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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 another 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 temperature sensors are computerised computerized for gettig getting more accurate results . These devices have typically two forms> 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.
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- 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 The different types of sensors can be sub-divided into the following three groups of sensors, Electro-mechanical, Resistive and Electronic. All these are discussed below:the temperature sensors are described below. The working principle, advantages, disadvantages, limitations along their applications are described.
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.
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Figure: Working principle of bi-metallic Strip, Source:http://www.electronics-tutorials.ws/io/io_3.html [4]
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Figure: Expansivity values for some metals in units of K*-1 X 0*-5, Source: http://sensorsandtransducers.wordpress.com/2012/02/06/bimetallic-strip/[6]
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.
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Thermocouple works on the basis of Seebeck effect .If two conductors of different materials are joined at one point, an EMF (electromotive force) 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.
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Types of Thermocouple
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Source: http://www.thomasnet.com/articles/automation-electronics/Thermocouples-Types[3]
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Advantages
- 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
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A thermistor is type of resistor whose resistance varies significantly with the temperature. it is constructed of semi conductor material with a resistivity that is especially sensitive to temperature.
Working principle of
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Thermistor:
A thermistor is a type of resistor whose resistance varies significantly with temperature. Thermistors use internal electrodes that sense surrounding heat and measure it through electrical impulses. They also help to control heat to some extent, usually causing the device to which they are attached to heat up much more slowly than it normally would. They are made with semi-conductor materials using temperature-sensitive resistanceThe material used in thermistor is generally a ceramic or polymer. There are two kinds of thermistors: NTC (Negative Thermal Coefficient) and PTC (Positive Thermal Coefficient). With the former, the resistance of the thermistor decreases as the temperature increases, and with the latter, the resistance increases as the temperature rises.
The basic mathematical model used for thermistors is the Steinhart-Hart equation, discovered by oceanographers I.S. Steinhart and S.R. Hart. In its simplest form it is:
1/T = a + b(ln R) +c(ln R)3
where T is the = temperature, a,b , and c are = coefficients that are measured, ln is the natural log, and R is the R= resistance in ohms.Ohms.
Advantages:
1) They are cheap.
2) Stable
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RTD stands for Resistance Temperature Detector. RTDs are sometimes referred to generally as resistance thermometers. An RTD is a temperature sensor that measures temperature using the principle of the resistance of a metal changes with temperature.In practice a small electric current transmitted through a piece of a metal which are platinum ,copper or nickel(the rtd element or resisitor ) located in the area where temperature is to be measured.The resistance value of the RTD elements then measured by an instrument. This resistance value is then correlated to temperature.They are used in many industrial applications below 600 °C, due to higher accuracy and repeatability. Platinum is the most stable resistance-temperature relationship over the largest temperature range and best metal for RTDs because it follows a very linear resistance-temperature relationship and it follows the R vs T relationship in a highly repeatable manner over a wide temperature range and the .The unique properties of platinum make it the material of choice for temperature standards over the range of -272.5 °C to 961.78 °C, and is used in the sensors that define the International Temperature Standard. Platinum is chosen also because of its chemical inertness
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The relative change in resistance (temperature coefficient of resistance) varies only slightly over the useful range of the sensor. The R vs T relationship is defined as the amount of resistance change of the sensor per degree of temperature change. Then estimate of RTD sensitivity can be noted from typical values of the linear fractional change in resistance with temperature. For platinum, this number is typically on the order of 0.004/°C, and for nickel a typical value is 0.005/°C. Thus, with platinum, for example, a change of only 0.4 W would be expected for a 100-W RTD if the temperature is changed by 1°C. Usually, a specification will provide calibration information either as a graph of resistance versus temperature or as a table of values from which the sensitivity can be determined. For the same materials, however, this number is relatively constant because it is a function of resistivity.
References
Curtis [1]Curtis D. Johnson, Process Control Instrumentation Technology, Resistance Temperature Detectors, 2006. http://zone.ni.com/devzone/cda/ph/p/id/208#toc3
The [2]The Michigan Chemical Process Dynamics&Control, 2006. https://controls.engin.umich.edu/wiki/index.php/TemperatureSensors
[3]Thomasnet, Types of Thermocouples, 2014. http://www.thomasnet.com/articles/automation-electronics/Thermocouples-Types
[4]Wayne Storr, Basic Electronics Tutorials. http://www.electronics-tutorials.ws/io/io_3.html
[5] Web material, Sensors and Transducers. http://sensorsandtransducers.wordpress.com/2012/02/06/bimetallic-strip/
[6]Web material, Temperature Sensor- The Thermistor. http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Sensors/TempR.html