• Accurate measurement of fluid or solid height within a
• Required by many industrial processes
Level measuring devices are generally used for
the accurate continuous measurement of
volume of fluid in containers
LEVEL MEASURING INSTRUMENTS
Hydrostatic Pressure Measurement
A visual indicator of the level of liquid having a
sealed cavity with at least one transparent wall
• used in conjunction with other industrial level
• serves as a direct indicator for an operator for
Basic Principle : The Manometer
• equal pressures applied to both liquid
• height of liquid column is same in both the
storage tank and the gauge sight glass
Liquid Interface Error
False indication of liquid interface of two liquids
• Presence of less denser liquid interface
gives a false indication of water level in
• Indicates presence of a single liquid in
Different liquid-liquid interface columns
may have same hydrostatic
pressures, balancing the liquid columns.
Liquid inside the vessel is hotter than
the liquid inside the gauge.
both columns contain the same
liquid of different densities.
Unequal heighted liquid columns
balance each other, if filled with
liquids of different densities
motion balance devices that move up and down with
• float must be of substantially lesser
density than the substance of interest
• must not corrode or react with the
• manual level measurement
hazardous and not applicable in case
of pressurized vessels
Floats – contd.
Spring reel’s angular position may
be converted to an electronic
signal by a multi turn
potentiometer or rotary encoder
Spring reel constantly tensions the
cable holding the float
HYDROSTATIC PRESSURE BASED
A vertical column of fluid generates a pressure at the bottom
of the column owing to the action of gravity on the fluid.
Greater the vertical height of the fluid, greater the pressure
A simple pressure gauge
attached to the bottom of the
vessel may be calibrated to
indicate the level of liquid.
transmitters connected in this
manner respond to an increased
liquid level by driving their
output signal high and vice versa
for level indication in the vessel.
DISPLACEMENT TYPE INSTRUMENTS
These instruments exploit Archimedes’ Principle
Any object, wholly or partially immersed in a fluid, is
buoyed up by a force equal to the weight of the fluid
displaced by the object.
The weight of the displaced fluid is directly proportional to the volume of
the displaced fluid (if the surrounding fluid is of uniform density).
• Displacer is immersed in process liquid
• Weight of the displacer is continuously
• Increasing liquid level
• Decreasing liquid level
ECHO TYPE INSRUMENTS
Time of flight of a travelling wave reflected from
the surface of the process liquid is measured
indicating the distance travelled and hence, the
liquid height inside the vessel.
• immune to changes in liquid density
• demand a much higher level of technology than simple displacer or
hydrostatic level sensing instruments
• level of accuracy depends on the velocity of the wave travelling en
route to the liquid surface and back
• for accurate results, the velocity of the wave should remain fairly
constant during the timed interval through the medium
Ultrasonic Level Measurement
Fillage = Total height – Ullage
The sound waves time of flight is the direct
function of how much empty space exists
between the liquid surface and top of the
vessel i.e. “ullage”.
Fillage becomes the natural measurement
and ullage a derived parameter
Ullage = Total height - Fillage
Increase or decrease in level of process
fluids, changes the capacitance between a
conductive rod and process vessel walls.
The basic principle behind the capacitive level
instruments is the capacitance eqn.
Only changes in and „d‟ cause the change in the capacitance
with change in liquid level
Capacitive Probe Varieties
Non- conductive Liquids
• Probes are coated with plastic or
some other dielectric medium
• Metal probe forms one plate of the
capacitor and the conducting
liquid forms the other
• Variables: and d
• The liquid itself is used as the
• Vessel walls form the second plate
of the capacitor