ENGT 5214 Research Methods: Thermistor Based Temperature Control

This report discusses the development and implementation of a thermistor-based temperature control system, focusing on maintaining a predetermined temperature for an alexandrite laser pole within a specific range. The report begins with an introduction to thermistors, highlighting their properties and types, particularly negative temperature coefficient (NTC) thermistors suitable for temperature control. It delves into the theory behind thermistor operation, including the Steinhart-Hart equation, and reviews existing temperature measurement techniques and instruments. The methodology involves two experiments: aligning the thermistor and testing the setup as a temperature controller, using a National Instruments Lab computer with a mercury thermometer for calibration. The experimental setup includes a radiator and thermistor enclosed in a metal box to enhance measurement precision. The report presents results from heating and cooling processes, noting temperature changes and calculating corresponding resistance values. A prototype temperature moderator circuit diagram is included, outlining the system's functionality and performance in maintaining the target temperature. This document is available on Desklib, a platform offering various AI-based study tools and resources for students.

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

Temperature measurement is defined as the measurement of velocity of fluid
particles. Temperature is a property of a body which is generally used in order to find the
degree of hotness or coldness or the heat intensity level of an object. The second law of
thermodynamics is relates the temperature to heat. The second law of Thermodynamics is
also known as the law of entropy which states that the entropy of a system keep on increases
and it never decreases. The clausius law stated that there is no heat engine which transfers
heat from a cold body to hot body, which means the heat cannot flow from a cold body to hot
body it can only flow from a hot body to cold body. Thus the heat flow can only be from a
higher temperature element to a lower temperature element.
In today’s world the temperature measurement is a very vital thing to run our day
today life the temperature measuring instruments are all around the society in the Air
conditioners, HVAC systems, Cars, two wheelers, etc. There are wide variety of temperature
measuring instruments that available these days some of the most important and widely
adopted instruments are the thermometers, pyrometers, bimetallic strips, pressure
thermometer, thermocouples, Electrical resistance thermometers, filled system temperature
measurements, etc.
The thermometers are the instrument that are used to measure the temperatures in
ordinary range in other word lower temperature ranges, the thermometer generally uses a
liquid metal such as mercury which will expand when exposed to a temperature that
expansion will be within the given area for the mercury to rise that area is a tubular section
marked with gradual readings of temperature, thus a temperature of a body is found. The
mercury thermometers are not the only thermometers there are wide variety of thermometers
available in the market commercially today they are the thermometers are not suitable for
higher temperature measurements.
The instruments used to measure high temperature range is known as pyrometers. The
pyrometers measures the temperature based on the change in intensity of the radiation and the
radiation colour. The temperature measurement method by radiation is the basic principle of
pyrometers. If we can find the thermal radiation emitted by the body we can be able to find
the temperature of the body. There are three types of pyrometers used widely and available
commercially they are Total radiation pyrometers, which measures the body temperature with
the help of a concave mirror focused to the thermocouple the second type of pyrometer is the
Infra-red pyrometer, which work on the basis that at high temperatures the body starts to emit

infrared rays also some visible rays which are visible to naked eye, with the help of a detector
and some electronic circuits the intensity of the Infrared rays are measured which is
proportional to the temperature of the particle. The infrared pyrometers can be used to
measure the temperature of particles up to a maximum range of 400 degree Celsius. The third
and the last type of pyrometer is known as the Optical radiation pyrometer, which uses a
lamp which is connected to an ammeter at the middle, lens at one end and eye piece at the
other end. The body to which the temperature to be measured is placed behind the optical
pyrometer instrument, then the filament (lamp) is adjusted to match the brightness of the heat
source, which is the object whose temperature to be measured, the outcome would be equal
brightness with the brightness level of the heat source or the lamp will be darker than the heat
source or the lamp might be in a more brighter stage than the heat source. The requirement is
to bring the filament brightness equal to the brightness of the heat source then the current
input to the filament is measured using the ammeter thus the temperature corresponding to
the current flow can be measured with the help of optical pyrometers.
The pressure, electrical resistance, expansion coefficient, etc are all interrelated with
the temperature in its basic molecular structure itself so these variables can also be used in the
measurement of temperature. We have seen the usage of pressure and expansion application
for the temperature measurement in case of thermometers and electrical resistance in case of
pyrometers, thermocouples, etc.
Temperature is a standout amongst the peak imperative bounds in control process.
The measurement of the exact temperature isn't simple and in order to get correctness
superior to anything 0.5°C incredible care is required. Mistakes happen because of a few
sources, for example, temperature angles, faulty sensors, alignment blunders, and poor heat
conduction. There are numerous kinds of equipment to measure the temperature called
temperature sensors. A few sensors, for example, the thermocouples, Negative temperature
coefficient thermistors, positive temperature coefficient thermistors, Resistance temperature
detector and the thermopiles are the more established traditional sensors and they are utilized
mainly because of their big favourable conditions. The latest sensors, for example, the
incorporated circuit sensors, the radiation thermometry equipments are established just for
constrained applications. Resistance temperature detectors are utilized as a part of medium
temperature range, going from -190 to +595°C. They offer high exactness, commonly with
deviation of 0.2°C. Resistance temperature detectors can more often than not be utilized as a
part of most compound and physical conditions; however they are not as strong as

thermocouples. The activity of Resistance temperature detectors requires outside power
supply. The choice of proper sensor isn't generally simple and it relies upon variables, for
example, the precision, the temperature run, speed of reaction, heat coupling, nature, and the
cost. Ecological applications, hardware pastime advertise, and car enterprises for the most
part utilize thermistors or coordinated circuit sensors.
Thermistor temperature sensor
Thermocouples are straightforward temperature sensors comprising of two different metals
combined. In 1821, the generation of thermoelectric voltage was found and an electric current
was discovered streaming in a shut circuit of two unique metals, if the two intersections were
held at various temperatures. One of the intersections was assigned the hot intersection and
the other intersection was assigned as the chilly or reference intersection. The current created
in the close circle was relative to the kinds of metals utilized and the distinction in
temperature between the hot and the icy intersections. Thermocouple wires are generally
made of various metals from the measuring equipment wires and subsequently, an extra
match of thermocouples is shaped at the association focuses. In spite of the fact that, these
extra thermocouples appear to cause an issue, the utilization of the Law of Intermediate
Metals demonstrate that these thermocouples have no impact, on the off chance that they are
kept at a similar temperature. There are around 12 standard thermocouple composes that are
generally utilized. Each compose is given a universally affirmed letter that shows the
materials from which the thermocouple is fabricated.
Procedure and apparatus
There were two experiments which were conducted in order to accomplish the final goal of
the task: the first step is to align the thermistor alongside above equations, the second is to
test the first setup as a temperature controller. The program is executed in a national
Instrumental Lab computer microchip, which is powered by a program executed with an Intel
Core 2 Duo 2.93 Ghz processor with 2 GB of DDR2 ram running an operating system of
windows 7. The adjusting equipment which is used to adjust or to calibrate is a mercury
thermometer.

Figure: ADC2 for the program
Each of the radiator and thermistor are stuck against a small opening in a little metal box in
order to enhance the precision of temperature measurement which is carried out at the
surroundings of the heating element. The electric potential across the thermistor, denoted by
VT, is calculated at by the Lab computer microchip consists the port ADC2. When the
temperature varies, the adjustment at VT will be estimated.
The change in RT is given by,
RT = V T
V 0−V T
The thermistor temperature changes are noted along the process of heating as well as
cooling. The value of the RT are calculated and noted for temperature ranging from 30 degree
Celsius to 89 degree Celsius and at 3 degree Celsius augmentations for the heating process
and down up to 29 degree Celsius for amid cooling. The temperature moderator circuit
diagram is shown below.

Figure: Prototype temperature moderator
Result
The model is worked by writing the objective temperature, TSET, into the computer
charge provoke, to initiate temperature measurement. In chances that the present temperature
is lower than that of the objective temperature. At the port PA0 of the chip would yield 5
Volts, by which the radiator is turned ONN. The flag is kept up till at the point that the
temperature surpasses TSET by a counterbalance of temperature change. Then the voltage of
the port PA0 would be set to zero, which in turn will turn off the radiator. In this way, when
temperature goes low under the TSET due to the temperature change, the radiator is betrayed.
In the current usage, temperature change equal to 1˚C serves to diminish the recurrence of
killing the radiator on and, to broaden its life expectancy, while as yet controlling the
temperature sufficiently tight.

Figure: Heating and cooling
The heating and the cooling curves are plotted on a graph which having (1/T) along X
axis and the (ln R) along Y axis, we can see that the plot is exhibiting a hysteresis circle
which is situated near the mean relapse line, this might be due to the usage of the mercury
thermometer during the experiment. The thermometer exhibits change in temperature in a
slower manner while heating and rapid and speeded up manner while cooling.

Figure: Temperature regulation
Conclusion
Thus the experiment is carried out with a Negative temperature coefficient thermistors
which will exhibit a character as with an increase of temperature the resistance decreases, that
if it is suitable for the usage as a temperature controller for the alexandrite laser pole which
should be working under a predefined range of temperature between the 29 degree Celsius to
89 degree Celsius, with an accuracy of plus or minus 3 degree Celsius. The thermistor is
arranged in order to comply with the Stenhart- Hart Equation which is appropriate for
thermistors. This condition is selected due to its level of adjustments. Thus the experiment is
carried out using a microchip lab computer powered by an Intel core 2 duo personal computer
with 2 GB of DDR2 RAM and running on a windows 7 Operating system. Then the results
are obtained and the graphs are plotted. The result came as that, the selected Negative thermal
coefficient thermistor is suitable for the operation of the laser pole as a temperature
controller. In order to achieve accuracy in the experimental results, it is highly suggested to
take more than one reading and finally taking the average among them by this method way
we can achieve high precision.

A more grounded radiator ought to be utilized to check if the thermistor conduct at
temperatures from 88 degree Celsius to 100 degree Celsius, in order to decide whether the
selected negative thermal coefficient thermistor meets the necessities of the purpose, however
there ought to be no motivation behind why it can't. Also, the test for the temperature control
carried out with the objective temperature set at 50 degree Celsius. Furthermore tests ought to
be kept running along the temperature ranging from 30˚C to 100˚C to additionally confirm its
execution. The number of times that the heater being turned on and off plays an important
role in case of increasing the life expectancy of the radiator, the largest measure of the
counter temperature from the objective temperature, and the temperature change.