Training

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Level Measurement
Instrument training material
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Aim

• Knowing the different type of level measuring
  instrument.
• Knowing the basic operating principle of
  different type of level measuring instrument.

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Manual Level Measurement Methods
Method Description Uses
Dipstick manual method of detecting level. Measurements of liquid with slow changes in level. (e.g. Oil Sump).
Hook Gauge another form of Dipstick method providing greater level accuracy. Measurement of liquids as with the Dipstick where smaller changes in level are required to be measured.
Sight glass. the liquid level is directly displayed against a scale. Liquid level measurement of vessels where level is not required to be recorded.
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Electrical Level Measurement Methods
Method Description Uses
Pressure differential pressure between the surface and the bottom of the liquid. Can be used for measurement of liquids in open or closed vessels.
Bubbler uses a pipe submersed into the liquid (to the bottom of the vessel). This pipe is connected to a pressure transducer and constant air supply. The pressure instrument measures the level, the higher the level the greater the back pressure on the pressure transducer. Liquids and slurries (including corrosive liquids).
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Electrical Level Measurement Methods
Method Description Users
Weight mounting the vessel onto weigh scales or load cells. greater the level the greater the weight. used for liquids (including corrosive), slurries and solids.
Conductive measure level giving discrete readings of the level. uses two electrodes, one immersed into the liquid the second to the switching point. The liquid must be conductive, when the level covers both probes a current flows between the electrodes. The current flowing is detected providing the level switch. Used to switch at level points giving discrete signals at level points (e.g. High and Low Level detection).
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Electrical Level Measurement Methods
Method Description Users
Capacitance two cylinders that are mounted vertically in the tank. These act as the plates of two capacitors in parallel, one with air between the plates and the second with the liquid. As the level changes the total capacitance is changed. Level measurement of liquids (including corrosive and high temperatures) can be achieved accurately with the capacitance gauge.
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Electrical Level Measurement Methods
Method Description Users
Ultrasonic uses a transmitter and receiver. waves at approximately 20kHz. waves reflect off the material the level is being measured. time it takes for the emitted waves to be reflected and detected by the receiver. The higher the level the quicker the waves are reflected to the receiver. used for liquids (including corrosive) and solids.
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Level Sensing Family

• Force
• Diaphragm
• Weighing
• Buoyancy
• Pressure
• Hydrostatic head
• Bubbler
• Differential pressure

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Hydrostatic Head Level Measurement
Open Tank
URV LRV Span
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Hydrostatic Head Level Measurement
The same hydrostatic effect as the Open Tank
because the LP Side of the transmitter is
equalized to the vapor pressure of the process
liquid.
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Hydrostatic Head Level Measurement
Closed System - Wet Leg
Both LP HP Side must be full of the liquid
(wet). Can be the same process liquid or other
higher density liquid, normally water or glycol.
Seal Pot is used to maintain correct level of
seal liquid.
URV LRV Span
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• Applications
• Extreme hot and cold temperatures
• Corrosive applications
• Clogging or solidifying
• Sanitary requirements

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Hydrostatic Head Level Measurement
Example An open tank containing water (density
1000 kg/m3) with the minimum level at 0.125 m
above the tapping point and the maximum level at
2 m above the minimum level point. Assume that
the transmitter is located 0.25 m below the
tapping point.
Span (1000)(9.81)(2000)/100000 196.2 mBar
LRV (1000)(9.81)(125)/100000
(1000)(9.81)(250)/100000 36.8 mBar
URV 36.8 196.2 233 mBar
Range 36.8 to 233 mBar
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Hydrostatic Head Level Measurement
Span (850)(9.81)(2000)/100000 166.8 mBar
LRV 0 (1050)(9.81)(1000)/100000 -
(1050)(9.81)(4000)/100000 103.0 - 412.0
-309.0 mBar
URV -309.0 166.8 -142.2 mBar
Range -309.0 to - 142.2 mBar
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LEVEL DISPLACEMENT TORQUE TUBE PRINCIPLE

• Changes in liquid level or density are
  transmitted from the displacer through the torque
  tube to the transmitter.
• Input 0 - 100 Level

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LEVEL DISPLACEMENT TORQUE TUBE PRINCIPLE

• A Hall-effect sensor converts his rotary motion
  to an electronic signal.
• Changes in magnetic field produces a signal
  proportional to torque tube movement.
• Output 4 - 20 mA

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Level Displacement Torque Tube Principle

• For pneumatic instruments, the torque tube
  rotation is converted to 20 - 100 kPa output
  signal by flapper-nozzle mechanism.

• On-board electronic circuit produces analog
  output signal proportional to the liquid level.
• Output 4 - 20 ma.

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Level Displacement Torque Tube Principle

• In smart LT,
• the analog signal from hall effect sensor is
  converted into an error-free digital signal
• processed by the on-board micro-controller
• the digital result is converted to analog output
  signal
• 4 - 20ma DC signal

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Displacement Type Level Measurement
Fisher 2390 Series Electronic Transmitter
Masoneilan 12300 series, 2-wire, loop-powered
Smart Level transmitter or Controller with HART
Communication
Pneumatic Level Transmitter or Controller
All operates according to the fully proven liquid
displacement and torque tube principle...
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Take a Break
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Capacitance Level Measurement

• The probe is one plate of the capacitor and the
  tank wall is the other plate (ground reference),
  therefore the area of the conductive plates (A)
  and the distance (D) are practically constant.
  The variable is the dielectric (E) of the
  insulating material that separates the plates.

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Capacitance Level Measurement

• As a tank is filled with media, the amount of
  capacitance being generated between the probe and
  the tank wall increases.
• The capacitance change, corresponding to the
  rising or falling of media in a tank, is
  converted into a pulse wave form proportional to
  the change in level.
• The amplifier then converts the pulse signal into
  a proportional 420 ma output signal, or in case
  of a point level device yields a contact output
  change.

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Float Type Level Switch

• buoyancy principle - "a body (float) immersed in
  a liquid is buoyed upward by a force equal to the
  weight of the displaced liquid".
• normally used for narrow level differential
  applications such as high level alarm or low
  level alarm.
• process isolation using non-magnetic enclosing
  tube
• top mounted, side mounted, or external cage.
• Switch mechanism - mercury, dry contact, or
  pneumatic.

• A permanent magnet (1) is attached to a
• pivoted switch actuator (2). As the float (3)
• rises following the liquid level, it raises the
• attraction sleeve (4) into the field of the
• magnet, which then snaps against the
• non-magnetic enclosing tube (5), actuating the
  switch.

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Magnetic Level Gauge
Visual liquid level indication has been dominated
by sight glasses for many years. During the past
ten years, magnetic coupled liquid level
indicators have gradually been replacing sight
glasses as the preferred visual level
indicator. The appeal of this type of level
indicator is total isolation of the process
within a sealed piping column. Elimination of
leaking seals, clouded glasses, broken glass
tubing, plus easy access cleaning and adaptation
to a variety of mounting styles and process
connections. Availability of switches and
transmitters adds to their desirability by
providing a much less expensive system than a
build up of sight glasses, alarm switches and
transmitters, all separately piped to the vessel.
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Magnetic Level Gauge
A single high strength magnet assembly is
centered inside the float at the level set point.
It is allowed to rise and fall with level change
within a non-magnetic piping column. A visual
indicator is strapped to the outside of the
column, totally isolated from the process liquid.
With rising level the flags rotate, changing
color. Each flag contains an alignment magnet
which reacts to the float magnet and also
protects against false actuation. The indicator
will always signal true level without field
calibration of the visual indicator. Alarm
switches and transmitter react to the float's
magnetic field through a similar magnetic
coupling. Field calibration of set point is
achieved by positioning the switch/transmitter on
the piping column at the desired set points.
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Vibrating Rod (Tuning Fork) Level Measurement

• The principle.
• Detecting the dampening effect when solids are
  brought into contact with the vibrating rod.
• This vibration is created by exciting the
  transmit piezoelectric crystal attached at the
  base of one rod.
• The piezoelectric crystal converts electrical
  energy to mechanical (vibratory) energy.

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Vibrating Rod (Tuning Fork) Level Measurement

• The principle. Cont
• The vibratory energy from one rod is transmitted
  to the other rod, setting it into vibration, and
  establishing resonance vibration between the
  rods.
• In free space, both the rods vibrates at their
  natural frequency.
• When solids come into contact with the rod, the
  vibration is dampened, decreasing the electrical
  signal, providing relay actuation.

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Vibrating Rod (Tuning Fork) Level Measurement

• The vibration,
• Detected by the receive piezoelectric crystal,
• Converted to low level electrical energy.
• The electronics receive the low voltage signal
  from the receive crystal and amplify it.
• The amplified signal is compared to the set point
  in the comparator circuit which determines relay
  operation.
• The signal is then filtered to remove unwanted
  frequency
• It is amplified and sent to the transmit crystal
  to create the vibration.

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Ultrasonic Level Measurement

• A non conductive type transmitter.
• The transmitter (electronics) sends an electrical
  signal to the transducer sensor crystal, which
  causes the crystal to vibrate and emit an
  ultrasonic pulse.
• The sound pulse is directed toward the liquid
  surface where it is reflected as an echo back to
  the transducer, again causing it to vibrate.
• The electronics detects when the return echo is
  received, and converts the time interval into a
  distance.

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Ultrasonic Non-contact Level Measurement

• Ultrasonic level measurement utilizes the simple
  equation.

• D distance .
• t function of the time required for an
  ultrasonic pulse to travel at the speed of
  sound(Va) from the face of the transducer to the
  reflecting surface and back to the transducer.
• The instrument is, therefore, performing a timing
  function to determine the level.

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Ultrasonic Non-contact Level Measurement

• As the level of the media moves, the time
  interval for the signal return also changes.
• The transmitter converts this time interval to a
  direct reading of level.
• Condition for measurement
• The transducer is mounted above the maximum level
  of the.
• Media to be measured with the ultrasonic pulse
  directed at the surface of the media.

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Radar Level Measurement

• Transmitting high frequency GHz electromagnetic
  radiation
• Time the transit time to from level surface
• Similar to ultrasonic instrument
• Not affected by sound where ultrasonic is

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Radar Level Measurement

• The signal strength reflected back from liquid
  surface to instrument is directly related to the
  dielectric constant of the liquid.
• Liquid with low dielectric reflect very little of
  the signal.
• Non-contact type will have energy loss on the
  return.
• Guided-wave radar is based on Time Domain
  Reflectometer (TDR) technology which use pulses
  of electromagnetic energy that transmitted down
  the probe tube.

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The End of Level

• Temperature to come..

See You.