Introduction

A temperature sensor is a device that gathers data concerning the temperature from a source and converts it to a form that can be understood either by an observer or anther device. It measures the temperature of a medium. The sensors are used for different purposes from the simple home use to extremely accurate and precise scientific use.

The best well known example is mercury-in-glass thermometer. The mercury in the thermometer expands and contracts on the change in temperature. As the volume change is quantified the temperature is measured with a fair degree of accuracy. These temperature sensor are used from home to the university labs for the temperature measurement.

The complex temperatur sensors are computerised  for gettig more accurate results . These devices have typically two forms> contact sensors and non contact sensors.

Even human and most animals  have biological temperature sensors in their body which works as the same function as the artificial does. It collects data and transfers it into a understandable form. For eg, human sensory nerves sends sensory information from the skin to the brain in the form of electrical impulses. 

Types

Types: Temperature Sensors measure the amount of heat that produces from the object. It detects the changes of the temperature either as output of analog or digital. The temperature consists of two basic types of features:

• Contact Temperature Sensor Types – These types of sensors are needed to be in contact with the object or system that is to be measured. They can measure solid, liquid or gas in a wide range.
• Non-contact Temperature Sensor Types – These types of temperature sensor use convection and radiation to measure temperature. They can measure liquid or gaseous state of object that emits radiant energy in the form of infra-red radiation.

These two basic types of sensors can be sub-divided into the following three groups of sensors, Electro-mechanical, Resistive and Electronic. All these are discussed below:

 1. Thermostat

This is a type electro-mechanical temperature sensor, which is consists of two different types of metal; for example nickel, copper, tungsten or aluminum. These are bonded together to make a bi-metallic strip. When this is connected with heat, the different linear expansion of two different metals produces a mechanical bending.

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The thermostat consists of two thermally different metals. When this is connected with cold object, the two metals are closed together and current passes through the thermostat. When it gets hot, one metal expands more than the other and the bonded bi-metallic strip bends up (or down) opening the contacts preventing the current from flowing.

There are two main types of bi-metallic strips, which change their expansion according to temperature changes of the object. These are “snap-action” and “creep-action”. The “snap-action” types give instantaneous action “ON/OFF” or “OFF/ON” on the electrical contacts at a set temperature point, and the slower “creep-action” types that gradually change their position as the temperature changes.

Snap-action type thermostats are commonly used in our homes for controlling the temperature set point of ovens, irons, immersion hot water tanks and they can be found on walls to control the domestic heating system. But it has large hysteresis range on the opening and closing of electrical contracts. That’s why commercially available bi-metallic thermostats have temperature adjustment screws, by which it is possible to adjust the temperature at desired level and pre-set hysteresis level.

 2. Thermocouple

Thermocouples are the most common temperature measuring device. It consists of two dissimilar metals, joined together at one end.

Working principle

  Thermocouple works on the basis of Seebeck effect .If two conductors of different materials are joined at one point, an EMF is created between the open end which is dependent upon the temperature of the junction. As T1 increases, so does V. The EMF also depends on the temperature of the open ends T2. The junction is placed in the process, the other end is in iced water at 0C. This is called the reference junction. The voltage is directly proportional to temperature.

Types of thermocouple                       

 http://www.thomasnet.com/articles/automation-electronics/Thermocouples-Types

Advantage

• It is simple and rugged in construction
• It can measure wide range of temperatures up to 2600°C
• Fast Response
• Inexpensive
• Calibration can be checked easily
  
  

Limitations

• It is least stable and least repeatable.
• It requires cold junction compensation for accurate temperature measurement.
• The emf induced verses temperature characteristics is non linear.
• Lowest accuracy.

Application

• Thermocouple is extensively used in steel processing, turbine and diesel engine for temperature measurement
• It is used in gas feed heat appliances such as ovens and water heaters.
• It is used in power production.
• A thermocouple can be used as a vacuum gauge over the range of approximately 0.001 to 1 torr absolute pressure
• Thermoelectric cooling
• Medical equipment and Packaging equipment 

 3. Thermistor

A thermistor is type of resistor whose resistance varies significantly with the temperature. They are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors and self-regulating protectors.

It is cheap and easily abundant temperature sensors which is easy to use and adaptable also. Circuits with thermistor can have reasonable output voltages- not the millivolt output as thermocouples have. Because of these qualities they are widely used for simple temperature measurements but not for the high one. But in the temperature ranges where they work are widely used.

How Does A Thermistor's Resistance Depend Upon Temperature?

Steinhart-Hart equation gives the reciprocal of absolute temperature as a function of the resistance of a thermistor.

Using the Steinhardt-Hart equation, you can calculate the temperature of the thermistor from the measured resistance.

The Steinhardt-Hart equation is:

1/T = A + B*ln(R) + C*(ln(R))³ R in W, T in ^oK

The constants, A, B and C can be determined from experimental measurements of resistance, or they can be calculated from tabular data.

How Do You Use A Thermistor?

In this bridge circuit, three resistors are constant, R_a, R_b, and R_c, while the resistive sensor, R_s, varies depending upon some physical variable - like temperature, light level, etc.  That's where the thermistor can be used.

        The thermistor can be placed anywhere in the bridge with three constant resistors, but different placements can produce different behaviour in the bridge.  For example, different placements might cause the output voltage to go in different directions as the temperature changes.

References

 The Michigan Chemical Process Dynamics&Control, 2006, https://controls.engin.umich.edu/wiki/index.php/TemperatureSensors

 Thomasnet, Types of Thermocouples, 2014, http://www.thomasnet.com/articles/automation-electronics/Thermocouples-Types

  Wayne Storr, Basic Electronics Tutorials, http://www.electronics-tutorials.ws/io/io_3.html

  Web material, Temperature Sensor- The Thermistor, http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Sensors/TempR.html