Temperature sensors


As we discuss earlier about the sensor and according to that knowledge we know that sensor is a device which can check the change in environmental conditions.
  the temperature sensor is a device or thermocouple  which can measure the temperature of  required system in some different measurable quantity(voltage) .these measurable quantity are some how related with the temperature.the simple example is mercury in glass thermometer.


1.contact type 
2.non contact type

1.contact type  
in this type the temperature sensors are use to detect the temperature of the solid,liquid and  gases . physical touch is required to measre the temperature of the system.mainly conduction process is take place in these types of temperature sensors.example : bi-metallic temperature sensor.
Here  will discuss about the bi-metallic temperature sensor

Bi-Metallic Temperature Sensor
                                                              Bi-metallic temperature basically consist of  two metallic strips.these strips have different -2 thermal expansion coefficient . copper , tungsten ,nickel ,aluminium are the most popular metals that are used in bi– metallic temperature sensors. two metallic strips  are bonded together to form a bi-metallic strip.due to the different-2 thermal expansion  coefficient bending stresses are produced in the bi-metallic metal.

 Working process
                                   we can easily understand the process of  the bi-metallic temperature sensor with the help of  diagram which is given above.Here a elctro mechanical switch is shown.in starting condition when the temperature of the bi-metallic strip is low in this condition it is physically touches the contact point which is responsible for  closing the electrical power supply and current is flow through the bi-metallic strip to the destination of the electrical supply.but when strip become hot in this condition thermal expansion will take place and due to the bending stresses bi-metallic strip become bend and it looses the physical touch from the circuit and then current will not flow and circuit brake down take place.

 There are two type of bi-metallic strips depending upon the nature of the movement when subjected to high temperature.
1.snap action 
2.creep action

snap action 
                        when the bi-metallic strip is subjected to veriable temperature then bi-metallic strip response instantaneously  on or off .these are mostly used in the iron, microwave oven,and also in immersion hot water tanks.

 creep action
                      in this type the bi-metal is in coil or in spiral form.coil become up when the temperature of the  coil  changes.this is more sensitive than the on or off action. mainly used in temperature gauges and dial gauges. 

 Non contact type of temperature sensor
                                                                              these types of temperature sensors  basically  use radiation or convection to detect the temperature of the system. so  these are used to measure the temperature of the liqid,gas,and any system which can able to emits the radiation.

                         Thermistor is a special type of resistor.these are made up of ceramics material oxides of manganese ,nickel or cobalt coated with glass.As the temperature of these materials changes resistor also change depends upon the thermal expansion coefficient of these materials.these are constructed by ceramic type of semiconductor using the metal oxide of mn,co,and ni technology they are more reliable and sensitive than other one.           

there are two types of semiconductors are used
1.semiconductor which have negative thermal expansion coefficient
2.semiconductor which have positive thermal expansion coefficient

semiconductors which have negative thermal expansion coefficient 
 these types of semiconductors looses there resistance when their is increament in temperature.

semiconductors which have positive thermal expansion coefficient 
 As the  temperature is increase then theresistance resistance of semiconductor also increases.


RTD(resistance temperature detector
                                                                                   RTD‘s are also the sensors that are use to measure the temperature of system.they are made up of high purity conducting metals like platinum,copper,nickel.these metals wound into a coil.resistance may vary as the as function of temperature.they possesses positive thermal coefficient..they have poor thermal sensitivity.RTD consist of a length of fine colied wire wrapped around a ceramic or glass core.they are more brittle so easily brakes.due to this they are usually placed inside the a sheathed probe to prove it.Platinum  is mostly used in RTD because it measure the accurate temperature .so this is also known as platinum resistance temperature detector sensor.

Working of platinum  RTD
                              As the platinum RTD sensor placed in a   measurable condition or any system (electric circuit)then due to the temperature variation ,platinum become hot and the molecules  started to vibrate which  decreases the flow of free electrons.(these electron are responsible for  flowing of current.).so the voltage also drop in this condition.this voltage drop is proportional to the temperature of the system which can easily measurable.
                                                           you can easily understand the working process of the Platinum RTD sensor from this image diagram.

                          these are also used to measure the temperature .these are sensor or an electric device.mainly thermocouple consist of two different types of  metal wires.these wires are welded at one end with each other.this junction point where temperature is measure.According to seeback when two different metal wires joint with each other at one end and if there is temperature different between joint a magnetic field is produced which is also known as thermo-magnetic.


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