Thermistor Based Temperature Control for Precision Temperature Measurement

Thermistor based temperature control

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Introduction
An electrical instrument which aids in the measurement of temperature is known as
Thermistor. A thermistor is a temperature sensitive variable resistor made up of a ceramic
such as semi conducting material. Generally they are made up of the oxides of metals and
their mixtures such as cobalt oxide, nickel, copper, etc. The thermistors respond in a way
different that metals by negatively, the metals respond in a positive manner. Thermistors
behave like a resistor with high negative resistance coefficient. The resistance in thermistor
decreases by five percentage for each degree rise in the temperature. The thermistors are used
in precision temperature measurement because of its high sensitivity to temperature. The
thermistor resistance ranges from 0.5 ohm to 0.75 Mega ohm.
There are two types of thermistors they are Negative Temperature Coefficient (NTC)
and Positive Temperature Coefficient (PTC) thermistors. In a Negative Temperature
Coefficient thermistor with an increase of temperature the resistance decreases while in case
of positive temperature coefficient there is an increase in resistance with the increase in
temperature.
Depending on the adjustment in the situation obstruction with evolving temperature.
When the thermistor is adjusted against its conditions, it is easy to measure the atmospheric
temperature outside the thermistor, it is achieved by measuring the adjustment in the voltage
crosswise over its obstruction variations. The thermistor consists of combination of circuit-
controlled radiator and microchip control. The thermistor is used to direct a predefined
temperature in the range of 25 ̊C to 99 ̊C, to an exactness up to the range of positive to
negative 3 degrees, which shows it is capable to be used in the alexandrite laser pole. The
objective of this task is to find a method (method B) to keep up the alexandrite laser pole at
some predetermined temperature in the range of 25 ̊C and 99 ̊C, with a precision ranging
from positive to negative 3 degrees.
The most commercially available and the most suitable negative coefficient of temperature
thermistors are used for the purpose of temperature controlling. It has a little impression
permitting advantageous implanting into different equipments and is exceedingly receptive to
little temperature changes.

Compared with the positive coefficient of temperature thermistors the negative coefficient of
temperature thermistors efficiently perform under various range of temperatures between 40
to 140 0C which is the required and recommended range of temperature for the alexandrite
laser. These artistic semi-conductors show a decrease in electricity as the temperature rises,
they also have a steady and repeatable voltage and temperature trademark bench.
Theory
The name thermistor gets from the words "heat" and "resistor". Thermistors are temperature
delicate aloof semiconductors which display a substantial change in electrical opposition
when subjected to a little change in body temperature. Thermistors are fabricated in different
sizes and shapes. Globules, circles, washers, wafers, and chips are the most generally utilized
thermistor sensor writes. Plate thermistors are made by mixing and compacting different
metal oxide powders with appropriate fasteners. The plates are shaped by packing under high
weight on pelleting machines to create round, level clay bodies. At long last, the thermistors
are subjected to an extraordinary maturing procedure to guarantee high steadiness of their
qualities.
The relationship between the temperature and the resistance of a thermistor is given by the
Stinhart-Hart condition.
R= 1
T =a+blnR + c ( lnR ) 3
Where-
a, b and c are constants
This can also be expressed as-
lnRt=ln R0 + T 0
T
Temperature and measurement literature review