Level Measurement
Instrument training material
Aim
Knowing the different type of level
measuring instrument.
 Knowing the basic operating principle of
different type of level measuring
instrument.

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.
Electrical Level Measurement Methods
Method
Description
Uses
Pressure
differential
Can be used for
measurement of
liquids in open or
closed vessels.
Bubbler
uses
pressure between
the surface and the bottom of
the liquid.
a pipe submersed into the Liquids and
liquid (to the bottom of the
slurries (including
vessel). This pipe is connected corrosive liquids).
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.
Electrical Level Measurement Methods
Method
Weight
Conductive
Description
Users
mounting
the vessel onto weigh
scales or load cells.
greater the level the greater the
weight.
used for liquids (including
corrosive), slurries and
solids.
measure
Used to switch at level
points giving discrete
signals at level points (e.g.
High and Low Level
detection).
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.
Electrical Level Measurement Methods
Method
Description
Users
Capacitance
two
Level measurement
of liquids
(including
corrosive and high
temperatures) can
be achieved
accurately with the
capacitance gauge.
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.
Electrical Level Measurement Methods
Method
Description
Ultrasonic
uses
Users
a transmitter and
used for liquids
receiver.
(including corrosive)
and solids.
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.
Level Sensing Family

Force
Diaphragm
Weighing
Buoyancy

Pressure
Hydrostatic head
Bubbler
Differential pressure
Hydrostatic Head Level Measurement
Maximum Level
Normally contains process
media but in some application it
is filled with a higher density
liquid (water or glycol).
x
y
L
H
z
Minimum Level
Open Tank
Span = p P g x
means the zero
Suppressed-Zero
LRV = p P g y + p F g z
value of the
measured variable
URV = LRV + Span
is less than the
Where pP = density of process liquid
LRV.
pF = density of liquid-filled in impulse line
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.
LP Side must be dry,
or no liquid.
Maximum Level
x
y
z
Minimum Level
L
H
Span = p P g x
Closed System - Dry Leg
LRV = p P g y + p F g z
Condensate Pot
collects liquid or
URV = LRV + Span
condensate from
p
Where P = density of process liquid
impulse line.
pF = density of liquid-filled in impulse line
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.
Maximum
Level
d
x
y
Span = pP g x
LRV = (p P g y + pF g z) - (pF g)(d + z)
L
H
URV = LRV + Span
Where pP = density of process liquid
p F = density of liquid-filled in impulse line
Minimum Level
z
Elevated-Zero
means the zero value
of the measured
variable is greater than
the LRV.
Diaphragm Seal DP Transmitter
replaces wet legs to reduce
maintenance on applications
where the wet leg is not stable or
often needs to be refilled.
Applications:
•Extreme hot and cold temperatures
• Corrosive applications
• Clogging or solidifying
• Sanitary requirements
Maximum Level
d
x
y
Span = p P g x
LRV = p P g y - p F g d
URV = LRV + Span
L
Where
pF
pP
Minimum Level
H
= density of process liquid
= density of capillary fill liquid
Hydrostatic Head Level Measurement
Open Tank
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.
Maximum Level
x=2000 mm
Minimum Level
y=125 mm
L
H
z=250 mm
Span = (1000)(9.81)(2000)/100000 = 196.2 mBar
LRV = (1000)(9.81)(125)/100000 + (1000)(9.81)(250)/100000
= 36.8 mBar
Suppressed-Zero
URV = 36.8 + 196.2 = 233 mBar
Range = 36.8 to 233 mBar
Hydrostatic Head Level Measurement
Maximum Level
4.0 m
2.0 m
Minimum Level
L
H
1.0 m
Process Liquid Density = 850 kg/m3
Fill Liquid Density = 1050 kg/m3
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
Elevated-Zero
URV = -309.0 + 166.8 = -142.2 mBar
Range = -309.0 to - 142.2 mBar
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
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
Level Displacement & Torque Tube Principle


On-board electronic
circuit produces
analog output signal
proportional to the
liquid level.
Output = 4 - 20 ma.

For pneumatic
instruments, the torque
tube rotation is converted
to 20 - 100 kPa output
signal by flapper-nozzle
mechanism.
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
Displacement Type Level Measurement
Fisher 2390 Series Electronic
Transmitter
Masoneilan 12300
series, 2-wire, loop-powered
Smart Level transmitter or
Controller with HART
Communication
All operates according to the fully proven
liquid displacement and torque tube principle...
Pneumatic
Level
Transmitter or
Controller
Take a Break
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.
Capacitance = Eo E * A
D
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 4–20 ma output signal,
or in case of a point level device yields a
contact output change.

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.
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.
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.
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.
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.
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.
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.
Ultrasonic Non-contact Level Measurement

Ultrasonic level measurement utilizes the simple
equation.
Va * t
D =
2

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.


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.
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

120 Vac
Lined Powered
24 Vdc, 4 – 20 mA
Loop Powered
Air, r = 1.0
Transmitted
Signal
Liquid, r >1.4
Non-contact
type
Reflected
Signal
Guided Wave
type
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.

The End of Level
Temperature to come…..
See You….