Level Measurement
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)
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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 non-contact?
• Continuous measurement or point measurement?
• Direct or Indirect measurement?
• What type of material is being measured? Liquid or
Solid? Clean or Slurry?
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
available)
RodGauge - similar to a dipstick found in a car, it has weighted line
markings to indicate depth or volume
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.
Sight Glass
Another simple direct
method of
measuring liquids.
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)
Magnetic Level Sensors
Used where the sight
glass level gauge
can not be.
Magneto-resistive
types can provide
an electrical output.
Liquid/liquid interface (such as water and oil) can be measured by changing
the buoyancy of the magnetic float
Floats
Float rides the surface level to provide the
measurement. Many different styles are available.
Liquid density does not affect measurement
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
level.
• Different liquid densities or closed tank
applications must be accounted for.
General Theory for Head Measurement
The Pressure exerted by the
Height of the liquid is:
Liquid Density (D)
P = H x Density*
Height (H)
Pressure PSI
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.
Example
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)
Height
(H)
Water
Density (D)
Height
(H)
Oil
Density (D)
Tank 2
Tank 1
PSI
PSI
P = ? psi
P = ? psi
Example
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)
Height
(H)
Water
Density (D)
Height
(H)
Oil
Density (D)
Tank 2
Tank 1
PSI
PSI
P = H x Density
= 30 ft x 0.0361 lbs/in3
= (30 x 12) x 0.0361
= 13 psi
P = ? psi
Example
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)
Height
(H)
Water
Density (D)
Height
(H)
Oil
Density (D)
Tank 2
Tank 1
PSI
PSI
P = H x Density x SG
P = H x Density x SG
= 30 ft x 0.0361 lbs/in3 x 1
= 30 ft x 0.0361 lbs/in3 x 0.85
= (30 x 12) x 0.0361
= (30 x 12) x 0.0361 x 0.85
= 13 psi
= 11 psi
Practical Considerations when using
head type instruments
The reference point of the tank vs instrument
input must be considered.
Liquid Density
(D)
Liquid Density
(D)
Height
(H)
Height
(H)
Pressure
PSI
Pressure PSI
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
Vertical rises and drops contribute to the overall
height and therefore head pressure. Horizontal
runs have no effect.
Height
(H)
Water
Density (D)
P
P
Bubblers
P
Bubblers allow the
indicator to be
located anywhere.
The air pressure in
the tube varies with
the head pressure
of the height of the
liquid.
Instrument
input does
not matter
Regulated
purge system
(air or nitrogen)
Bottom of tube
determines
reference point
Can’t 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.
Closed Tank Applications
P (atmospheric)
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.
P Head = h x D
P vapour
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.
P Head = (h x D) + P vapour
Using a d/P Cell Transmitter
The differential pressure cell is one of the most common
methods of measuring level.
4 – 20 mA
24 VDC
mA
To PLC or
Controller
Lo side open to
atmosphere
Open Tank Measurement
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Lo side of the d/P cell is left open to atmosphere.
Hi side measures the hydrostatic head pressure which is proportional to the
height of the liquid and its density.
Using a d/P Cell Transmitter
In a closed tank, the Low side of the d/P cell is connected
to the top of the tank and will cancel the effects of the
vapour pressure above the surface.
4 – 20 mA
24 VDC
H
mA
To PLC or
Controller
L
Closed Tank Measurement
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Lo side of the d/P cell measures the vapour pressure above the surface.
Hi side measures the hydrostatic head pressure which is proportional to the
height of the liquid and its density + vapour pressure
Displacers
• 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
Conductivity Level Measurement
Point Level Measurement
Continuous Level Measurement
DC Excitation voltages may create problems with some liquids
(polarization) AC excitation voltages can be used but some frequencies
are not suitable for all liquids.
Advantages and disadvantages
Low Cost
Conductive, non-coating liquids only
Insulating coatings can cause problems
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
measured.
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:
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Electrode Coating
Low sensitivity
Temperature
Different materials
Shielding & noise
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 reflections
• High Pressure & High Temperatures affect the signal speed
• Vapour and condensate create false echo’s
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.
Guided wave Radar Sensor
A generated pulse of
electromagnetic
energy travels
down the probe.
Upon reaching the
liquid surface the
pulse is reflected.
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.
Summary
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Hydrostatic Head
Float
Load Cells
Magnetic Level
Gauge
Capacitance
Conductivity
Ultrasonic
Radar
Guided Wave
Radar
Dip Stick
Summary
• 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
• Detect if the level is
at a predetermined
point
• Generally used to
detect either a lowlevel 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.