LEVEL MEASURING SENSORS

A Presentation

of

“Industrial Instrumentation”

Presented to: Dr. Ing. Naveed Ramzan

Presented By: Yasir Javed 2007-Chem-30

LEVEL MEASURING SENSORS

Level measuring sensors detect the level of substances that flow, including liquids, slurries, granular materials, and powders.

The substance whose level to be measured can be Inside a container or in its natural form (e.g. a

river or a lake). In open tank or closed tank

CLASSIFICATION OF LEVEL MEASURING SYSTEMS

Liquid or Solid Level Measurement Point or Continuous Level Measurement * Electromechanical or Electromagnetic Level

Measurement Contacting or Non-Contacting Level

Measurement

GENERAL CONSIDERATIONS IN LEVEL MEASURING SENSOR

SELECTION

Density and viscosity Chemical composition Ambient temperature Process temperature Process pressure Regulated environment Process agitation

POINT/SWITCH LEVEL MEASUREMENT AND DETECTION

SYSTEMS

Point level measurement sensors are used to mark a single discrete liquid height–a preset level condition.

They are: Floats Displacers Conductivity Probes Vibrating fork Resonance

FLOATS

A float operated liquid level control operates on the basic buoyancy principle, which states, "a body (float) immersed in a liquid is buoyed upward by a force equal to the weight of the displaced liquid". As a result, floats ride on the liquid surface partially submerged and move the same distance the

liquid level moves.

ADVANTAGES AND DISADVANTAGES

The Advantages of this method are that it is relatively simple, suitable for various products, and very accurate.

The Disadvantages are that it requires a certain amount of mechanical equipment, especially in pressure vessels.

DISPLACERS

Displacer level sensors are force balance devices in accordance with Archimedes' principle which states that “the buoyant force acting on an object equals the weight of the fluid displaced”.

ADVANTAGES AND DISADVANTAGES

The obvious Advantage of this method is that the absolute level does not change.  

The Disadvantages are that it is dependent on relative density and requires a significant amount of mechanical equipment. Cost of installation for displacers is high and many refineries are now replacing them due to the inaccuracies experienced under process density changes especially on interface duties.

CONDUCTIVITY

The principle for this form of level measurement is that the presence of a product will cause a change in the resistance between two conductors.

ADVANTAGES AND DISADVANTAGES The Advantages of this method are simple,

inexpensive and suitable for dual or multiple point control.

  The Disadvantages are probe cannot

become contaminated with grease or other deposits and has limited suitability for products of varying conductivity.

VIBRATING FORKS RESONANCE

Frequency changes which shift tuning fork Piezo crystal which oscillates forks and Solution causes a reduction in resonant frequency Second crystal detects.

ADVANTAGES AND DISADVANTAGES

Advantages: No moving parts, thus no maintenance. Can be mounted in any position. Small and can be mounted in a 1-in or

smaller mounting boss.

Disadvantages: They are prone to material buildup between

the tines. Build up material can cause inaccurate

reading.

CONTINUOUS LEVEL MEASUREMENT AND DETECTION

SYSTEMS

Continuous level sensors are more sophisticated and can provide level monitoring of an entire system.

They include; Radar Systems Ultrasonic Systems RTD Magnetostrictive

RADAR Extremely short microwave impulses with low

emitted power in the 6.3 GHz frequency range are emitted by the antenna system to the measured product, reflected by the product surface and received again by the antenna system.

ADVANTAGE AND DISADVANTAGE The Advantage of radar is its broad

applicability on most liquids and measurement independent of pressure, temperature, vapor, and (to a degree) product dielectric constant.

  The Disadvantage is that the measurement

may be lost because of heavy agitation of the liquid or the formation of foam.

ULTRASONIC

Ultrasonic transmitters work on the principle of sending a sound wave from a peizo electric transducer to the contents of the vessel.

The device measures the length of time it takes for the reflected

sound wave to return to the transducer.

ADVANTAGES AND DISADVANTAGE

The main Advantages of ultrasonic level instrumentation are that the transducer does not come into contact with the process material, they have no moving parts and a single top of vessel entry makes leaks less probable than fully wetted techniques.

The main Disadvantage is that there are various influences that affect the return signal. Things such as powders, heavy vapors, surface turbulence, foam and even ambient noise can affect the returning signal. Temperature can also be a limiting factor in many process applications.

TIME-DOMAIN REFLECTOMETRY TD

The principle of this technology is to launch an extremely short microwave impulse on a waveguide, which can be a cable or rod.

This waveguide, or probe, contains the signal in a close area around the cable or rod.

ADVANTAGES AND DISADVANTAGES

The Advantages of TDR level measurements are disregard of the presence of vapors, steam, dust, gas layers, buildup, temperature changes, pressure changes, acoustic noise, changing density of the material to be measured, changing dielectric constant of the material to be measured, and changing conductivity of the material to be measured.

Some Disadvantages of this measuring system

are that it does come in contact with the process and is an intrusive form of measurement. The system is not suitable for highly agitated liquids.

MAGNETOSTRICTIVE

A magnetostrictive system consists of a magnetostrictive wire in a probe and a donut-shaped float containing a permanent magnet. The float is the only moving part and travels up and down the probe.

ADVANTAGES AND DISADVANTAGES

Advantages are high accuracy and independence from material characteristics such as conductivity and dielectric constants.

Disadvantages are limitations to relatively clean liquids and anything that would cause the float to hang up on the probe.

Any Question?