Instrumentation measurement principles

Instrumentation & Control:
The art and science of measurement and control of process variables within a production
or manufacturing area.
Process Variables Pressure
Temperature
Flow
Level
Density
Humidity
Force
PH
…
Speed

Pressure:
Is the ratio of force to the area over which that force is distributed.
Type of Pressure
Measurement
Absolute Pressure Measurement
Gauge Pressure Measurement
Differential Pressure Measurement
Perfect Vacuum
Atmospheric Pressure
Gauge Pressure
Absolute Pressure
Gauge

Pressure:
Sensors of Pressure
Measurement
Manometer
Bourdon Tube
Bellows
Piezoelectric
Piezoresistive (Strain Gauge)
Capacitive
Unit Conversion:
Diaphragm

Pressure/Manometric Measurement:
Is based on pressure ability to displace a column of a
liquid in a manometer (Pressure head calculations)

Pressure/Manometric Measurement:
Manometers
Fluid
Mercury (Hg)
Water (H2O)
Application
Higher Density –Small Manometers
Toxic
Availability
Non-Toxic
Low Density-Accuracy
Blood Pressure
Lung Pressure
Fluid Pressure in Pipeline
Disadvantages Low Range
Capillary Rise
Slow Response to Pressure Fluctuations
Min 15mm diameter

Pressure/Inclined Manometer:
For low pressure measurement or high accuracy applications.

Pressure/Bourdon Tube Measurement:
Is based on the principle that a flattened tube tends to straighten or regain its
circular form in cross-section when pressurized
C-Shape
Helical
Spiral
Twisted
Bourdon Tube
Types

Pressure/Bellows:
Are elastic vessels that can be compressed when pressure is applied to the outside
of the vessel, or extended under vacuum. When the pressure or vacuum is released,
the bellows will return to its original shape.

Pressure/Diaphragm:
Pressure exerted by the fluid causes elastic deflection of the diaphragm.

Pressure/Piezoelectric:
Piezoelectric materials produce electricity charge when exposing to mechanical
stress.

stress.
Advantages
-Very high frequency response and sensitivity.
-Self generating, so no need of external source.
-Simple to use, small dimensions, large measuring range, low cost.
Disadvantages
-It is not suitable for measurement in static condition.
-Affected by temperature variation.
- Exceptional linearity and repeatability.

stress.
Applications - Ultrasonic transmitters and receivers.
- Frequency references.(Crystal Oscillators)
- Temperature sensors (resonant frequency changes with temperature)
- Accelerometers
- Microphones and loudspeakers (small loudspeakers with poor audio
characteristics =beepers)
- Pressure sensor
- Force sensor

Pressure/Piezoresistive (Strain Gauge):
The resistance of Piezoresistive sensors (either metal or semiconductors types)
changes when pressure is applied.

Advantages
-Bond excellently to most surfaces, Rugged, Small size
-High frequency response
-High linearity, Low impedance
Disadvantages
-Strain gauge grid expands and contracts at a different rate than
the surface it is attached to
-Compared to piezoresistive sensors strain gauges have much lower
sensitivity
- Can be wrapped around curved surfaces unlike the piezoresistor
Applications - Mechanical Engineering
- Load cell
- Tactile sensors in robots

Pressure/Capacitive:
Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain
due to applied pressure.

Temperature:
Is a comparative objective measurement of hot and cold.
Sensors of
Temperature
Measurement
Thermocouple (TC)
Resistance Temperature Detector (RTD) (TE)
Bimetal
Filled Thermal Systems
Optical
Unit From Celsius To Celsius
Fahrenheit [F°] = [C°] × 9⁄5 + 32 [C°] = ([F°] − 32) × 5⁄9
Kelvin [K°] = [C°] + 273.15 [C°] = [K°] − 273.15
Thermistor
IC Sensors

Temperature/Thermocouple:
Consists of two dissimilar conductors (or semiconductors) that contact each other at one
or more spots, where a temperature is experienced. It produces a voltage when the
temperature of one of the spots differs from the reference temperature at other parts of the
circuit.
Thomas Johan Seebeck
Seebeck effect:
The temperature difference between hot and cold junctions
produces an electric potential (voltage) which can drive an
electric current in a closed circuit.

Temperature/Thermocouple:
Advantages
Temperature-Voltage Curve
Self Powered
Simple, Rugged
Inexpensive
Disadvantages
Wide Variety
Wide Range
Non Linear
Low Voltage
Reference Required
Least Stable
Least Sensitive vs. Temperature Change

Temperature/RTD:
Are sensors, used to measure temperature by correlating the resistance of the RTD
element with temperature. They are typically platinum, copper or nickel.
RTD Main
Categories
Thin Film
Wire-wound
Coil Elements

Temperature/RTD:
RTD
Configurations
Two-wire configuration
Three-wire configuration
Four-wire configuration

Temperature/RTD:
Advantages
High Accuracy
High Linearity
Wide Operating Range
Disadvantages
Self Heating
Slow Response Time
Power Source Required

Temperature/Thermistor:
A thermistor`s output is based on the resistance change in a metal-oxide semiconductor
material as its temperature changes.
Thermistors
Type
NTC (Negative Temperature Coefficient)
PTC (Positive Temperature Coefficient)
Advantages
High output
Fast response time
Low cost
Accurate over small ranges
Disadvantages
Non Linear
Limited Temperature Range
Power Source required
Self Heating

Temperature/IC Sensors:
The newest type of temperature sensor to be developed is the integrated circuit (IC)
temperature transducer.
Advantages
Most Linear
High Output
Inexpensive
Disadvantages
Temperatures limited to 200 degrees C
Power supply required
Slow response time
Self Heating

Temperature/Bimetal (Thermostat):
Refers to an object that is composed of two separate metals joined together, which
converts a temperature change into mechanical displacement.

Temperature/Bimetal:
Refers to an object that is composed of two separate metals joined together, which
converts a temperature change into mechanical displacement.

Temperature/Filled Thermal Systems:
Consists of a liquid enclosed in a tube. The volume of the fluid changes as a function of
temperature.
Advantages
Powerless
Not Hazardous for Explosive Atmosphere
Stable, High Repeatability
Disadvantages
Only Visual
Low Accuracy
Slow response time

Temperature/Optical Systems:
These sensors measure electro magnetic radiations related to temperature.
Optical
Sensors
Optical pyrometer
IR Thermometer

Flow:
Is volume/mass of fluid which passes through cross section per unit time.
Methods of
Flow
Measurement
DP Flowmeters
Velocity Flowmeters
Mass Flowmeters
Positive Displacement Flowmeters

Flow:
DP Flowmeters
Orifice Plate Flowmeters
Venturi Tube Flowmeters
Flow Nozzle Flowmeters
Pitot Tube Flowmeters
Annubar Flowmeters
Elbow Flowmeters
Bernoulli's Principle: For an inviscid flow of a nonconducting fluid, an increase in the
speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the
fluid`s potential energy .
Daniel Bernoulli

Flow/Orifice Plate Flowmeters :
This sensors measure differential pressure
caused by orifice plates. The differential
pressure is related directly to flow.
Characteristics:
- Turn down ratio less than 5:1
- Poor accuracy at low flow rates
- Accuracy depend on orifice plate shape
- Plugging with slurries
- High pressure drop
- Low cost
Flow/Venturi Tube Flowmeters :
caused by Venturi Tube. The differential
pressure is related directly to flow.
Characteristics:
-Lower pressure drops
- Higher turn down ratio 10:1
- High cost
- No slurry plugging

Flow/Flow Nozzle Flowmeters :
This sensors measure differential pressure caused
by flow nozzle. The differential pressure is related
directly to flow.
Characteristics:
- Mostly air and gas flow measurement
- Turn down ratio and accuracy like orifice
- Intermediate pressure loss
- Higher cost than orifice plate
- Good for slurry services
Flow/Pitot Tube Flowmeters :
between stagnation pressure and static
pressure
Characteristics:
- Low pressure low
- Poor performance with slurry services

Flow/Flow Annubar Flowmeters :
This sensors measure differential pressure between
stagnation pressure and static pressure
Characteristics:
-Less pressure drop than orifice plate (1/25)
-Less clogging due to fluid flow direction around
meter
Flow/Elbow Flowmeters :
Base on Fluid centrifugal force that is related
directly with its speed.
Characteristics:
- Low cost
- Very poor accuracy
- Low pressure loss

Flow:
Velocity
Flowmeters
Vortex Flowmeters
Turbine Flowmeters
Ultrasonic Flowmeters
Magnetic Flowmeters
Variable Area Flowmeters (Rotameter, Vane-Style)

Flow/Vortex Flowmeters :
This measurement involves placing a bluff body (called a shedder bar) in the path of the
fluid. As the fluid passes this bar, disturbances in the flow called vortices are created. The
frequency at which these vortices alternate sides is essentially proportional to the flow rate
of the fluid. a sensor measures the frequency of the vortex shedding. This sensor is often a
piezoelectric crystal, which produces a small, but measurable, voltage pulse every time a
vortex is created. Since the frequency of such a voltage pulse is also proportional to the
fluid velocity, a volumetric flow rate is calculated using the cross sectional area of the flow
meter.
Characteristics:
- Not affected by pipe vibration, pressure surge, temperature shocks
-Low pressure loss
-High accuracy and repeatability

Flow/Turbine Flowmeters :
Turbine flowmeters use the mechanical energy of the fluid to rotate a “pinwheel” (rotor) in
the flow stream. The rotor shaft spins on bearings. When the fluid moves faster, the rotor
spins proportionally faster. 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.
Characteristics:
-Less accurate at low flow rates
- Not suitable for dirty fluids
- High repeatability and accuracy
- Fast response
- Expensive

Flow/Ultrasonic Flowmeters :
Measures the velocity of a fluid with ultrasound to calculate volumetric flow.
Using ultrasonic transducer, 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.
1- Transit time (1~2MHz):
- Measures the upstream and down
stream time
- Linear, accurate, wide measuring
span, high repeatability
- No pressure drop
- Low cost

Flow/Ultrasonic Flowmeters :
Measures the velocity of a fluid with ultrasound to calculate volumetric flow.
Using ultrasonic transducer, 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.
2- Doppler effect (640kHz~1MHz):
Is the change in frequency of a wave
for an observer moving relative to its
source.
Characteristics:
-Needs 100 PPM or more particles in fluid
-Particles must be large enough (>λ/4)
- Particle velocity often differs noticeably
from the velocity of the liquid.
-The velocity needs to be far higher than
the critical velocity at which particles
settle.

Flow/Magnetic Flowmeters :
Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction to determine the
flow of liquid in a pipe. In a magnetic flow meter, a magnetic field is generated and
channeled into the liquid flowing through the pipe. Following Faraday’s Law, flow of a
conductive liquid through the magnetic field will cause a voltage signal to be sensed by
electrodes located on the flow tube walls. When the fluid moves faster, more voltage is
generated. Faraday’s Law states that the voltage generated is proportional to the movement
of the flowing liquid.
Characteristics:
- No Moving Parts
- Very Wide Rangeability
- Ideal For Slurries
- Unobstructed Flow Path
- Liquid Must Be Conductive

Flow/Variable Area Flowmeters (Rotameter) :
Variable area flowmeters measure flow by allowing the flow stream to change the opening
within the flow meter by moving an internal part. When the flow increases, the fluid
generates more force and moves the internal part farther. Spring-opposed float designs
allow this type of flow meter to be installed in horizontal pipes, because the functioning of
the float is not dependent upon gravity.
Characteristics:
- No external power source
- Not suitable for slurry fluids
- Not expensive
- Approximately linear
- Must always be vertically oriented
unless it be a Vane-Style or Piston
type
Vane-Style

Flow/Variable Area Flowmeters (Rotameter) :
Variable area flowmeters measure flow by allowing the flow stream to change the opening
within the flow meter by moving an internal part. When the flow increases, the fluid
generates more force and moves the internal part farther. Spring-opposed float designs
allow this type of flow meter to be installed in horizontal pipes, because the functioning of
the float is not dependent upon gravity.
Bypass Design:
- Rotameters are not generally
manufactured in sizes greater than 6
inches /150 mm, but bypass designs are
sometimes used on very large pipes.

Flow:
Mass
Flowmeters
Coriolis Flowmeters
Thermal Flowmeters
Mass Flow Rate (m) : In physics and engineering, mass flow rate is the mass of a
substance which passes per unit of time. (Kg/h)

Mass Flow Meters/ Coriolis:
The Coriolis mass flow meter uses a tube that is designed to vibrate up and down due to
angular momentom of fluid at its natural frequency while all of the fluid flows through
it. A strong magnet is used to make the tube vibrate.
Characteristics:
- High accuracy and repeatability
- No obstructions in the fluid path
- Suitable for applications where
temperature and pressures fluctuate
-Suitable for Custody transfer

The Coriolis mass flow meter uses a tube that is designed to vibrate up and down due to
angular momentom of fluid at its natural frequency while all of the fluid flows through
it. A strong magnet is used to make the tube vibrate.

Coriolis
Flowmeter
Components
Drive Coil
Pick off Sensor
Tube
RTD
Processor
Flow Splitter
Case
Process Connection

Mass Flow Meters/ Thermal:
Thermal flowmeters use the thermal properties of the fluid to measure the flow of the
fluid in a pipe or duct. In a typical thermal flow meter, a measured amount of heat is
applied to the heater of the sensor. Some of this heat is lost to the flowing fluid. As flow
increases, more heat is lost. The amount of heat lost is sensed using temperature
measurements in the sensor.
Characteristics:
- No flow rate limitations
- Excellent turn down ratio, typically
50:1
- No moving parts
- Relatively high initial cost
- Suitable for applications where
temperature and pressures fluctuate

Flow / Positive Displacement Flow Meters:
Positive
Displacement
Flowmeters
Positive Displacement (PD) Flow meters are volumetric flow measurement instruments
that measure flow by passing a precise volume of fluid with each revolution.
Characteristics:
- No flow rate limitations
- Not affected by flow viscosity,
density and turbulence
Gear Flowmeters
Piston Flowmeters
Helical Flowmeters

Level:
Refers to instrumentation techniques designed to measure the height of a fluid or solid
within a containing vessel.
Methods of
Level
Measurement
Manual / Mechanical
Electro Mechanical Contacting
Electronic Non-Contacting

Level / Manual or Mechanical Level Measurement:
Methods
Float Systems
Rod Gauging / Dip Probe / Dip Stick
Sight / Gauge Glass
Tape Systems

Level / Manual or Mechanical Level Measurement:
Level
Gauges
Transparent
Reflex
Bi-Colour
Magnetic

Level / Electro Mechanical Contacting Level Measurement:
Methods
Displacer
Magnetostricitive
Servo Operated Displacer
Resistance Tape (Metritape)
Archimedes' principle :
Archimedes' principle indicates that the upward buoyant force that is exerted
on a body immersed in a fluid, whether fully or partially submerged, is equal
to the weight of the fluid that the body displaces.
Conductivity
Capacitance
Hydrostatic Pressure Level Meter
Optical
Vibration Fork Level Switch

Level / Displacer Level Meter:

Level / Magnetostricitive Level Meter:
Principle :
A low-current interrogation pulse is generated in the
transmitter electronics and transmitted down the
waveguide creating an electromagnetic field along
the length of the waveguide. When this magnetic
field interacts with the permanent magnetic field of a
magnet mounted inside the float, a torsional strain
pulse, or waveguide twist, results. This waveguide
twist is detected as a return pulse. The time between
the initiation of the interrogation pulse and the
detection of the return pulse is used to determine the
level measurement with a high degree of accuracy
and reliability.

Level / Metritape Level Meter:
The outer envelope jacket (1) is compressed by the
hydrostatic pressure of the liquid (2). This causes the gold
wire winding to contact the gold plated base strip. The
resulting change in the resistance of the gold wire (3)
indicates the length of active helix and the distance from
sensor top to liquid surface.

Level / Servo Operated Displacer Level Meter:
The displacer is suspended from a
strong and flexible measuring wire
wound on a measuring drum. A
transducer measures the apparent
weight of the displacer partly
immersed in liquid. When the level
starts moving downwards, the
transducer will sense the change in
weight. The servo motor drives the
measuring drum to unwind
the measuring wire until the displacer
is partly immersed in liquid. When the
level rises, the servo motor drives the
measuring drum to wind up the
measuring wire until the displacer is
again partly immersed in liquid.

Level / Conductivity Level Meter:

Level / Capacitance Level Meter:

Level / Optical Level Meter:
The sensor contains an infrared light emitter or LED, and a photocell receiver. Light is
transmitted across a gap to the receiver. As the sensor is lowered into the clarifier
sludge blanket, the instrument will indicate a gradual decrease in opacity.

Level / Hydrostatic Pressure Level Meter:
Methods
Differential Pressure Measurement
Bubbler Systems

Level / Vibration Fork Level Switch:
When the service material covers the tines,
they cause damping of the vibrations. This
stoppage of vibrations is sensed by the
electronic circuitry and the signal after
processing is used to operate a relay.

Level / Electronic Non-Contacting Level Measurement:
Methods
Radar
Ultrasonic
Load Cell
Laser
Radiometric

Level / Radar Level Measurement:
Characteristics:
- Needs high dielectric constant
- High accuracy
- Not affected by ambient conditions
- Very expensive
- Restricted pressure rating

Level / Guided Wave Radar Level Measurement:
Characteristics:
- 20 times more efficient than Radar
level transmitter due to more focused
energy path
- Can measure level for liquids with
dielectric constant less than 1.4
TDR (Time Domain Reflectometry)

Level / Ultrasonic Level Measurement:
When ultrasonic pulse signal is targeted towards an object, it is reflected by the object
and echo returns to the sender. The time travelled by the ultrasonic pulse is calculated,
and the distance of the object is found.
Characteristics:
- No mechanical movement and no contact
- High accuracy
- Affected by air quality and temperature

Level / Laser Level Measurement:
Operates on a principle very similar to that
of ultrasonic level sensors. Instead of using
the speed of sound to find the level,
however, they use the speed of light.
Characteristics:
- No beam spread, can be targeted on a point
- Good for bulk, slurry and opaque fluids
-Accurate even in vapor and foam environments
- Very Expensive

Level / Load Cell Level Measurement:
Is a transducer that is used to create an electrical signal whose magnitude is directly
proportional to the force being measured.
Characteristics:
- Requires support structure
- Poor turn down ratio
- Very Expensive

Level / Radiometric Level Measurement:
It employs a radioactive source(usually Cesium-137 or Cobalt-60 isotopes) which emits
gamma radiation that passes through the walls of the pipe or vessel containing the medium
being monitored. A detector is mounted on the opposite side of the container which senses
the radiation that is not absorbed by the medium and is directly related to the parameter
being measured.
Characteristics:
- No vessel penetration is needed
- suitable for hazardous and corrosive materials
- Very Expensive

Instrumentation measurement principles

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