers exist in several types. Glass thermometers cancontain either ethanol or mercury, although nowadays the mainliquid used in these thermometers is ethanol.

Bi-Metal Thermometer

This type of thermometer consists of a connected gauge and stem.There is a spring attached to a rod on the tip of the sensor,leading up to the gauge’s needle. Inside the stems, the springsits sensing the end.

When heat is applied on the sensing coil, there is a creation ofmovement in the coil, which causes the movement of the needle inthe gauge – thus the temperature is displayed.

Gas-Filled and Liquid Thermometer

These types of thermometers work the same. Both have a bulbfilled with either gas or liquid located inside the sensing endof the probe. When heat is applied, the gas expands or theliquid heats up, which signals the rod attached to move theneedle to the measured temperature.

Digital Thermometer

This type of thermometer utilizes a probe such as a thermocoupleor an RTD. The temperature is measured by the probe (sensingend) and displayed as a digital reading.

Infrared Sensors

These types of temperature sensors detect temperatures from adistance by measuring the amount of thermal radiation that isbeing emitted by a heat source or object.

These temperature sensors find their application in hightemperatures or hazardous environments, where a safe distancemust be maintained away from a particular body.

Thermal Imaging

These temperature sensors are the most common type ofnon-contact temperature sensors. They are used in the followingcircumstances: when the target object is in motion (like withinmoving machinery on a conveyor belt); if the object is far away;if the surrounding environment is dangerous; or in extremetemperatures where a contact sensor is not suitable.

Negative Temperature Coefficient (NTC) Thermistor

A thermistor is a temperature sensor that reacts to minutetemperature changes. At very low temperatures, it provides highresistance. As the temperature increases, the resistance quicklydrops. The glass coated NTC thermistor offers the highestaccuracy. The smallest change in resistance per degrees ofCelsius is immediately displayed.

Negative temperature coefficient thermistors requirelinearization due to their use of the exponential workingprinciple, where the temperature range is between -58°F to482°F (-50°C to 250°C). NTC thermistors also requirelinearization because of the size and speed involved.

The use of a NTC thermistor in a detection circuit requires theuse of a small voltage to be passed across the thermistor. Thetemperature will be reflected by the thermistor’s resistance,which rapidly drops as the temperature increases.

For applications in the range of -40°F to 257°F(-40°C to 125°C), NTC thermistors provide an option thatis less expensive than platinum RTDs, thermocouple sensors, andsemiconductor based sensors. NTC’s fast changing resistance basemakes them capable of providing superior accuracy, powerefficiency, stability, reliability and responsiveness. They caneasily be integrated into any system.

Semiconductor-Based Sensors

Semiconductor-based temperature sensors work with dualintegrated circuits. They consist of two diodes that aresimilar, with voltage and temperature-sensitive currentcharacteristics for measuring the changes in temperatureeffectively.

Semiconductor-based sensors give a linear output; however, theyare less accurate at 1 to 5°C. These types of temperaturesensors have the slowest responsiveness (5 to 6 seconds) acrossthe narrowest temperature range (-94°F to 302°F or-70°C to 150°C).

Vibrating Wire Temperature Sensor

This type of temperature sensor is used to measure the internaltemperature in water or concrete structures. It exhibits aresolution of better than 0.1°C and its working is similarto that of a thermocouple temperature sensor.

Its temperature range is also high, ranging from -4°F to176°F (-20°C to 80°C).

Chapter 3: Applications and Benefits of Temperature Sensors

This chapter will discuss the applications and benefits oftemperature sensors. It will also discuss temperature controlusing temperature sensors.

Applications of Temperature Sensors

The function of temperature sensors is to measure temperature inmany various applications and industries. Temperature sensorsare all around us, present in both industrial settings andeveryday life. The following are examples of the applications oftemperature sensors:

Industrial Applications

Temperature sensors are utilized to monitor various environmentsand machinery, power plants, and manufacturing. Temperaturesensors are used to measure water temperatures in reservoirs andboreholes. They can also be used to interprettemperature-related stress and changes in volume in dams.Temperature sensors are also utilized in the study of thetemperature effect on other installed instruments.

Scientific and Laboratory applications

Temperature sensors are utilized in science and biotechmonitoring.

Medical Applications

Temperature sensors are utilized in the monitoring of patients,in medical devices, in thermodilution, in humidifiers, gasanalysis, cardiac catheters, ventilator flow tubes, and dialysisfluid temperature.

Uses in Motorsports

Temperature sensors are used for measuring inlet airtemperature, exhaust gas, engine temperature, and oiltemperature.

Domestic Appliances

Temperature sensors are used in kitchen appliances (ovens,kettles, etc.) and also in white goods.

HVAC Applications

Temperature sensors are utilized in air conditioning devices andheating ventilation devices, either domesticated or commercial.

Sensors in Transit

Temperature sensors are used in refrigerated vans and trucks.

Benefits of Temperature Sensors

The benefits of temperature sensors include:

• Temperature sensors are precise, extremely reliable, and havea low cost.
• Temperature sensors are suitable for both embedded and surfaceapplications.
• They provide low thermal mass resulting in a fast responsetime.
• The vibrating wire temperature sensor is completelyinterchangeable; all sensors can be read by one indicator.
• Temperature sensors are available with indicators for directdisplay of temperature.
• Temperature probes exhibit excellent hysteresis and linearity.
• The technology of the vibrating wire ensures long termstability, easy and quick readout.
• Temperature sensors perfectly suit remote scanning, reading,and data logging.

How to Choose the Right Temperature Sensor

There are some factors that must be considered when selecting atemperature sensor for a certain application. The following arefactors that must be considered when opting for a temperaturesensor:

Temperature Range

Different types of temperature sensors are capable of measuringdifferent ranges and might be more accurate within a certainrange. It is important to make sure that the range of thetemperature sensor is checked, and also the range of yourapplication that is expected before purchasing. The temperaturerange of the temperature sensor is provided on the datasheet.

Accuracy and Stability

An application may require a certain degree of accuracy; beaware that the variance of thermocouples in long term stabilityis higher than that of thermistors and RTDs.

Size and Package

The type of temperature sensor selected is determined by thespace available within the application. If there is limitedspace, a smaller device is required. How the temperature sensorwill be connected to the application and how the temperature isgoing to be measured is determined by package style; therefore,package style is an important consideration.

Environmental Conditions

A critical factor in the placement and choice of a temperaturesensor is the conditions where it will be used. Humidity,vibrations, and other environmental factors radically affect thestability and accuracy of a temperature sensor. In addition,electrical noise affects its readings especially if thetemperature changes are minute and small. Attention to the noiselevel where a temperature sensor is used ensures accuratemeasurements.

The conditions and environment where a temperature sensor is tobe placed has to be closely examined to ensure optimumperformance. Aside from the obvious structure of the temperaturesensor, any connections, such as lead wires, have to beprotected from hostile and unstable environments.

Various measures can be taken to protect a temperature sensorand its wiring from the conditions of the environment such asprotective metal sheaths that are resistant to wear andcorrosion. The use of sheathing increases the cost of atemperature sensor and affects its sensitivity. Additionally,certain sensor configurations have mounting considerations toensure a solid thermal connection.

Effective Temperature Control Using Temperature Sensors

Temperature sensors are utilized in the effective control oftemperature. Five keys to effective temperature control arediscussed below:

Select the Right Sensor

The last decision made in the system design is often theselection of a temperature sensor. Typically, the decision isdriven by the compatibility of the process controller of thesystem and often comes down to availability and cost.Unfortunately, the critical selection criteria that includesensor mass, operating temperature, signal strength, readingsensitivity, and operating ambient are overlooked frequently.

There are three types of sensors used in commercial temperaturecontrol: thermistors, thermocouples, and resistive thermaldevices (RTDs). Each type of temperature sensor exhibits its owncharacteristics and various styles within each type are designedfor specific temperature ranges, output signals and processcompatibility based on the construction materials.

The sensor must be selected in such a way that it matchesspecific design application parameters. An importantcharacteristic is the sensor mass, and it is important toconsider it when making a final selection. Heavy sensor bodiesoffer a slower, dampened response to process changes, whereas alight sensor mass offers a quicker response to processtemperature changes. Typically, if the temperature control ismore exacting, the sensor will be lighter and will respond morequickly.

Install the Sensor in the Correct Location

The location of the temperature sensor of the system is crucial,even though it might be obvious. In some situations, fieldinstallation of a temperature sensor is affected by physicalobstacles that are responsible for the initial design locationto be modified. When the placement of the temperature sensor inthe correct location is difficult, there must be a considerationof compromises and alternatives.

However, the sensor’s position is the key to the success ofevery other part of the system. There must never be a compromiseof the system efficiency for ease of sensor installation. Listedbelow are a few tips to avoid common pitfalls:

• The sensing location must be made sure that it is at the entrypoint of the position of the critical temperature.
• The mixing of the process fluid must be ensured, and thereshould be no cold or hot currents that could fool the sensor.
• It should be ensured that the location of the sensor is notnegatively affected by fluid velocity changes.
• It should be ensured that no heating or cooling sources willaffect the process fluid. For instance, an uninsulated pipingsection after the sensor but prior to the critical point ofthe temperature of the process can affect the temperaturedelivered to the critical point.

Balance the Cooling and Heating Capacity

The goal of any temperature control system is to balance theheating and cooling capabilities of the system. If the energybalance is mismanaged, this may result in temperatureinstability. There are two basic types of controls, which areproportional output control and on/off output control.

The easiest type of control to understand is the on/off control.Its decision making on temperature deviation is based on thetemperature setpoint of the system. At a temperature that ispredetermined, either below or above the setpoint, there is anactivation of the full capacity of the heating or cooling systemby the system. The only way to stop further cooling or heatingis by turning off the conditioning equipment. If the massprocess is larger than the heating or cooling capacity and thetemperature fluctuation that is allowed is wide, on/off controlcan be an ideal cost-effective control method.

However, there are some practical limitations with it. Becausethe capabilities of a system are often significantly higher thanthe load in different situations, only turning the cooling orheating equipment on and off is generally impractical.

One approach that can be used to enhance the capacity control ofon/off control systems is the addition of different levels orstages of capacity to the cooling or heating system.

Keep Process Variables Consistent

The process control can be more manageable by keeping all of theequipment subsystem functions predictable. With the advent ofenergy and management improvements, some energy-saving schemescan have a destabilizing effect on the temperature control ofthe process. As loads change, many new systems willautomatically compensate several variables in response to thischange.

The most commonly adjusted variables are supply fluidtemperature, process fluid volume, and heating or coolingcapacities. The adjustment of several subsystems such as supplytemperatures, process pressures, and fluid flow will change thesystem’s characteristics, and these changes are capable ofcreating temperature instability and cause problems in control.For instance, consider a plant chiller system in which the rateof flow of the fluid of the process is changed.

The reduced rate of flow can change heat-transfercharacteristics exponentially in some components of the systemand heat exchangers. The loss of heat transfer may result intemperature instability at the critical point and can cause themalfunctioning of the compressor or other components.

Use the Correct Proportional Controller

Many of the proportional controllers fall into three designactions or types: (PID) proportional plus integral andderivative, (PI) proportional plus integral, and (P)proportional only. Engineers have been provided with moreflexibility in their designs with technical advances in smallprocess controllers.

The improvement of controllers has increased to such a degreethat they are capable of often compensating for system designflaws and unruly characteristics. The simplest controllers areproportional controllers, and they are the basis for most smallelectronic devices. A proportional controller is able to workoff a deviation of temperature and applies an output signal onthe basis of a linear calculation in which a change in a givendeviation results in a linear change in output.

Conclusion

A temperature sensor can also be defined as a simple instrumentthat measures the degree of coldness or hotness and thenconverts it into a readable unit. There are different types oftemperature sensors, including thermocouples, thermistors, RTDs,etc. Each temperature sensor has its own unique characteristics,which makes it suitable in specific types of applications.However it is important to consider the temperature range,accuracy, size, and stability of a temperature sensor foroptimal performance within a specific application.