TEMPERATURE SENSOR
In previous days the thermometer was only used for measuring human body temperature and atmospheric temperature. Now a days due to drastic industrial developments, it is essential to measure temperature of various situations and conditions much accurately and precisely.
Various Types Liquid and gas thermometer Bimetallic strip thermometer Resistance thermometers, which include RTD and Thermistor Thermocouple Junction semiconductor sensor Radiation Pyrometer
Liquid and Gas Thermometer Charles Law of Gas is applied, this law states that volume of gas is directly proportional to Kelvin temperature of the gas when pressure is kept constant / When volume is kept constant.
Bimetallic Strip Thermometer
RTD The electrical resistance of an electrical conductor is a measure of the difficulty to pass an electric current through that conductor. The resistance of a conductor changes when its temperature is changed. This property is utilized for measurement of temperature.
Resistance Thermometers R = Ro(1+α1T+α2T2+….+ αnTn+…)
Circuit Platinum is especially suited for this purpose. Gold & Silver are rarely used for construction of RTD Most common TRD Platinum, nickel or nickel alloy, copper. 100Ω at 0°C with a resistance temperature coefficient 0.00385/ 0°C
Characteristics of RTD Metal RTD Coefficient Temperature Range Melting Point Min Max Platinum 0.39 -260 110 1773 Copper 180 1083 Nickel 0.62 -220 300 1435 Tungsten 0.45 -200 1000 3370
Thermistors Two or more semiconductor powders are mixed with a binder to form a slurry. After getting the slurry from the previous process, small drops of that slurry is formed over the lead wires. For drying purpose, we have to put it into a sintering furnace. During this process, that slurry (metallic oxides) will shrink onto the lead wires and it helps to make an electrical connection.
Thermistors Application Measurement T Control T Compensation T Power High F Thermal conductivity Level, Flow, pressure Composition of gases Vacuum Sintered mixture of metallic oxides such as manganese, nickel, cobalt, copper, iron and uranium. Construction type Bead Probe Disc Rod
Thermocouple Whenever two different types of metals are connected together, a thermoelectric potential is generated across the two free ends the metals according to the temperature of the joint. This is known as the thermoelectric effect.
Thermocouple Materials Copper and iron, Base-metal alloys of Alumel (Ni/Mn/Al/Si), Chromel (Ni/Cr), Constantan (Cu/Ni), Nicrosil (Ni/Cr/Si) Nisil (Ni/Si/Mn), noble metals platinum and tungsten, noble-metal alloys of platinum/rhodium and tungsten/rhenium.
Laws of Thermocouple The first law, homogeneous circuit, states that in a circuit composed of a single homogeneous metal, cannot generate an electric potential by the application of even sufficient temperature difference between two different points of the circuit.
Second Law Thermocouple
Third Law of Thermocouple
Junction Semiconductor A p-n junction diode and bipolar junction transistor have very strong dependency on temperature. Diode or a BJT is connected a constant current source and if the forward biased voltage of these devices are measured, it would be observed that due to change in temperature there would be very much change in forward biased voltage. Hence this forward biased voltage can easily be calibrated in temperature such as the value of this voltage gives direct temperature measurement. these type of temperature sensors are very sensitive and mainly used in electronics purposes.
Radiation Pyrometer Very modern type of temperature sensor. Direct contacting the object whose temperature to be measured with the thermometer. Very hot furnace where it is not possible place a conventional thermometer. In that case radiation pyrometer is used to measure its temperature from far away only by sensing the electromagnetic radiation comes out from the object. This technology of temperature measurement can also be used in high voltage system. If temperature of any point in the on load power network to be measured from a safe distance, this technology is quite usable. A pyrometer thermometer catches and senses the thermal electromagnetic radiation, emitted from an object and gives the temperature of the object as the instrument is calibrated so