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Title: sensor s transducers 2


1
Sensorsand Actuators
PE-4030 Chapter 2/b Part two
  • Professor Charlton S. Inao
  • Mechatronics
  • Defence Engineering College
  • Bishoftu, Ethiopia

2
Instructional Objectives
  • To understand the working principle and
    applications of the following sensors
  • 1. Liquid Flow Sensor
  • 1.1 Orifice
  • 1.2 Turbine Flow Meter
  • 2. Level Sensor
  • 2.1 Floats
  • 2.2 Differential Pressure
  • 3. Temperature Sensor
  • 3.1 Liquid in Glass
  • 3.2 Bimetallic Strip
  • 3.3 Thermistors
  • 3.4 Electrical Résistance Thermometers
  • 3.5 Thermocouples
  • 4. Light Sensor
  • To practice how to select sensor based on
    industrial requirements.

3
Flow Sensors
4
1.1 Differential Pressure Flowmeter
1.1.1 Orifice
The orifice plate is simply a disc , with a
central hole, which is placed in the tube through
which the fluid is flowing The pressure
difference is measured between a point equal to
the diameter of the tube upstream and a point
equal to half of the diameter downstream. It does
not work well with the slurries. The accuracy is
typically about 1.5 of full range and is
non-linear.
5
A concentric orifice plate is the simplest and
least costly of the differential pressure
devices. The orifice plate constricts the flow of
a fluid and produces a differential
pressure across the plate (see Figure ) This
results in a high pressure upstream and a low
pressure downstream that is proportional to the
square of the flow velocity. An orifice plate
usually produces a greater overall pressure loss
than other flow elements. One advantage of this
device is that cost does not increase
significantly with pipe size
6
  • 1.1.2 Venturi meter

Venturi tubes are the largest and most expensive
differential pressure device. They work by
gradually narrowing the diameter of the pipe, and
measuring the pressure drop that results
(see Figure ). An expanding section of the
differential pressure device then returns the
flow to close to its original pressure. As with
the orifice plate, the differential pressure
measurement is converted into a corresponding
flow rate. Venturi tubes can typically be used
only in those applications requiring a low
pressure drop and a high accuracy reading. They
are often used in large diameter pipes.
7
1.1.3 Flow Nozzle
Flow nozzles are actually a variation on the
Venturi tube, with the nozzle opening being an
elliptical restriction in the flow, but having no
outlet area for the pressure . Pressure taps are
located approximately 1/2 pipe diameter
downstream and 1 pipe diameter upstream. The
flow nozzle is a high-velocity flow meter used
where turbulence is high (Reynolds numbers above
50,000), as in steam flow applications. The
pressure drop of a flow nozzle is between that of
a Venturi tube and the orifice plate (30 to 95
percent).
8
1.2 Ultrasonic Flow Transducer
An ultrasonic flow meter is a type of flow meter
that measures the velocity of a fluid with
ultrasound to calculate volume flow. Using
ultrasonic transducers, the flow meter can
measure the average velocity along the path of an
emitted beam of ultrasound, by averaging the
difference in measured transit time between the
pulses of ultrasound propagating into and against
the direction of the flow or by measuring the
frequency shift from the Doppler effect.
What is the Doppler Effect? The Doppler effect is
observed whenever the source of waves is moving
with respect to an observer. The Doppler
effect can be described as the effect produced by
a moving source of waves in which there is an
apparent upward shift in frequency for observers
towards whom the source is approaching and an
apparent downward shift in frequency for
observers from whom the source is receding.
9
. Ultrasonic flow meters are affected by the
acoustic properties of the fluid and can be
impacted by temperature, density, viscosity and
suspended particulates depending on the exact
flow meter. They vary greatly in purchase price
but are often inexpensive to use and maintain
because they do not use moving parts, unlike
mechanical flow meters.
Ultrasonic-used to describe sounds that are too
high for humans to hear (16KHz- 1 GHz) .
10
Sound is the propagation of smallest pressure and
density variations in an elastic medium (gas,
liquid, solid-state body). For example, a noise
is generated when the air in a specific spot is
compressed more than in the surrounding area.
Subsequently, the layer with changed pressure
propagates remarkably fast in all directions at
speed of sound of 343 m/s. Acoustic frequencies
between 16 kHz and 1 GHz are referred to as
ultrasound in industrial settings we call it
ultrasonics. To clarify people are able to
hear frequencies between 16 Hz and 20 kHz i.e.
the lower frequencies of industrial ultrasonics
are audible, especially if secondary frequencies
are generated. And what is more, ultrasonics is
palpable when touching the weld tool. For
ultrasonic welding, the frequency range is
between 20 kHz and 70 kHz. Additional fields of
application Imaging ultrasound in the field of
medical diagnostics ranges between 1 and 40 MHz.
It is not audible or palpable. In the field of
industrial material testing, ultrasonics is used
at frequencies from 0.25 to 10 MHz.  
NOTE
11
1.3 Drag Force Flowmeter
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1.4 Turbine Meter
  • The turbine flowmeter consists of a multi-bladed
    motor that is supported centrally in the pipe
    along which the flow occurs.The fluid rotates the
    motor , the angualr velocity being approximately
    proportional to the flow rate. The rate of the
    revolution of the rotor can be determined using a
    magnetic pick up which produces an induced emf
    pulse every time the rotor blade passes it as th
    e blades are made from magnetic material or have
    small magnets mounted at their tips.

The pulses are counted and so the number of
revolutions of the rotor can be determined. The
meter is expensive with a n accuracy of typically
about 0.3
15
Turbine-Based Flow Sensors Turbine and propeller
type meters use the principle that liquid flowing
through the turbine or propeller will cause the
rotor to spin at a speed directly related to flow
rate. Electrical pulses can be counted and
totaled. These devices are available in full
bore, line-mounted versions and insertion types
where only a part of the flow being measured
passes over the rotating element. Turbine flow
meters, when properly specified and installed,
offer good accuracy, especially with low
viscosity fluids. Insertion types are used for
less critical applications however, they are
often easier to maintain and inspect because they
can be removed without disturbing the main piping.
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  • Turbine flowmeters use the mechanical energy of
    the fluid to rotate a pinwheel (rotor) in the
    flow stream. Blades on the rotor are angled to
    transform energy from the flow stream into
    rotational energy. The rotor shaft spins on
    bearings. When the fluid moves faster, the rotor
    spins proportionally faster.
  • Turbine flowmeters now constitute 7 of the
    world market.
  • Shaft rotation can be sensed mechanically or by
    detecting the movement of the blades. Blade
    movement is often detected magnetically, with
    each blade or embedded piece of metal generating
    a pulse.
  • Turbine flowmeter sensors are typically located
    external to the flowing stream to avoid material
    of construction constraints that would result if
    wetted sensors were used. When the fluid moves
    faster, more pulses are generated. The
    transmitter processes the pulse signal to
    determine the flow of the fluid. Transmitters and
    sensing systems are available to sense flow in
    both the forward and reverse flow directions.

17
1.5 Electromagnetic Flow Sensor
Electromagnetic Flow Sensors Operation of these
sensors is based upon Faradays Law of
electromagnetic induction, which says that a
voltage will be induced when a conductor moves
through a magnetic field. The liquid is the
conductor, and the magnetic field is created by
energized coils outside the flow tube. The
voltage produced is proportional to the flow
rate. Electrodes mounted in the pipe wall sense
the induced voltage, which is measured by the
secondary element. Electromagnetic flow meters
are applied in measuring the flow rate of
conducting liquids (including water) where a high
quality, low maintenance system is needed. The
cost of magnetic flow meters is high relative to
other types of flowmeters. They do have many
advantages, including they can measure difficult
and corrosive liquids and slurries, and they can
measure reverse flow.
18
1.6 Laser Doppler Anemometter
The laser doppler anemometer (LDA) is a
well-established technique that has been widely
used for fluid dynamic measurements in liquids
and gases for well over 30 years. The directional
sensitivity and non-intrusiveness of LDA make it
useful for applications with reversing flow,
chemically reacting or high-temperature media,
and rotating machinery, where physical sensors
might be difficult or impossible to use. This
technique does, however, require tracer particles
in the flow.
The laser doppler anemometer (LDA) is a
well-established technique that has been widely
used for fluid dynamic measurements in liquids
and gases for well over 30 years. The directional
sensitivity and non-intrusiveness of LDA make it
useful for applications with reversing flow,
chemically reacting or high-temperature media,
and rotating machinery, where physical sensors
might be difficult or impossible to use. This
technique does, however, require tracer particles
in the flow.
19
1.7 Hot Wire Anemometter
The Hot-Wire Anemometer is the most well known
thermal anemometer, and measures a fluid velocity
by noting the heat convected away by the
fluid. The principal of a hot wire anemometer is
based on a heated element from which heat is
extracted by the colder impact airflow. Thermal
anemometry is the most common method used to
measure instantaneous fluid velocity.
20
Typically, the anemometer wire is made of
platinum or tungsten and is 4 10 µm (158
393 µin) in diameter and 1 mm (0.04 in) in
length. Typical commercially available hot-wire
anemometers have a flat frequency response
(lt 3 dB) up to 17 kHz at the average velocity of
9.1 m/s (30 ft/s), 30 kHz at 30.5 m/s
(100 ft/s), or 50 kHz at 91 m/s (300 ft/s). Due
to the tiny size of the wire, it is fragile and
thus suitable only for clean gas flows. In liquid
flow or rugged gas flow, a platinum hot-film
coated on a 25 150 mm (1 6 in) diameter
quartz fiber or hollow glass tube can be used
instead, as shown in the schematic .
21
The Hot-Wire Anemometer is the most well known
thermal anemometer, and measures a fluid velocity
by noting the heat convected away by the fluid.
The core of the anemometer is an exposed hot wire
either heated up by a constant current or
maintained at a constant temperature (refer to
the schematic ). In either case, the heat lost to
fluid convection is a function of the fluid
velocity. By measuring the change in wire
temperature under constant current or the current
required to maintain a constant wire temperature,
the heat lost can be obtained. The heat lost can
then be converted into a fluid velocity in
accordance with convective theory.
22
L E V E L
SENSORS
23
Level Sensors
2.0 Indirect Method
  • Monitoring of the weight of the vessel by load
    cell
  • Weight Ah?g
  • Note h?g P
  • 2. Measurement of pressure at some
  • point in the liquid P h?g

2.1 Floats
A direct method of monitoring the level of
liquid in a vessel is by monitoring the movement
of the float. . The displacement of the float
causes a lever arm to rotate and so move a slider
across the potentiometer. The result is an
output of voltage related to the height of the
liquid.
24
Float Swtich
25
2.2Differential Pressure
The differential pressure cell determines the
pressure difference between the liquid at the
base of the vessel and atmospheric pressure, the
vessel being open to atmospheric pressure. The
differential pressure cell monitors the
difference in pressure between the vase of the
vessel and the air or gas above the surface of
the liquid.
26
Level Transmitters
27
Level Sensors
28
Radar Level Sensor
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Guided-Wave Radar (GWR)Guided-wave radar (GWR)
is a contacting level measurement method that
uses a probe to guide high-frequency
electromagnetic waves from a transmitter to the
media being measured (Figure 2). GWR is based on
the principle of time domain reflectometry (TDR).
With TDR, a low-energy electromagnetic pulse is
guided along a probe. When the pulse reaches the
surface of the medium being measured, the pulse
energy is reflected up the probe to circuitry
that then calculates the fluid level based on the
time difference between the pulse being sent and
the reflected pulse received. The sensor can
output the analyzed level as a continuous
measurement reading via an analog output, or it
can convert the values into freely positionable
switching output signals. Unlike older
technologies, GWR offers measurement readings
that are independent of the chemical or physical
properties of the process media with which it is
in contact. Additionally, GWR performs equally
well in liquids and solid
36
GWR is suitable for a variety of level
measurement applications including those that
involve Unstable process conditionsChanges in
viscosity, density, or acidity do not affect
accuracy. Agitated surfacesBoiling surfaces,
dust, foam, and vapor do not affect device
performance. GWR also works with recirculating
fluids, propeller mixers, and aeration
tanks. High temperatures and pressuresGWR
performs well in temperatures up to 315C and can
withstand pressures up to 580 psig. Fine powders
and sticky fluidsGWR works with vacuum tanks
filled with used cooking oil as well as tanks
holding paint, latex, animal fat, soybean oil,
sawdust, carbon black, titanium tetrachloride,
salt, and grain.
GWR technology measuring liquid level in process
vessel  
37
Ultrasonic Technology
Ultrasonic transmitters operate by sending a
sound wave generated from a piezoelectric
transducer to the surface of the process material
being measured. The transmitter measures the
length of time it takes for the reflected sound
wave to return to the transducer. A successful
measurement depends on the wave, reflected from
the process material and moving in a straight
line back to the transducer. Because factors such
as dust, heavy vapors, tank obstructions, surface
turbulence, foam, and even surface angles can
affect the returning signal when using an
ultrasonic level sensor, you must consider how
your operating conditions can affect the sound
waves.
Ultrasonic transmitter mounted on top of tank  
38
GWR is suitable for a variety of level
measurement applications including those that
involve Unstable process conditionsChanges in
viscosity, density, or acidity do not affect
accuracy. Agitated surfacesBoiling surfaces,
dust, foam, and vapor do not affect device
performance. GWR also works with recirculating
fluids, propeller mixers, and aeration
tanks. High temperatures and pressuresGWR
performs well in temperatures up to 315C and can
withstand pressures up to 580 psig. Fine powders
and sticky fluidsGWR works with vacuum tanks
filled with used cooking oil as well as tanks
holding paint, latex, animal fat, soybean oil,
sawdust, carbon black, titanium tetrachloride,
salt, and grain.
39
Gravimetric Level Sensor
40
Description Gravimetric measurement of level
with SIWAREX weighing technology produces
high-precision weight measurement results without
any contact with the material. The weight of your
product is correctly determined independently of
the temperature, container shape, material
density, shift in the center of gravity and
agitators or the like. Bridging, heaped objects,
hopper flow, foam, steam and dust have no effect
on the gravimetric measurement. These advantages
enable SIWAREX weighing technology to be used in
legal-for-trade plants. Measuring points in
potentially explosive areas can be very easily
realized with standard components.
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We put together the correct load
cells and built-in components from our product
range for gravimetric measurement to match the
particular application, load range and accuracy
requirements. Our range extends from platform
load cells, bending beams and shear beams to can
compression cells in the load classes from 3kg to
280t. Service-proven load cell technology and
separation of the medium mean maximum service
life with no special maintenance. This improves
plant availability and reduces the operating
costs on a permanent basis. The load cell
signals are evaluated by SIWAREX weighing
electronics which are seamlessly integrated in
the SIMATIC automation system. This enables very
easy handling and use of the advantages provided
by SIMATIC such as flexibility, a diagnostic
interrupt system and much more. Dont forget that
the safest engineered level measurement solution
includes switches for back-up, overfill, low
level and dry run protection.
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Capacitance Level Sensor
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  • With the tank empty, the insulating medium
    between the two conductors is air. With the tank
    full, the insulating material is the process
    liquid or solid. As the level rises in the tank
    to start covering the probe, some of the
    insulating effect from air changes into that from
    the process material, producing a change in
    capacitance between the sensing probe and ground.
    This capacitance is meas ured to provide a
    direct, linear meas urement of tank level.

47
Hydrostatic Level Sensor
  • Principles of OperationA hydrostatic level
    sensor is a submersible or externally mounted
    pressure sensor that determines level by
    measuring pressure above it, which increases with
    depth. From this measurement, together with
    knowledge of the liquid's density / specific
    gravity, it is possible calculate the liquid
    level above the sensor in the vessel. Temperature
    compensation will take into account changes in
    specific gravity due to variations in temperature.

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Advantages of Hydrostatic Level Sensors Easy to
install and relatively low cost Good overall
accuracy and long-term stability Applicable to
a wide variety of fluids Limitations of
Hydrostatic Level Sensors Not suitable for
solids or liquids with suspended solids Can
only read level above the transmitter Need to
know the density / specific gravity of the liquid
being measured
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Hydrostatic Head Level Sensor
50
  • For decades, DP-type instrumentslong before the
    DP cellwere used to measure liquid level.
    Orifice meters, originally designed to measure
    differential pressure across an orifice in a
    pipeline, readily adapted to level measurement.
  • Todays smart DP transmitters adapt equally well
    to level measurements and use the same basic
    principles as their precursors.
  • With open vessels (those not under pressure or a
    vacuum), a pipe at or near the bottom of the
    vessel connects only to the high-pressure side of
    the meter body and the low-pressure side is open
    to the atmosphere.
  • If the vessel is pressurized or under vacuum,
    the low side of the meter has a pipe connection
    near the top of the vessel, so that the
    instrument responds only to changes in the head
    of liquid .

51
  • DP transmitters are used extensively in the
    process industries today. In fact, newer smart
    transmitters and conventional 4 20 mA signals
    for communications to remote DCSs, PLCs, or other
    systems have actually resulted in a revival of
    this technology. Problems with dirty liquids and
    the expense of piping on new installations,
    however, have opened the door for yet newer,
    alternative methods.
  • Hydrostatic Tank Gauging. It is an emerging
    standard way to accurately gauge liquid inventory
    and to monitor transfers in tank farms and
    similar multiple-tank storage facilities. HTG
    systems can provide accurate information on tank
    level, mass, density, and volume of the contents
    in every tank. These values can also be networked
    digitally for multiple remote access by computer
    from a safe area.

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  • The level transmitter, with its probe installed
    at an angle into the bottom portion of the tank,
    is an innovative way to detect accumulation of
    water, separated from oil, and to control
    withdrawal of product only. Moreover, by
    measuring the water-oil interface level, the LT
    provides a means of correcting precisely for the
    water level, which would incorrectly be measured
    as product.

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Though the DP transmitter is most commonly used
to measure hydrostatic pressure for level
measurement, other methods should be mentioned.
One newer system uses a pressure transmitter in
the form of a stainless steel probe that looks
much like a thermometer bulb. The probe is simply
lowered into the tank toward the bottom,
supported by plastic tubing or cable that carries
wiring to a meter mounted externally on or near
the tank. The meter displays the level data and
can transmit the information to another receiver
for remote monitoring, recording, and
control. Another newer hydrostatic measuring
device is a dry-cell transducer that is said to
prevent the pressure cell oils from contaminating
the process fluid. It incorporates special
ceramic and stainless steel diaphragms and is
apparently used in much the same way as a DP
transmitter.  
54
Temperature Sensing Devices
55
Temperature Scales
Temperature Measurements
  • Celsius(º C)- common SI unit of relative temp
  • KC 273
  • Kelvin(K)-Standard SI unit of absolute
    thermodynamic temperature
  • Fahrenheit-(º F)English unit of relative
    temperature. T 9/5C 32
  • Rankine(ºR) English system unit of absolute
    thermodynamic temperature. RF 460

56
Temperature Sensors
  • 3.1 Liquid in Glass
  • -A simple non electrical temperature measuring
    device which typically uses alcohol or mercury as
    the working fluid, which expands and contracts
    relative to the glass container. When making
    measurements in a liquid, the depth of immersion
    is important

57
Temperature Sensors
  • 3.2 BiMetallic Strip
  • Another nonelectrical temperature measuring
    device. I tis composed of two or more metal
    layers having different coefficient of thermal
    expansion. Since these layers are permanently
    bonded together, the structure will deform when
    temperature changes, due t to the difference in
    the thermal expansions of the two metal layers.
    The deflection can be related to the temperature
    of the strip.

The mechanical motion of the strip makes or
breaks an electrical contact to turn a heating or
cooling system On or OFF.
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Temperature Sensors
  • 3.3Resistance Temperature Detector(RTDs)
  • RTD is constructed of metal wire wound around a
    ceramic or glass core and hermetically sealed.
    The resistance of the metallic wire increases
    with temperature. The resistance Temperature
    relationship is approximated by the following
    linear expression
  • RRo1 a(T-To)

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  • Where Toreference temperature
  • Ro resistance at the reference
  • temperature
  • acalibration constant
  • The reference temperature is usually the ice
    point of the water(0º C).
  • The most commonly used metal in RTD is platinum,
    because of its high melting point, resistance to
    oxidation, predictable tem characteristics, and
    stable calibration values.
  • The operating range of typical platinum RTD is
    220 deg centigrade to 750 deg centigrade.

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  • 3.4 Thermistor-is a semiconductor device whose
    resistance changes exponentially with
    temperature. Thermistors have much narrower
    operating ranges than RTDs.
  • Its resistance temperature relationship is
    usually expressed in the form
  • R Roe ß(1/T-1/To)
  • Where To reference temperature
  • ß a calibration constant called
    the characteristic temperature of the
  • material

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Temperature Sensors
  • 3.5 Thermocouples
  • Two dissimilar metals in contact form a
    thermoelectric junction occur in pairs, resulting
    in what is called thermocouple.This is known as
    Seebeck effect.The thermocouple voltage is
    directly proportional to the junction temperature
    difference
  • V a(T1-T2)

Where a is called the Seebeck coefficient T1 and
T2 is the junction temperature of metals A and
B.
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Thermocouple Circuit
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Thermocouple Configuration
Thermocouple Data
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Thermocouple
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Thermocouple Type, Materials, Range, Sensitivity
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Thermocouple Junction Temperature and Output
voltage
Junction Temperature (C) Output Voltage (mV)
0 0
10 0.507
20 1.019
30 1.536
40 2.058
50 2.585
60 3.115
70 3.649
80 4.186
90 4.725
100 5.268

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Light Sensors
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The Electromagnetic Spectrum
  •  
  • The electromagnetic spectrum is divided into
    radio waves and light waves by frequency. Light
    waves are further divided by into infrared,
    visible, ultraviolet and X-rays. The spectrum is
    either expressed in frequency or wavelength.
    Wavelength is the distance that an
    electromagnetic wave travels through space in one
    cycle of its frequency.

Since distance is velocity multiplied by time,
wavelength can be expressed as the velocity of
electromagnetic waves multiplied by the time of
one cycle of frequency f. Since the accepted
speed of light is 186,000 miles per second or
300,000,000 meters per second, this is ë(in
meters) 300,000,000 meters/sec 1/f(in
seconds) or, ë(in meters) 300/f(in MHz) If
visible light (white light) is passed through a
prism, , the visible light separates into its
color components.
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The frequency of visible light is from 400
million megahertz to 750 million megahertz. The
wavelength is from 750 nanometers (10-9) to 400
nanometers. Light sensors extend into the
infrared frequency range below visible light and
into the ultraviolet light frequency range above
visible light. Cadmium sulfide sensors are most
sensitive in the green light region of visible
light, while solar cells and phototransistor
sensors are most sensitive in the infrared region.
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Light Sensors
  • Used in control of street lamps
  • Used in the automatic /digital camera
  • Used in the automotive and military industry

Light sensor diodes make the resistance of the
circuit decreases and the current increases as
the light/illuminance increases, at constant
voltage.
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Selection of Sensors
  • 1. Identify the nature of the measurement
    required
  • Variable to be measured
  • Nominal value
  • Range of Value
  • Accuracy required
  • The required speed of measurement
  • Reliability required
  • Environmental conditions

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  • 2. Identify the nature of the output required
    from the sensor, this determining the signal
    conditioning requirements in order to give
    suitable output signals from the measurement.
  • 3. Identify the possible sensors, taking into
    account such factors as range, accuracy,
    linearity, speed of response, reliability,
    maintainability, life, power supply requirements,
    ruggedness, availability and cost.
  • 4.Identify the signal conditioning requirements.
    Eg. Measurement of level of a corrosive acid in a
    vessel.
  • Using a load cell, which gives an electrical
    output, calibrated to the level, ie. When empty
    and when full.

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