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Measurement of liquid level in tanks and vessels

Direct Methods

Indirect Methods

Float
Sight glass

Liquid level position to electrical signal

Resistive

Capacitive

Inductive

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Sight/Gauge Glass

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Simple construction and installation
Less maintenance
Low cost
Accurate and consistent
No moving parts
Long life

Manual
Not for dusty, viscous/sticky liquids
Ambient Temperature sensitive
Eye level
Not for high pressure tanks

Adv

Disadv

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Float Type Level Indicator

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Pros and Cons

Low in cost, reliable and operate over a large temperature range
Corrosive environment

Small changes in level is difficult
Thread elongation /elasticity
Damage in float material
Mechanical obstructions
Friction in pulleys

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Resistive method

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Suitable for indication and initiation of control action
Warning and alarm systems
Signal can be transmitted to any desired place
Low voltages to be used to prevent arcing
Can be used in high pressure systems
Simple to install and calibrate

Unsafe to use in highly volatile or explosive nature liquids
Possibility of corrosion in contact rods
More contact rods are needed for continuous measurement of level
Conductivity of liquids interrupt the measurement

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Capacitive method

Change in capacitance with change in area of plates, distance between plates and the dielectric constant

Three methods:

Variable area methods
Capacitive voltage divider methods
Variable dielectric constant methods

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Variable area methods

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Capacitive voltage divider method

Conductivity of a liquid is high, its surface can be used as one electrode of the capacitor
Other electrode is a fixed reference plate parallel to the surface of the liquid
AC voltage is applied between the liquid and the electrode P₁
Capacitance C₂ is inversely proportional to the distance between the liquid surface and P₂. Thus the output voltage decreases with rise of liquid level and therefore the relationship between them is non-linear.

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Variable dielectric constant methods

Non conducting liquids, can be used as a dielectric in a capacitor
An insulated metal electrode firmly fixed near and parallel to the metal wall of the tank.
Electrode and the tank wall form the plates of a parallel plate capacitor with the liquid in between them acting as the dielectric

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Inductive methods

Inductive level transducers are mainly used for measurement of level of liquids which are conductive
Based on variable permeability
Inductance of the search coil changes rapidly as the conducting liquid moves into the plane of the coil.

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Very sensitive
Measurement very small ranges
Moving parts are not exposed to systems
Suitable for continuous measurement
Good for use slurries
Signal can be transmitted for further monitoring and control

Performance affected by dusty environment or electrical fluctuations
Changes in temperature affects sensitivity
Changes in composition of liquids affects the measurement
Liquids should possess proper dielectic strength
Mounting or installation is difficult

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Ultrasonic Method

An ultrasonic transmitter receiver can be mounted on the top of tank for measurement of level of either solids or liquids.

The time taken by the beam is a measure of the distance travelled by the beam.

Time between transmitting and receiving a pressure pulse is proportional to the distance between the ultrasonic set and surface of the contents of the tank.

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Easy to install
Minimum maintenance
No moving parts
Non contact measurement
Signal processing and monitoring
Works well for corrosive liquids

High Power requirement
Regular calibration required
Interference by vapours

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Measurement of Liquid Level with Gamma Rays

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Point level for liquids:

Capacitance,

Conductive level probes,

Float,

Thermal, and

Light beam.

Continuous for liquids:

Pressure,

Float,

Nuclear,

Ultrasonic, and

Radar.

Point level for solids: