Level measurement systems

LEVEL MEASUREMENT SYSTEMS

• Level measurements is an integral part of process control, and may be used in a variety of industries

• Level measurement may be divided into two categories:

¾ point level measurement

¾ continuous level measurement

• Point level sensors are used to mark a single discrete  liquid height, a preset level condition (as a high alarm or a low alarm condition)

• Continuous level sensors provide an analog output that directly correlates to the level within the containing vessel. 

This analog signal may be directly linked to a visual indicator or to a process control loop, forming a level management system.

The material to be measured

• Liquid

  from pure, clean water to viscous, sticky, and corrosive and abrasive fluids

• Bulk material

 from free-flowing, dry crystals to moist, lumpy solids

The processing environments for level sensors extend from vacuum to high-pressure service, and from subzero to elevated temperatures.

Level sensors

• Mechanical sensors

• Float methods
• Buoyancy method
• Vibrating level systems

• Hydrostatic pressure methods

• Differential pressure level detectors
• Bubbler systems

• Electrical methods

• Conductivity probes
• Capacitance probes
• Optical level switches
• Ultrasonic level detectors
• Microwave level systems
• Nuclear level systems

Floats

• The basic float arm indicator comprises very simply a float connected to a pivoted arm that drives pointer or a switch. 

• The unit can be made for either side- or top- entry.

• Moving parts present a very definite disadvantage, since they are situated in the liquid and are thus prone to corrosion and seizing

• Methods of the providing indication other then by linkage to a pointer include the use of a potentiometer, or of magnetic or inductive coupling

Design of floats 

Buoyancy method

• These devices use Archimede's principle

• The mechanical level indicator consists of the immersion body with calibrated measuring spring which transmits the change of level to the mechanical or electrical indicator.

Vibrating level switches

Principle: A vibrating fork or blade 

• An electronic circuit excites the blade of probe to its resonant frequency, and when material comes into contact with the blade, vibration is damped causing switching of the relay

• This device is suitable for control maximum levels of solids and liquids in many types of applications (e.g. foods, grains, granules, pellets, cement, powder)

Hydrostatic pressure methods

• The hydrostatic pressure at the bottom of a container is directly proportional to the liquid height

Bubbler systems

• Clean air from a compressor is forced through a restriction into a tube that leads to the bottom of the tank

• The air pressure after the restriction is equal to the hydrostatic pressure at the bottom of the tank

Conductivity level probes

•  For electrically conductive liquid

• An insulated electrode is used (e.g. an insulated rod)

• The insulator may be polytetrafluorethylene or polyethylene

• The conductive liquid forms the second electrode 

• With increasing liquid level increases the area of the second electrode as well as capacitance 

• An electronic transducer converted capacitance changes into a voltage or current signal

• For nonconductive liquids

Applications

• Suitable for measuring of liquids and bulk (loose) materials

• Suitable for wide temperature ranges (from -40 to +200) °C and high pressures

• Unsuitable for measuring foaming liquids

Optical level switches

• Light must be transmitted and then received

• Optical level sensors consist of:

– Light source (bulb, LED)

– Photoelectric detector (photodiode, phototransistor, photoresistor)

• Devices works with infrared or visible radiation

Ultrasonic level measurement

The measuring equipment consists:

• A transmitter that periodically sends a sound pulse to the surface of the liquid

• A receiver that amplifies the returning pulse

• A time interval counter that measures the time elapsing between the transmission of a pulse and receipt of the corresponding pulse echo

Ultrasonic level-meter with compensation

• Compensation of the influence of the gas density changes

• Cyclical measurement of the sonar pulse velocity in the environment

• Automatic compensation

Microwave level systems

• They are parallels in the operating principles of ultrasonic systems with microwave radar level systems

• Much higher frequencies (around 10 GHz) are used in radar system

• The radar beam is not affected by density changes

Pulse method:

• Microwave pulses are transmitted in short cycles

• The time is measured (ps)

• Demanding at the time measuring accuracy

c - microwave velocity [m.s-1]

t - time [s]

L - distance [m]

• When the reflected signal f0 returns to the receiver, it is mixed with the outgoing signal f1.

• There will be a difference in frequency between the transmitted and the reflected signals Δ f = f1 − f0

• The difference in frequency is proportional to the time difference Δ t = t1 − t0and also to the distance to the liquid surface.

• The frequency difference can be measured very accurately.

Radar level gauge

Nuclear level systems

• Nuclear radiation from a selected source can be related to liquid or solids levels in a vessel

• Cobalt-60, cesium-137 or radium-226 is used as the gamma radiation source

• The radioactive source is capable of transmitting through the container wall

• As a detector for converting nuclear gamma ray radiation into electrical quantities related to level, some systems use Geiger counters

Some guidelines for selecting instruments
to be used for the indication or control of level

• The material to be measured must be looked at to determine its compatibility with the instrumentation. For instance: Is the liquid hot, cold, under pressure, viscous, corrosive, abrasive, hygienic ?

• Is the area hazardous, requiring intrinsically safe or flameproof products ?

• Can the sensor contact the material being measured?

• Can the sensor be inserted into an existing entry or does it require new "hole" ?

• Does the instrument have to be top-enter or can be mounted in the side ?

• Is point or continuous measurement desirable ? 

• Is remote control or indication desirable ?

• Are there objection to the mechanical moving parts?

• The question must be asked whether or not the equipment need becompatible with data loggers microprocessor or computers.

• What is the required accuracy of the measurement ?

• What are costs ?