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PEC – UET Lahore Dr. Shahid Naveed 14. August 2013 / Dr. –Ing Naveed Ramzan 1


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Title: PEC – UET Lahore Dr. Shahid Naveed 14. August 2013 / Dr. –Ing Naveed Ramzan 1

Industrial Instrumentation
Dr. Ing. Naveed Ramzan
Pressure Sensors
  • In any given plant, the number of pressure
    gauges used is probably larger than all other
    instruments put together

Practical Considerations in Pressure Measurements
  • In industrial applications, good repeatability
    often is more important then absolute accuracy.
  • If process pressures vary over a wide range,
    transducers with good linearity and low
    hysteresis are the preferred choice.
  • Ambient and process temperature variations also
    cause errors in pressure measurements,
    particularly in detecting low pressures and small
    differential pressures. In such applications,
    temperature compensators must be used

Practical Considerations in Pressure Measurements
  • Selection Criteria
  • Pressure transducers usually generate output
    signals in the millivolt range (spans of 100 mV
    to 250 mV). When used in transmitters, these are
    often amplified to the voltage level (1 to 5 V)
    and converted to current loops, usually 4-20 mA
    dc. The transducer housing should be selected to
    meet both the electrical area classification and
    the corrosion requirements of the particular
  • If the installation is in an area where
    explosive vapors may be present, the transducer
    or transmitter and its power supply must be
    suitable for these environments. This is usually
    achieved either by placing them inside purged or
    explosion proof housings, or by using
    intrinsically safe designs.
  • The single most important decision in selecting a
    pressure transducer is the range
  • When high process temperatures are present, one
    can consider the use of various methods of
    isolating the pressure instrument from the
    process. These include loop seals, siphons,
    chemical seals with capillary tubing for remote
    mounting, and purging.

Practical Considerations in Pressure Measurements
Maintenance Without exception, pressure sensors
require scheduled, periodic maintenance and/or
Practical Considerations in Pressure Measurements
  • Calibration
  • Pressure transducers can be recalibrated
  • 1. on-line or
  • 2. in a calibration laboratory.
  • Laboratory recalibration typically is preferred,
    but often is not possible or necessary.
  • In the laboratory, there usually are two types of
    calibration devices
  • deadweight testers that provide primary,
    base-line standards, and
  • laboratory or field standard calibration devices

Self Assessment Test
Question No. 1
  • The difference between gauge and absolute
    pressure is
  • a Vacuum
  • 0.433 psia
  • atmospheric pressure
  • zero

Question No. 2
  • Pressure is defined as
  • Force per unit area
  • F. A
  • A/F
  • None of the above

Question No. 3
  • Metals used in the bourdon tubes should not
  • Fatigue
  • hysteresis
  • creep
  • All of the above

Question No. 4
  • A capsule diaphragm is made by welding two
  • to a solid base
  • together at the center
  • together around the edges
  • To two other diaphragms

Question No. 5
  • Dead weight tester is used for
  • testing dead weights
  • measuring process pressures accurately
  • producing high pressure
  • calibrating pressure instruments

Question No. 6
  • One torr is defined as
  • One mm Hg
  • One inch Hg
  • One atmosphere
  • One kilopascal

Question No. 7
  • Which gauge measures pressure by sensing changes
    in the thermal conductivity of the gas
  • Pirani gauge
  • Slack diaphragm gauge
  • Mcleod gauge
  • None of them

Question No. 8
  • A thermocouple gauge is one type of
  • Ionization gauge
  • Thermal conductivity gauge
  • Mcleod gauge
  • None of these

Question No. 9
  • Diaphragms used in pressure applications are
  • light
  • Small in size
  • slack
  • bimetallic

Question No. 10
  • A Mcleod gauge can measure pressure as low as
  • 0.05 torr
  • 0.005 torr
  • 0.0005 torr
  • 0.00005 torr

Question No. 10
  • What type of manometer is best for measuring low
  • Well
  • Inclined
  • U-Tube
  • Multiple tube

Question No. 11
  • A capacitance pressure transducer indicates
    changes in pressure by changing the
  • Voltage output of an ac circuit
  • frequency
  • capacitance
  • alternating current

Home Work
  • Make similar 5 MCQs from
  • Temperature and Pressure Sensors by each member.
  • Solve Them and attached the sheet

A Quick View! What we have covered?
  • Total Lectures held 9
  • Fundamentals of Electrical Technology and
  • digital logic employed in the measurement
  • Review of Scientific principles employed in
  • Parts of Instrument
  • Performance Characteristics of Instruments
  • Selection and Calibration of Instruments
  • Instruments Identification and Line Symbols
  • Principle measurements desired in industry
  • (a) Temperature
  • (b) Pressure, Load
  • (c) Level
  • (d) Flow
  • (e) Others ( Weight, Composition, pH etc.)
  • (f) Transducers
  • Installation and Installation Costs
  • Case Studies
  • Quiz Held 1 Next Test due on

Level Sensors
Level Measurement
  • Level is another common process variable that is
    measured in many industries. The method used will
    vary widely depending on the nature of the
    industry, the process, and the application.

Inventory -- a constant supply or storage of
material Control -- continuous, batch,
blending, and mixing control -- stabilize flow to
the next process Alarming -- hi/lo limits,
safety shut down Data Logging -- material
quantities for inventory and billing purposes and
where regulatory requirements are necessary
What is measured?
  • The measured medium can be liquid, gas or solid
    and stored in vessels (open/closed tanks), silos,
    bins and hoppers.
  • Units of level can be expressed in
  • feet (meters)
  • gallons (liters)
  • pounds (kilograms)
  • cubic volume (ft3, m3)

Methods ---- Direct or Indirect (inferential)
  • Hydrostatic Head
  • Float
  • Load Cells
  • Magnetic Level Gauge
  • Capacitance Transmitters
  • Magnetostrictive
  • Ultrasonic
  • Microwave
  • Laser
  • Radar
  • Guided Wave Radar
  • Dip Stick
  • Vibration

Direct Methods
  • Direct methods sense the surface or interface of
    the liquid and is not affected by changes in
    material density (Specific Gravity)
  • Examples
  • Dip Stick
  • Resistance Tapes
  • Sight Glass
  • Floats
  • Ultrasonic

Indirect Methods (Inferential)
  • Indirect methods infer liquid level by
    measuring some other physical parameter such as
    pressure, weight, or temperature.
  • Changing materials means a corrective factor must
    be used or recalibrating the instrument.
  • Examples
  • Hydrostatic head methods
  • Load Cells
  • Capacitance
  • Conductivity

Selection Criteria
  • When determining the type of level sensor that
    should be used for a given application, there are
    a series of questions that must be answered
  • Open tank or closed tank?
  • Can the level sensor be inserted into the tank or
    should it be completely external? Contact or
  • Continuous measurement or point measurement?
  • Direct or Indirect measurement?
  • What type of material is being measured? Liquid
    or Solid? Clean or Slurry?

Selection Criteria
  • For all liquids you will need
  • The system operating temperature with max. and
    min. excursions?
  • two wide range expensive the sensor
  • The system operating pressure?
  • Check that system T and P do not conflict
    with the materials of construction?

Selection Criteria
  • For Solids
  • Bulk density
  • Be careful with very large silos as
    compaction at the bottom can greatly change
    assume bulk densities
  • Flow characteristics?
  • Expected particle size distribution?
  • Is solid abrasive and/or corrosive and what is
    the moisture/solvent content?

For Liquids
Dip Stick
  • Simple and cheap
  • Can be used with any wet material and not
    affected by density.
  • Can not be used with pressurized tanks
  • Visual indication only (electronic versions are

RodGauge - similar to a dipstick found in a car,
it has weighted line markings to indicate depth
or volume
For Liquids
Sight Glass
  • Another simple direct method of measuring
  • Can be used in pressurized tanks (as long as the
    glass or plastic tube can handle the pressure)

Good for applications where non-contact
measurement is needed (like beverages)
For Liquids
  • Float rides the surface level to provide the
    measurement. Many different styles are available.
    Usually used for pump control, high/low level
    alarms and emergency shut-off

Liquid density does not affect measurement
For Liquids
Conductivity Level Measurement
Point Level Measurement
Continuous Level Measurement
Advantages and disadvantages Low
Cost Conductive, non-coating liquids
only Insulating coatings can cause problems
For Liquids
Resistance Tape
  • The pressure of the fluid in the tank causes the
    tape to short-circuit, thus changing the total
    resistance of the measuring tape. An electronic
    circuit measures the resistance it's directly
    related to the liquid level in the tank.

For Liquids
  • Bubblers allow the indicator to be located
  • The air pressure in the tube varies with the head
    pressure of the height of the liquid.

Cant be used in closed tanks or where purging a
liquid is not allowed (soap). Very popular in the
paper industry because the air purge keeps the
tube from plugging.
For Liquids
  • Advantages
  • -- Easy installation
  • -- Continuous reading providing analogue or
    digital signal
  • -- No moving parts
  • -- Good accuracy and repeatability

For Liquids
  • Limitations
  • -- Not suitable for pressurized tanks
  • -- Sediments may block tube or probe
  • -- Tanks must be freely vented

Hydrostatic Head Level Sensors
  • These methods infer level by measuring the
    hydrostatic head produced by the liquid column.
  • A pressure sensing element is installed at the
    bottom of the tank and pressure is converted to
  • Different liquid densities or closed tank
    applications must be accounted for.

General Theory for Head Measurement
Hydrostatic Head Level Sensors
  • The Pressure exerted by the Height of the liquid
  • P H x Density
  • If the Density of the liquid is known then
  • H Pressure
  • Density

Note For liquids other than water, use the
density of water 0.0361 lb/in3 as a reference and
multiply by the SG of the liquid.
A dip stick measurement of the level of these 2
tanks indicates 30 feet of liquid in both tanks.
Calculate the pressure that each gauge will read
if tank 1 contains water (S.G. 1) and tank 2
contains oil (S.G. 0.85)
P ? psi
P ? psi
A dip stick measurement of the level of these 2
tanks indicates 30 feet of liquid in both tanks.
Calculate the pressure that each gauge will read
if tank 1 contains water (S.G. 1) and tank 2
contains oil (S.G. 0.85)
P ? psi
P H x Density 30 ft x 0.0361 lbs/in3
(30 x 12) x 0.0361 13 psi
A dip stick measurement of the level of these 2
tanks indicates 30 feet of liquid in both tanks.
Calculate the pressure that each gauge will read
if tank 1 contains water (S.G. 1) and tank 2
contains oil (S.G. 0.85)
P H x Density x SG 30 ft x 0.0361 lbs/in3
x 1 (30 x 12) x 0.0361 13 psi
P H x Density x SG 30 ft x 0.0361 lbs/in3
x 0.85 (30 x 12) x 0.0361 x 0.85 11 psi
Practical Considerations when using head type
Hydrostatic Head Level Sensors (Contd)
  • The reference point of the tank vs instrument
    input must be considered.

This may not be practical in some applications
where the tank elevation is below grade or where
a remote visual reading is required.
Tank Elevations
Hydrostatic Head Level Sensors (Contd)
  • Vertical rises and drops contribute to the
    overall height and therefore head pressure.
    Horizontal runs have no effect.

Closed Tank Applications
Hydrostatic Head Level Sensors (Contd)
  • Open tanks are vented to atmosphere so the
    pressure at the bottom of the tank is only due to
    the head pressure of the liquid.

Closed tanks are not vented to atmosphere so the
pressure at the bottom of the tank is due to the
head pressure of the liquid the vapour pressure
above the surface.
Magnetic Level Sensor
  • Used where the sight glass level gauge can not be
  • Magneto-resistive types can provide an electrical

Liquid/liquid interface (such as water and oil)
can be measured by changing the buoyancy of the
magnetic float
  • Not the same as a float.
  • The displacer is immersed in the tank and the
    buoyant force of the liquid produces a torque
    which is proportional the amount of liquid level.

The output force can be converted to provide a
proportional pneumatic or electrical continuous
output of tank level.
Displacer side mounted unit
  • The displacer float size and diameter is selected
    according to the size of tank and the height of
    the level to be measured.
  • The output force can be converted to provide a
    proportional pneumatic or electrical continuous
    output of level.
  • Advantages and disadvantages
  • Very little movement of displacer float
  • Affected by different liquid densities
  • Limited to smaller tanks sizes

UltraSonic Level Measurement
  • Non-Contact direct level sensor
  • Level is a function of the time it takes an
    ultrasonic pulse to hit the surface and return
  • Limitations include
  • Surface foam absorbs signal, agitation create
  • High Pressure High Temperatures affect the
    signal speed
  • Vapour and condensate create false echos

Radar Level Sensors (Microwave)
  • Similar to ultrasonic but at a much higher
    frequency (6.3 GHz)
  • Various designs
  • -- Frequency Modulated
  • Continuous Wave
  • -- Pulsed Wave
  • -- Guided Wave

These sensors have better performance in
applications where vapour, dust or uneven
surfaces exist.
Load Cells
  • Tank level is determined by the weight of the
    quantity of material
  • Load Cells (strain gauge transducers) placed at
    the bottom of the tank measure the weight and
    then convert it to an electrical signal.

Capacitance Level Detection (RF Level Probes)
RF Capacitance level sensors are similar in
operation to conductivity sensors and are
typically used when non-conductive liquids are
The only variable is the dielectric of the tank
material that varies in proportion to the level.
Conductive liquids like mineral water will short
out the probe to the tank wall. Good for solids
Can be used for non-liquids
  • RF capacitance probes are used in a wide range of
    applications and are subjected to certain
    limitations such as
  • Electrode Coating
  • Low sensitivity
  • Temperature
  • Different materials
  • Shielding noise

  • Level is measured by locating the boundary
    between two media, called the interface
  • Level can be measured directly or indirectly
  • Noninvasive devices are preferred when the
    material is corrosive, hazardous, sterile, or at
    a high temperature or pressure

Batch application using point level measurement
Creative Assignment! HAZOP?????
  • Detect if the level is at a predetermined point
  • Generally used to detect either a low-level
    condition or high-level condition
  • The output of a point level sensor is on-off, or
    1 or 0 state digital signal

A batch process does NOT use a continuous level
measurement device.
  • Discussion Questions?

Dr. Shahid Naveed 27. April
2013 / Dr. Ing Naveed Ramzan