There are different method for level measurements for indirect method, here we will be discussing in details some and short words. Purge level system, Displacer type level measurement, FLOAT AND CABLE Level Measurement, DISPLACEMENT (BUOYANCY) LEVEL MEASUREMENT, Magnetic Reed Switches for Level Measurment, DIFFERENTIAL PRESSURE METHOD for Level Measurement, CAPACITANCE TYPE LEVEL MEASUREMENT, CAPACITANCE TYPE LEVEL MEASUREMENT, RADIATION TYPE LEVEL MEASUREMENT, ULTRASONIC TYPE LEVEL MEASUREMENT.
Purge level system
The method above is suitable for open tank applications. When a liquid is in a pressure vessel, the liquid column pressure can’t be used unless the vessel pressure is balanced out. This is done through the use of different pressure meters.
Differential pressure meter
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Displacer type level measurement
The common methods employed for automatic continuous liquid level measurement are as follows.
- Float and cable
- Displacement (Buoyancy)
- Head (Bubble tube, Diaphragm box, Pressure, Differential pressure)
- Capacitance
- Radiation (Nucleonic)
- Ultrasonic
FLOAT AND CABLE Level Measurement
A float and cable instrument measures liquid level by transmitting to a mechanism the rise and fall of a float that rides on the surface of the liquid. These methods can be used on both open and closed tanks. Advantage of float is simplicity and
insensitive to density changes. Disadvantage is limitation to clean liquid and turbulence of liquid creates measurement problem.
insensitive to density changes. Disadvantage is limitation to clean liquid and turbulence of liquid creates measurement problem.
DISPLACEMENT (BUOYANCY) LEVEL MEASUREMENT
It is a type of force balance transmitter. It is based on Archimedes’ principle which states as “A body immersed in a
liquid will be buoyed upward by a force equal to the weight of liquid displaced”. This method is used to measure liquid level by sensing the buoyant force exerted on a displacer by the liquid in which it is immersed. The buoyant force on an object depends on how much liquid is displaced and the density of the liquid. The buoyant force always equals the weight of the displaced liquid. If the buoyant force becomes equal to the object’s weight, the object floats. Displacer element is a cylinder of constant cross sectional area and heavier than the liquid displaced.
This method is used for both open and closed tanks. Buoyancy transmitter is normally used in vessels where lower connection is not possible/permissible, fluctuating pressures or levels and high temperature service.
Magnetic Reed Switches for Level Measurment
If the liquid is hazardous chemical or at high temperature or pressure, magnetic reed switches are used. It is normally in open state. When the floating magnet outside the tube comes near the switch, it attracts the magnetic pole piece in the switch. This action closes the switch until the floating magnet moves away. These switches are not sealed in the tube, and they never come into contact with In this method a diaphragm box is suspended from a chain. Diaphragm is filled with air. The instrument that senses pressure changes and relates to level measurement is mounted above vessel. This method is normally used for open vessels.
DIFFERENTIAL PRESSURE METHOD for Level Measurement
In open tank pressure at high pressure side of cell is measure of liquid level. In closed tank, effect of tank pressure on measurement is nullified by piping this pressure to opposite side of cell. Any difference between the pressures sensed by the two legs is due to head pressure alone, and is used to measure the liquid level in the vessel.
In closed tank with liquid that produce vapors, the condensed liquid produces a head pressure on the low side of the instrument (called wet leg), causing the reading to be below zero. A zero elevation adjustment is carried out to compensate for this zero error. After draining the wet leg, the transmitter does not read correctly until the wet leg is refilled.
CAPACITANCE TYPE LEVEL MEASUREMENT
The amount of capacitance depends on the distance between the plates, the area of the plates, and the height of the dielectric between the plates. The equation is
C = K (A/D)
Where, C = capacitance, K = dielectric constant, A = area of plate, D = distance between plates.
In this method a probe is inserted in a tank and capacitance is measured between probe and tank. Capacitance varies with respect to tank level. This phenomenon is due to the difference between dielectric constant of air and liquid Concept of capacitance –
Sensing Liquid Level for Level Measurement
For sensor calibration reference measurement its empty and full tank condition must be taken .Generally we need to know the dielectric constants of the liquid being measured to calibrate the sensor at its full condition.
Application
Liquids level measurement.
Powered and granular solids.
Liquid metals at very high temperature.
Liquefied gases at very low temperature.
Corrosive material.
Very high pressure industrial processes.
Powered and granular solids.
Liquid metals at very high temperature.
Liquefied gases at very low temperature.
Corrosive material.
Very high pressure industrial processes.
Advantages of capacitance level measurement
Inexpensive.
Reliable.
Versatile.
Minimal maintenance.
Contains no moving parts.
Reliable.
Versatile.
Minimal maintenance.
Contains no moving parts.
Good range of measurement
Rugged.
Simple to use.
Easy to clean.
High temperature and high pressure application.
Rugged.
Simple to use.
Easy to clean.
High temperature and high pressure application.
Disadvantage capacitance level measurement
Not suitable for low dielectric material
RADIATION TYPE LEVEL MEASUREMENT
In this method a radioactive source is kept on one side of tank and detector on other side. As radiation passes through the tank, its intensity varies with amount of material in tank and can be related to level. Its advantage is that nothing comes
in contact with liquids. It is very costly and difficult to handle.
in contact with liquids. It is very costly and difficult to handle.
ULTRASONIC TYPE LEVEL MEASUREMENT
Ultrasonic sound waves with frequencies of 1 to 5 MHz can be used to detect liquid or solid levels. Ultrasonic are sound waves but at higher frequencies than 20 KHz (detected by human ear). It consists of an ultrasonic transducer (piezoelectric crystal). When voltage is applied to plates, the piezoelectric crystal expands or contracts. The crystal vibrates, and these
vibrations can be transferred to a diaphragm to produce ultrasonic sound waves. The liquid surface acts as a reflector, and the transducer receives the reflection of its transmitted pulses. The transmitter and receiver are both connected to an echo timer, which measures the amount of time between the emission of sound wave and the reception of the echo. Time required by sound wave to travel to the liquid and back to receiver is carefully measured and this time is related to level.
vibrations can be transferred to a diaphragm to produce ultrasonic sound waves. The liquid surface acts as a reflector, and the transducer receives the reflection of its transmitted pulses. The transmitter and receiver are both connected to an echo timer, which measures the amount of time between the emission of sound wave and the reception of the echo. Time required by sound wave to travel to the liquid and back to receiver is carefully measured and this time is related to level.
In case transmitter could not be installed in tank, a noninvasive (not in contact with liquid) sensor transmits an ultrasonic signal through the walls of a vessel. When the vessel is filled with liquid, the signal travels through the liquid and the opposite wall to a receiver transducer, where it is converted to and electrical signal. It has good accuracy. It is costly.
Advantages of uidGed Wave Radar Level transmitter
Application with density variation, pressure and temperature fluctuation. Long ranges with narrow setting. Interface measurement independent of density.
Guided Wave Radar Level transmitter Advantages –
1.Long range.
2.Top mounted with narrow measurement beam.
3.Low sensitivity to foam and build up.
4.Flexible interface measurement with no moving parts.(Simultaneous measurement of interface and total level).
Disadvantages of Guided wave radar transmitter
1.Contacting
2.Corrosion
3.Pulling forces
4.Buildup
2.Corrosion
3.Pulling forces
4.Buildup