I. Introduction Touch screen technology enables direct interaction with devices through touch. Using either capacitive or resistive methods, these screens detect user input, translating it into commands. Widely used in smartphones, tablets, and kiosks, touch screens offer intuitive navigation, eliminating the need for physical buttons and enhancing user experience across various applications. Understanding how touch screens work is crucial for users and developers alike. It enhances user experience by ensuring effective interaction, informs choices in device selection, and guides troubleshooting. For developers, knowledge of touch technology is vital for creating intuitive applications and improving interface design, ultimately leading to greater user satisfaction. This blog aims to explore why touch screens predominantly respond to fingers, delving into the science behind touch sensitivity. By examining the principles of capacitive and resistive technology, we will uncover the
Introduction:
Temperature may be defined as the condition of a body by virtue of which heat is transferred from one system to another.
a) Liquid
in glass thermometer:
A typical liquid-in-glass thermometer is shown in Figure and consists of
a sealed stem of uniform small-bore tubing, called a capillary tube, made of
glass, with a cylindrical glass bulb formed at one end. The bulb and part of
the stem are filled with a liquid such as mercury or alcohol and the remaining
part of the tube is evacuated. A temperature scale is formed by etching
graduations on the stem. A safety reservoir is usually provided, into which the
liquid can expand without bursting the glass if the temperature is raised
beyond the upper limit of the scale.
Principle of operation
The operation of a liquid-in-glass thermometer
depends on the liquid expanding with increase in temperature and contracting
with decrease in temperature. The position of the end of the column of liquid
in the tube is a measure of the temperature of the liquid in the bulb. Two
fixed points are needed to calibrate the thermometer, with the interval between
these points being divided into ‘degrees’. In the first thermometer, made by
Celsius, the fixed points chosen were the temperature of melting ice (0°C) and
that of boiling water at standard atmospheric pressure (100°C), in each case
the blank stem being marked at the liquid level. The distance between these two
points, called the fundamental interval, was divided into 100 equal parts, each
equivalent to 1°C, thus forming the scale.
• easy to
use
These thermometers use the
following two principles:
1. All metals change in dimension, that is expand or contract when there is
a change in temperature.
2. The rate at which this expansion or contraction takes place depends on
the temperature co-efficient of expansion of the metal and this temperature
coefficient of expansion is different for different metals. Hence the
difference in thermal expansion rates is used to produce deflections which are
proportional to temperature changes.
Construction:
The bimetallic thermometer consists of a bimetallic strip. A bimetallic strip is made of two thin strips of metals which have different coefficients of expansion. The two metal strips are joined together by brazing, welding or riveting so that the relative motion between them is arrested.
The bimetallic strip is in the form of a cantilever beam. An increase in temperature will result in the deflection of the free end of the strip as shown i diagram. This deflection is linear and can be related to temperature changes.
The following are the important properties a material should have to be selected for bimetallic thermometers.
1. Coefficient of expansion.
2. Modulus of elasticity.
3. Elastic limit after cold rolling.
4. Electrical conductivity.
5. Ductility.
6. Metallurgical ability.
Metals used in bimetallic strips are:
For high expansion
1. Brass
2. Nickel-iron alloyd with chromium & manganese.
For low Expansion
Invar (alloy of nickel &
iron).
Application of bimetallic strips and thermometers
1. The bimetallic strip is used in control devices.
2. The spiral strip is used in air conditioning thermostats.
3.
The helix strip is used for
process application such as refineries, oil burners, tyre vulcanisers etc.,
Advantages of bimetallic thermometers
1. They are simple, robust and inexpensive.
2. Their accuracy is between +or- 2% to 5% of the scale.
3. They can with stand 50% over range in temperaures.
4. They can be used where evr a mecury –in-glass thermometer is used.
Limitations of bimetallic thermometer:
1. They are not recommended for temperature above 400’C.
2. When regularly used, the bimetallic may permanently deform, which inturn will introduce errors.
c) Pressure thermometer:
These thermometers work on the principle of thermal expansion of the matter wherein the change in temperature is to be measured.
Depending on the filling medium, they can be classified as liquid, gas or vapour type.
Construction:
It consists of liquid, vapour or gas, a flexible capillary tube, a bourdon tube.
Due to variation in temperature, the pressure and volume of system change and the fluid either expands or contracts.
This causes the bourdon tube to move or uncoil which then actuates the pointer over a graduated scale, thus providing a measure of temperature.
Advantages:
• Direct reading is possible
• Less expensive
• Better dynamic response
• Easy maintenance
d)
Thermocouples:
Thermocouple is made up of two dissimilar metals, joined together at one end that produces a voltage with a change in temperature. The junction of two metals is called sensing junction which is connected to extension wires.
Principle:
• When two dissimilar metals, joined together at one end, a small voltage is generated at the junction. This is called as peltier effect on which thermocouple works.
• If the temperature of the junction changes, it causes voltage to change too, which can be measured by the input circuit. The output is proportional to the temperature difference between the junction and the free ends. This is called as Thompson effect.
• Both of the above effects can be combined together to measure temperature. By holding one junction at a known temperature and measuring the voltage, the temperature at the sensing junction can be deduced. The voltage generated is directly proportional to the temperature difference. The combined effect is known as the thermo-junction effect or seebeck effect.
FUNDAMENTAL THERMOCOUPLE LAWS:-
o Law of Homogeneous Materials: - A thermoelectric current cannot be sustained in a single homogeneous material by the application heat alone, regardless of how it might vary in cross-section.
o Law of Intermediate Material: - The algebraic sum of thermoelectric forces in circuit composed of any number of dissimilar materials is Zero if all of the circuit is at same temperature.
o Law of Successive or Intermediate Temperature:- If two dissimilar homogeneous materials produces thermal (emf)1 when the junctions are T1 and T2 and produces thermal (emf)2 when the junction are at T2 and T3 the emf generated when the junction are at temperature T1 and T3 will be (emf)1 + (emf)2.
Depending on the combination of materials used for
the conductors, thermocouples are classified in different types. The selection
of the material is based on cost, availability, convenience, melting point,
chemical properties, stability, and output. They are usually selected based on the
temperature range and sensitivity needed.
Advantages:
• Wide range of temperature can be measured
• Self powered. no external power sourse is requires.
• Good response
• They can be easily installed
• Inexpensive
e) Resistance temperature detectors(RTD):
A resistance temperature detector (RTD) can also be called a resistance thermometer as the temperature measurement will be a measure of the output resistance.
The main principle of operation of an RTD is that when the temperature
of an object increases or decreases, the resistance also increases or decreases
proportionally.
RTD types are broadly classified according to the
different sensing elements used. Platinum, Nickel and Copper are the most
commonly used sensing elements. Platinum is considered the best as it has the
widest temperature range. This is shown in the resistance versus temperature
graph below. Platinum type RTD is also known for its best interchange ability
than copper and nickel. It also has the highest time stability.
RTD’s are available with either two, three, or four output wires for connection to the secondary instrument as shown in the figure below. The various wiring arrangements are designed to reduce and/or eliminate any errors introduced due to resistance changes of the lead wires when they also undergo temperature changes. RTDs used for electrical equipment generally use either a three-wire system or a four-wire system having paired lead wires.
Copper lead wires are satisfactory for all the arrangements. For a given RTD, all the lead-wires should be of the same gauge and the same length, and should be run in the same conduit.
The four wire system is little affected by temperature induced resistance changes in lead-wires, and, of all the arrangements, it is affected least by stray currents. It, therefore, is used to measure temperature differences and is used generally for making very accurate measurements.
Advantages
•
Very high accuracy
• Excellent stability and reproducibility
• Interchangeability
• Ability to be matched to close tolerances for temperature difference measurements.
• Ability to measure narrow spans
• Suitability for remote measurement
Disadvantages
•
Susceptibility to mechanical damage
• Need for lead wire resistance compensation
• Sometimes expensive
• Susceptibility to self-heating error
• Susceptibility to signal noise
• Unsuitability for bare use in electrically conducting substance
• Generally not repairable
• Need for power supply
f) Thermistors:
Thermistors are temperature sensitive semiconductors that exhibit a
large change in resistance over a relatively small range of temperature. There
are two main types of thermistors, positive temperature coefficient (PTC) and
negative temperature coefficient(NTC). NTC thermistors exhibit the
characteristic of resistance falling with increasing temperature. These are
most commonly used for temperature measurement.
A thermistor is similar to a RTD, but a semiconductor material is used
instead of a metal. A thermistor is a solid state device and has larger
sensitivity than does an RTD. Unlike RTDs, the temperature resistance
characteristic of a thermistor is non linear and can not be characterised by a single
coefficient. Unlike RTD, the resistance of a thermistor decreases with
increasing temperature.
Thermistors can not be used to measure as high as RTD but the
temperature range remains restricted between 100 to 200 degree centigrade.
Advantages:
• Large resistance change with temperature
• Rapid response
• Good stability
• Low cost
• Interchangeable
Disadvantages:
• Non linearity
• Limited operating temperature range
• Inaccuracy due to over heating
g) Radiation pyrometer:
The wavelengths measured by the device are known to be pure radiation wavelengths, that is, the common range for radioactive heat. This device is used in places where physical contact temperature sensors like Thermocouple, RTD, and Thermistors would fail because of the high temperature of the source.
The main theory behind a radiation pyrometer is that the temperature is
measured through the naturally emitted heat radiation by the body. This heat is
known to be a function of its temperature. According to the application of the
device, the way in which the heat is measured can be summarized into two:
1. Total Radiation Pyrometer – In this method, the total heat emitted from the hot source is measured at all wavelengths.
2.
Selective Radiation Pyrometer – In this method, the heat radiated from
the hot source is measured at a
given wavelength.
As shown in the figure below, the radiation pyrometer has an optical system, including a lens, a mirror and an adjustable eye piece. The heat energy emitted from the hot body is passed on to the optical lens, which collects it and is focused on to the detector with the help of the mirror and eye piece arrangement. The detector may either be a thermistor or photomultiplier tubes. Though the latter is known for faster detection of fast moving objects, the former may be used for small scale applications. Thus, the heat energy is converted to its corresponding electrical signal by the detector and is sent to the output temperature display device.
Advantages
•
The device can be used to measure
very high temperatures without direct contact with the hot source (Molten
metal).
• The biggest advantage is that the optical lens can be adjusted to measure temperature of objects that are even 1/15 inch in diameter and that too kept at a long s=distance from the measuring device.
• The sight path of the device is maintained by the construction of the instrument components, such as the lens and curved mirrors.
h) Optical pyrometer:
In an optical pyrometer, a brightness comparison is made to measure the temperature. As a measure of the reference temperature, a color change with the growth in temperature is taken. The device compares the brightness produced by the radiation of the object whose temperature is to be measured, with that of a reference temperature. The reference temperature is produced by a lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the source object. For an object, its light intensity always depends on the temperature of the object, whatever may be its wavelength. After adjusting the temperature, the current passing through it is measured using a multimeter, as its value will be proportional to the temperature of the source when calibrated. The working of an optical pyrometer is shown in the figure below.
Optical
Pyrometer - Working
As shown in the figure above, an optical pyrometer has the following components.
1. An eye piece at the left side and an optical lens on the right.
2. A reference lamp, which is powered with the help of a battery.
3. A rheostat to change the current and hence the brightness intensity.
4. So as to increase the temperature range which is to be measured, an absorption screen is fitted between the optical lens and the reference bulb.
5. A red filter placed between the eye piece and the reference bulb helps in narrowing the band of wavelength.
Working
The radiation from the source is emitted and the optical objective lens
captures it. The lens helps in focusing the thermal radiation on to the
reference bulb. The observer watches the process through the eye piece and
corrects it in such a manner that the reference lamp filament has a sharp focus
and the filament is super-imposed on the temperature source image. The observer
starts changing the rheostat values and the current in the reference lamp
changes. This in turn, changes its intensity. This change in current can
be observed in three different ways.
1. The filament is dark. That is, cooler than the temperature source.
2. Filamnet is bright. That is, hotter than the temperature source.
3. Filament disappears. Thus, there is equal brightness between the filament and temperature source. At this time, the current that flows in the reference lamp is measured, as its value is a measure of the temperature of the radiated light in the temperature source, when calibrated.
Advantages
1. Simple
assembling of the device enables easy use of it.
2. Provides a very high accuracy with +/-5 degree Celsius.
3. There is no need of any direct body contact between the optical pyrometer and the object. Thus, it can be used in a wide variety of applications.
4. As long as the size of the object, whose temperature is to measured fits with the size of the optical pyrometer, the distance between both of them is not at all a problem. Thus, the device can be used for remote sensing.
5. This device can not only be used to measure the temperature, but can also be used to see the heat produced by the object/source. Thus, optical pyrometers can be used to measure and
view wavelengths less than or equal to 0.65 microns. But, a Radiation Pyrometer can be used for high heat applications and can measure wavelengths between 0.70 microns to 20 microns.
Disadvantages
1. As the measurement is based on the light intensity, the device can be used only in applications with a minimum temperature of 700 degree Celsius.
2. The device is not useful for obtaining continuous values of temperatures at small intervals.
Applications
1. Used to measure temperatures of liquid metals or highly heated materials.
2. Can be used to measure furnace temperatures.
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