THERMISTOR
It is
passive transducer used for temperature measurement. The measuring of
temperature depends on resistance of thermistor. Thermistor have positive and
negative temperature coefficient but mostly negative temperature is preffered
because
Resistance α 1/temperature
* It temperature increases the
resistance decreases and vice versa.
* The change in temperature values
observed using wheatstone bridge.
* Thermistors are made up of Metallic
oxides like copper, nickel, cobalt, manganese & uranium.
* Thermistors are used for measuring
vaccum flow, pressure and liquid levels.
Construction:
The below figure shows construction and measurement of temperature using
thermistor. The figure consists of sensing element, Metal tube, Insulation and
Leads, the combination of all elements is named as thermistor. The thermistor
sensing element and metal tube are separated by insulation. Thermistors are
available in shapes like beads, discs, rods and probes.
Working:
At starting the measurement process some current is introduced into the
sensing element and determine the initial resistance value by using wheatstone
bridge.
* Now place the equipment at media whose temperature is measured. If the temperature increases the sensing element gets heated by decreasing the resistance of thermistor and vice versa. The difference between these gives variations in output. The output characteristics are shown below:
* Now place the equipment at media whose temperature is measured. If the temperature increases the sensing element gets heated by decreasing the resistance of thermistor and vice versa. The difference between these gives variations in output. The output characteristics are shown below:
Improtance and Features:
1. High stability, sensitivity & reliability
2. Fast output response & Long
working life
3.High temperature and coefficient
resistance
4.Thermistors are rugged and less cost
5.self heating is prevented for low
currents.
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