LEVEL MEASURING DEVICES

LEVEL MEASUREMENT

• Accurate measurement of fluid or solid height within a vessel

• 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

• Level Gauges• Float type• Hydrostatic Pressure Measurement• Displacement Type• Echo Type• Capacitive Type

LEVEL GAUGES(Sight Glass)

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 instruments

• serves as a direct indicator for an operator for monitoring

Basic Principle : The Manometer

• equal pressures applied to both liquid columns

• height of liquid column is same in both the storage tank and the gauge sight glass

Liquid Interface Error

• Presence of less denser liquid interface gives a false indication of water level in the tank• Indicates presence of a single liquid in the tank

False indication of liquid interface of two liquids

Different liquid-liquid interface columns may have same hydrostatic pressures, balancing the liquid columns.

Temperature Error

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

FLOAT

motion balance devices that move up and down with liquid level

• float must be of substantially lesser density than the substance of interest

• must not corrode or react with the substance

• manual level measurement hazardous and not applicable in case of pressurized vessels

Floats – contd.

Spring reel constantly tensions the cable holding the float

Spring reel’s angular position may be converted to an electronic signal by a multi turn potentiometer or rotary encoder

HYDROSTATIC PRESSURE BASED INSTRUMENTS

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.

Differential pressure 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

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.

These instruments exploit Archimedes’ Principle

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

monitored

•Increasing liquid level Greater buoyant force

•Decreasing liquid level Smaller buoyant force

The Buoyant Force Instrument

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

CAPACITIVE INSTRUMENTS

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

Conductive Liquids 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 dielectric medium

• Vessel walls form the second plate of the capacitor

• Variables: