Temperature Sensors

Temperature is one of the most commonly used physical parameters. Temperature measurement always doesn’t need to involve complex constructions and measuring mechanisms. It also doesn’t need to be something that is done in a power plant or an industry. Temperature is a parameter that is used in homes, hospitals, hotels etc. The below article explains the importance of temperature and its measurement using various temperature sensors.

Contents:

1. What is a Temperature?
2. Why Temperature is to be measured?
3. Classification of Temperature Sensors
4. Thermocouples and their Types
5. Thermistors and their types
6. Resistance Temperature Detectors (RTD)
7. Applications of Temperature Sensors
8. Temperature Sensors used in Large Scale Applications

What is a Temperature?

Temperature is a physical parameter that is used to measure the amount of heat. An object is said to be hot or cold according to the temperature of that particular object. If the object’s temperature is high, it is considered a hot object and vice versa. Other important characteristics of temperature are listed below.

• Temperature is the measurement of the amount of heat energy in a particular object.
• Temperature is a common parameter to almost every object in the world irrespective of its material and size.
• The temperature of the human body, the temperature of a liquid, the temperature of place, etc. can be measured.
• Several scales are used to measure the temperature of an object.
• Some of the most commonly used scales are Celsius, Fahrenheit, and Kelvin.
• Temperature is measured in terms of degrees.
• Normally, temperature flows from the point with higher temperature to the point with lower temperature.

Why Temperature is to be measured?

Sometimes it becomes mandatory to measure the temperature of the object or a place as it may lead to failure of the process if the temperature is left unmeasured. The main reasons for the temperature to be measured and monitored are as follows:

• As stated before, temperature is a parameter that is used in almost every field of technology.
• Medically, temperature measurement is an important screening process to check whether the patient’s body temperature is normal or not.
• When the patient is under treatment, it becomes necessary for the body to maintain the correct temperature to work properly.
• High body temperature is described as fever and it may interfere with the other treatments that the patient is undergoing.
• In food industries, the temperature of the product is measured throughout the process to ensure the correct cooking and preserving of that product.
• Even in homes and offices, gadgets that involving heat and fire comes with a regulator which is used to control the amount of heat.

Classification of Temperature Sensors

The temperature is measured using a device called temperature sensors. Various temperature sensors are used to measure the temperature of various objects. Though all of the sensors work under the same principle. When the temperature of a metal changes, the resistance across the metal or the amount of voltage across the metal changes. By analyzing the resistance or voltage change, the temperature of the object is calculated. The main types of temperature sensors are shown in the figure below.

• As shown in the above figure, a thermocouple is a temperature measurement device that uses two different metals that are connected to a junction.
• The RTD or the Resistance Temperature Detector consists of a platinum rod and is used to measure temperature with high accuracy.
• A thermistor is a resistance type thermometer that uses resistance change to calculate the temperature of an object.
• The semiconductor-based temperature sensors or the integrated circuit are used in electronics to monitor the temperature.

Thermocouples and their Types

Thermocouples are temperature measurement devices that are used in simple circuits that are used in places with less harm. For instance, temperature monitoring in homes and offices. The working, construction, and types of a thermocouple is explained below.

• The working of the thermocouple is based on a phenomenon called the Seebeck effect.
• According to the effect, when the closed circuit is formed by joining two dissimilar metals at two junctions, and the junctions are maintained at different temperatures, then an electromotive force (EMF) is induced in the closed circuit.
• The construction of the thermocouple is pretty much similar to what is stated in the Seebeck effect.
• The thermocouple is classified into several different types based on the range of temperature they can measure.
• Some of the most commonly used thermocouples are Type E, Type J, Type K, Type N etc.
• A type of thermocouple named W-5 percent Re is specifically designed in such a way that it is used to measure temperature at vacuum.

Thermistors and their types

A thermistor is a device that uses resistance change to measure the temperature of an object. Thermistors are versatile instruments that can be used in various applications ranging from small fire alarms to complex applications like microwave ovens and refrigerators. Other such facts about thermistors are listed below.

• A thermistor is also known as a polymetric thermistor and resettable fuse as it can do the same work as a fuse when connected to a circuit.
• They can also be used to regulate temperatures along with measuring and monitoring temperature.
• They are the most sensitive sensors among all the other temperature sensors.
• Thermistors are mainly classified into types based on their temperature coefficient – Positive Temperature Coefficient thermistors (PTC) and the Negative Temperature Coefficient (PTC).
• The PTC thermistors measure temperature in a way that the resistance of the thermistor increases when the temperature is increased.
• The NTC thermistors measure temperature in a way that the resistance of the thermistor decreases when the temperature is increased.

Resistance Temperature Detectors (RTD)

The Resistance Temperature Detectors or the RTD is again a temperature sensor that uses resistance for temperature measurement. The RTD is one of the most expensive temperature measurement devices as it provides the maximum accuracy and precision. The working and the construction of the RTD is as follows.

• The platinum resistance temperature detector or the PT100 consists of a pure platinum resistance element that is placed inside an outer casing.
• The RTD consists of three wires which is used to measures the resistance of the lead wire and to measure the resistance of the object and to finally subtract he values to find the resistance change.
• The three-wire configuration which is connected to the platinum piece terminates at the cold end of the RTD.
• The working principle of the RTD is same as the working principle of the thermistor i.e., when the temperature of the metal increases, the resistance of the metal will also be increased.
• But when a pure metal is used in an RTD, the accuracy of the temperature measured becomes very high as the resistance change will be so accurate in pure metals.

Applications of Temperature Sensors

Temperature sensors are one of the mandatory electronic instruments in daily life. Some of the main applications of temperature sensors in our day-to-day life are listed below.

• Medically, temperature sensors like thermometers are used to measure the temperature of the patient.
• Other temperature sensors like thermistors are used to constantly monitor the temperature of the human body.
• Several scanning processes like magnetic resonance imaging require a steady temperature throughout the process.
• Hence, temperature sensors like pyrometers and thermistors are used for this purpose.
• Temperature sensors are also used for thermal imaging i.e., to monitor a moving object by detecting the temperature change.

Temperature Sensors used in Large Scale Applications

Temperature sensors are not only used in day-to-day applications as stated above. They are also used in complex large-scale applications. The below are few examples of uses of temperature sensors is large scale applications.

• In industries temperature sensors are used to monitor the temperature of constantly moving objects like gears and motors to prevent unwanted friction and ignition.
• In the automobile industry, temperature sensors play a major role in the radiator part of the vehicle. It becomes necessary to maintain a cooler temperature in the engine for better performance.
• Temperature sensors are also used in large scale heating, ventilation and air conditioning process which is also known as HVAC systems.
• In the food industries, temperature measurement is one of the key processes through the process of conversion of a raw material to a finished product.