Temperature Sensors: Types, How It Works, & Applications

July 10, 2019

We all use temperature sensors in our daily lives, be it in the form of thermometers, domestic water heaters, microwaves, or refrigerators. Usually, temperature sensors have a wide range of applications, geotechnical monitoring field, being one of them.

Temperature sensors are a simple instrument that measures the degree of hotness or coolness and converts it into a readable unit. But, have you ever wondered how the temperature of the soil, boreholes, huge concrete dams or buildings is measured? Well, this is accomplished through some of the specialised temperature sensors.

Temperature sensors are designed to keep a regular check on concrete structures, bridges, railway tracks, soil, etc.

Here we are going to tell you what is a temperature sensor, how does it work, where is it used, and what are its different types.

What are the temperature sensors?

A temperature sensor is a device, typically, a thermocouple or resistance temperature detector, that provides temperature measurement in a readable form through an electrical signal.

A thermometer is the most basic form of a temperature meter that is used to measure the degree of hotness and coolness.

Temperature meters are used in the geotechnical field to monitor concrete, structures, soil, water, bridges etc. for structural changes in them due to seasonal variations.

A thermocouple (T/C) is made from two dissimilar metals that generate an electrical voltage in direct proportion with the change in temperature. An RTD (Resistance Temperature Detector) is a variable resistor that changes its electrical resistance in direct proportion with the change in the temperature in a precise, repeatable and nearly linear manner.

What do temperature sensors do?

A temperature sensor is a device that is designed to measure the degree of hotness or coolness in an object. The working of a temperature meter depends upon the voltage across the diode. The temperature change is directly proportional to the diode’s resistance. The cooler the temperature, lesser will be the resistance, and vice-versa.

The resistance across the diode is measured and converted into readable units of temperature (Fahrenheit, Celsius, Centigrade, etc.) and, displayed in numeric form over readout units. In geotechnical monitoring field, these temperature sensors are used to measure the internal temperature of structures like bridges, dams, buildings, power plants, etc.

What is a temperature sensor used for? | What are the functions of a temperature sensor?

Well, there are many types of temperature sensors, but, the most common way to categorise them is based upon the mode of connection which includes, contact and non-contact temperature sensors.

Contact sensors include thermocouples and thermistors because they are in direct contact with the object they are to measure. Whereas, the non-contact temperature sensors measure the thermal radiation released by the heat source. Such temperature meters are often used in hazardous environments like nuclear power plants or thermal power plants.

In geotechnical monitoring, temperature sensors measure the heat of hydration in mass concrete structures. They can also be used to monitor the migration of groundwater or seepage. One of the most common areas where they are used is while curing the concrete because it has to be relatively warm in order to set and cure properly. The seasonal variations cause structural expansion or contraction thereby, changing its overall volume.

How does temperature sensor work?

The basic principle of working of the temperature sensors is the voltage across the diode terminals. If the voltage increases, the temperature also rises, followed by a voltage drop between the transistor terminals of base and emitter in a diode.

Besides this, Encardio-Rite has a vibrating wire temperature sensor that works on the principle of stress change due to temperature change.

The vibrating wire temperature meter is designed on the principle that dissimilar metals have a different linear coefficient of expansion with temperature variation.

It primarily consists of a magnetic, high tensile strength stretched wire, the two ends of which are fixed to any dissimilar metal in a manner that any change in temperature directly affects the tension in the wire and, thus, its natural frequency of vibration.

The dissimilar metal, in the case of the Encardio-Rite temperature meter, is aluminium (Aluminum has a larger coefficient of thermal expansion than steel.) As the temperature signal is converted into frequency, the same read-out unit which is used for other vibrating wire sensors can also be used for monitoring temperature also.

The change in temperature is sensed by the specially built Encardio-rite vibrating wire sensor and is converted to an electrical signal which is transmitted as a frequency to the read-out unit.

The frequency, which is proportional to the temperature and in turn to the tension ‘σ’ in the wire, can be determined as follows:

f = 1/2 [σg/ρ] / 2l Hz

Where:

σ = tension of the wire

g = acceleration due to gravity

ρ = density of the wire

l = length of wire

What Are The Different Temperature Sensors?

Temperature sensors are available of various types, shapes, and sizes. The two main types of temperature sensors are:

Contact Type Temperature Sensors: There are a few temperature meters that measure the degree of hotness or coolness in an object by being in direct contact with it. Such temperature sensors fall under the category contact-type. They can be used to detect solids, liquids or gases over a wide range of temperatures.

Non-Contact Type Temperature Sensors: These types of temperature meters are not in direct contact of the object rather, they measure the degree of hotness or coolness through the radiation emitted by the heat source.

The contact and non-contact temperature sensors are further divided into:

Thermostats

A thermostat is a contact type temperature sensor consisting of a bi-metallic strip made up of two dissimilar metals such as aluminium, copper, nickel, or tungsten.

The difference in the coefficient of linear expansion of both the metals causes them to produce a mechanical bending movement when it’s subjected to heat.

Thermistors

Thermistors or thermally sensitive resistors are the ones that change their physical appearance when subjected to change in the temperature. The thermistors are made up of ceramic material such as oxides of nickel, manganese or cobalt coated in glass which allows them to deform easily.

Most of the thermistors have a negative temperature coefficient (NTC) which means their resistance decreases with an increase in the temperature. But, there are a few thermistors that have a positive temperature coefficient (PTC) and, their resistance increases with a rise in the temperature.

Resistive Temperature Detectors (RTD)

RTDs are precise temperature sensors that are made up of high-purity conducting metals such as platinum, copper or nickel wound into a coil. The electrical resistance of an RTD changes similar to that of a thermistor.

Thermocouples

One of the most common temperature sensors includes thermocouples because of their wide temperature operating range, reliability, accuracy, simplicity, and sensitivity.

A thermocouple usually consists of two junctions of dissimilar metals, such as copper and constantan that are welded or crimped together. One of these junctions, known as the Cold junction, is kept at a specific temperature while the other one is the measuring junction, known as the Hot junction.

On being subjected to temperature, a voltage drop is developed across the junction.

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