ACADs (08-006) Covered
5.4.1.7
Keywords
Calibrate, direct, inferred, transmitter calibration, gravity correction,
density correction, hydrostatic head detector, float.
Description
Supporting Material
Measuring Level
Terminal Objective: Given the appropriate equipment
and procedures the I&C Technician will calibrate and
maintain level instrumentation. Mastery will be
demonstrated by successful completion of Lab
Performance Exercises and written Exam.
• Contrast direct and inferred methods of measuring level,
giving an example of each
• Describe the theory of operation of a given level
measuring device
• Given an example of a hydrostatic head detector, calculate
the transmitter calibration input pressure for a dry
reference leg system using specific gravity and density
corrections
• Given an example of a hydrostatic head detector, calculate
the transmitter calibration input pressure for a dry
reference leg with zero suppression
• Given an example of a hydrostatic head detector, calculate
the transmitter calibration input pressure for a wet
reference leg system
Direct versus Indirect
• Direct: Level is
measured by direct
contact with the fluid
surface. Either the fluid
surface is visible to be
compared directly to a
scale, or direct contact
is made with the fluid
surface.
• Indirect: Level is
inferred by means other
than direct contact with
the Fluid surface. Fluid
level is inferred by its
effect on some type of
device.
Classify as Direct or Indirect
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Dipstick
Float
Sightglass
Float & chain
Bubbler tube
Capacitance
• Pressure or DP
transmitter
• Ultrasonic
• HJTC
• Warrick Conductivity
Level
• Magnetrol
Ways to measure level
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Antenna level sensor
Bubbler
Capacitance probe
Conductivity probe
Diaphragm detector
Differential Pressure
Dipstick
Displacer
Float
Impedance probe
Level gauge
Magnetic Follower
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Optical level
Radiation level sensor
Resistance tapes
Rotating paddles
Sightglass
Slip tubes
Tape level devices
Thermal level sensor
Time domain
reflectometry
• Ultrasonic level detectors
• Vibrating reed switches
Considerations in level measurement
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Atmospheric vs. pressurized vessel
Conductive vs. non-conductive fluid
Temperature, specific gravity of the fluid
Turbulance, foam, fog, dust, or a mixed layer at fluid
surface
Open flow channels
Head correction, range suppression or elevation
Mixed fluids, fluid boundaries, slurries, solids
Continuous monitoring or point level detection
Dipstick
Dipsticks
Pneumatic Dipstick
• Combines a dipstick and a bubbler tube
Float & mechanical linkage
Archimides Principle
• A body wholly or partly immersed in a fluid
is buoyed up with a force equal to the
weight of the fluid displace by the body
What is the difference between a float and a displacer?
Chain Float
Bubbler Tube
Capacitive-Type
Transducers
Measuring level with pressure differential
Reference to Atmosphere
Level Instruments
Calculating Dry Reference Leg Calibration
Values
Calculate zero and span calibration values for
the following example:
• Vented tank (at atmospheric pressure)
• The tank is full when it has 12’ of water at
room temperature
• Pressure tap located 2’ above the bottom
of the tank
• Transmitter 2’ below the bottom of the
tank
Dry Leg Level Instrument
Calculating Dry Reference Leg Calibration
Values
Calculate zero and span calibration values for
the following example:
• Tank pressurized with 2 psi Nitrogen
• The tank is full when it has 30’ of water at
room temperature
• Pressure tap located 1’ above the bottom
of the tank
• Transmitter 4’ below the bottom of the
tank
Wet Reference Leg
Level Instrument
Specific Gravity
Water in our pressurizer
is xxx degrees but the
water in the reference
leg is only xxx degrees.
How does this effect
differential pressure
seen at the level
transmitter?
Compare indicated to
actual level.
What can be done to
compensate?
Wet Reference Leg
Calibration Factors
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Tank level
Reference leg level
Specific Gravity
Pressure
Temperature
• The ASME steam tables can be used to determine
specific gravity for pure water or steam
• http://www.higgins.ucdavis.edu/webMathematica/
MSP/Examples/SteamTable
Pressure = H x Sg x D
• H = height of fluid column
• Sg = Specific Gravity of the fluid
• D = Density
Calculating Wet Reference Leg Calibration
Values
Calculate zero and span calibration values for the
following example:
• Tank pressurized at 2250 pounds with 800 degree
F water and steam mixture
• The tank is full when it has 40’ of water at 800o F
• The tank is empty when it has 12’ of water
• Pressure tap located 1’ above the bottom of the
tank
• Transmitter 8’ below the bottom of the tank
Ultrasonic Level Measurement
How does it work?
• Ultrasonic sound waves are
pulsed from the transducer
about 3 times per second
• The sound waves reflect
against the process medium
and return to the
transducer
• The microprocessor based
electronics measure the
time of flight between
sound generation and
receipt
• This time is translated into
the distance between the
transducer and the process
medium below
Conductivity Probe Method
Capacitance Level Sensor
• Also called RF level sensors
• Uses the process material as the dielectric of a
capacitor
• As the dielectric (level) changes, capacitance
changes
• This change (picofarads) is conveted to a level
signal.
• We use these on RCP oil levels.
Radar Level Instruments
• Sometimes called microwave
• Similar to ultrasonic, but uses 1MHz+ RF
• Can be used in high temperature steam
environments (FW heaters)
• Mist, steam, dust and foam can be ignored
• Non-contact surface level measurement
• Low power (2 wire)
On to Flow Measurement