sensors transducers transmitters

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

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
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.
1.1.1 Orifice

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.

11. 1.3 Drag Force Flowmeter

14. • 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.
1.4 Turbine Meter
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.

16. 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 Faraday’s 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
(< 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
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.
2.1 Floats
2.0 Indirect Method
1. Monitoring of the weight of the
vessel by load cell
Weight= Ahρg
Note: hρg = P
2. Measurement of pressure at some
point in the liquid P= hρg

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

35. 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 conditions—Changes in
viscosity, density, or acidity do not affect
accuracy.
Agitated surfaces—Boiling surfaces, dust,
foam, and vapor do not affect device
performance. GWR also works with
recirculating fluids, propeller mixers, and
aeration tanks.
High temperatures and pressures—GWR
performs well in temperatures up to 315°C
and can withstand pressures up to 580 psig.
Fine powders and sticky fluids—GWR 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 conditions—Changes in viscosity, density, or acidity
do not affect accuracy.
Agitated surfaces—Boiling surfaces, dust, foam, and vapor do not affect
device performance. GWR also works with recirculating fluids, propeller
mixers, and aeration tanks.
High temperatures and pressures—GWR performs well in temperatures
up to 315°C and can withstand pressures up to 580 psig.
Fine powders and sticky fluids—GWR 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.

41. 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.
Don’t forget that the safest engineered level measurement solution includes
switches for back-up, overfill, low level and dry run protection.

42. Capacitance Level Sensor

46. • 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 Operation
A 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.

48. 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

49. Hydrostatic Head Level Sensor

50. • For decades, DP-type instruments—long before the DP cell—
were used to measure liquid level. Orifice meters, originally
designed to measure differential pressure across an orifice in
a pipeline, readily adapted to level measurement.
• Today’s 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.

52. • 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.

53. 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.

55. Temperature Scales
• Celsius(º C)- common SI unit of relative temp
• K=C +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. R=F +460
Temperature Measurements

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 Bi–Metallic 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.

58. 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:
R=Ro[1 +α(T-To)]

59. Where To=reference temperature
Ro= resistance at the reference
temperature
α=calibration 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.

60. 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 [β(1/T-1/To)]
Where To= reference temperature
β =a calibration constant called the characteristic temperature of the
material

61. 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= α(T1-T2)
Where α is called the Seebeck
coefficient; T1 and T2 is the junction
temperature of metals A and B.

62. Thermocouple Circuit

63. Thermocouple
Configuration
Thermocouple
Data

65. Thermocouple

66. Thermocouple Type, Materials,
Range, Sensitivity

67. 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

68. Light Sensors

69. 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.
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.
The Electromagnetic Spectrum

70. 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.

71. Light Sensors
Light sensor diodes make the
resistance of the circuit
decreases and the current
increases as the
light/illuminance increases, at
constant voltage.
•Used in control of
street lamps
•Used in the
automatic /digital
camera
•Used in the
automotive and
military industry

72. 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

73. 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.

74. The End