2. Level measurement:
Vertical measurement :of process fluid taken
from the surface, or interface, to a fixed
reference point.
Reference point :is normally the bottom of
the vessel holding the
substance.
3. Level Measurement Continuous:
Continuously indicates the changing level of
process fluid.
Required when it is necessary to observe the
position of fluid/interface continuously, e.g.
using mechanical device (glass gauge).
4. Point-to-Point measurement:
Some processes require only that the level of
a substance be maintain between two points
(e.g. high and low level).
Such system activates control devices only
when predetermined levels are
reached.
5. Direct Measurement:
Involves straightforward approach.
Actual level of process fluid is obtained directly.
- float
- dip stick
-glass gauges
ADVANTAGE: Simple & economical, less / no
maintenance.
DISADVANTAGE: Inadaptable to output signal
transmission for remote indication / control by itself
6. Indirect measurement:
Involves conversion the measurement of other
physical quantity into level quantity.
-radar
-nuclear
-ultrasonic
-capacitance
-differential pressure
ADVANTAGE: Adaptable to output signal transmission
for remote indication / control.
DISADVANTAGE: Accuracy is affected by changes in
temperature of the process fluid.
8. Level Gauge:
Common technique for direct level measurement
Used for several reasons:-
- economical.
- easy of maintenance.
- applicable to wide range of fluid
Categorized into 3 groups:
Flat ( glass gauge)
Tubular (glass gauge) {Reflex- Transparent }
Magnetic( glass gauge)
9. Float:
An object of lower density than the
process liquid is placed in the vessel,
causing it to float on the surface.
Float rises and falls with the level.
Position of the float is sensed outside the
vessel to indicate level measurement.
10. Displacer:
Working Principle :Based on Archimedes’ Principle
“ a body immersed in a liquid will be buoyed by a force
equal to the weight of the liquid displaced.”
Volume of the displacer = V= (3.14xd2 / 4)xL
Change in liquid level produce change in displacer
weight
12. Advantages / Applications:
Continuous level measurement.
Applicable to non-viscous & clean liquids.
High-accuracy, provided that liquid density is
constant.
Wide range of measurement span, but cost
increases relatively.
Liquid-liquid interface.
13. Disadvantages / Limitations:
Inapplicable to solid.
For coating media, the moving parts may be seized.
Affected by density changes of process liquid
application.
Leaking is possible (through torque tube).
14. Differential Pressure:
Based on the principle that hydrostatic pressure varies
with changes in level.
The principle of this measurement is that the
hydrostatic head, H exerted by a liquid is equal to the
specific gravity of that liquid, GL multiplied by the
height of the liquid, x.
H = (GL)(x)
Where H = hydrostatic head in inches or meters of water, as applicable
x = height of liquid in inches, as applicable
GL = specific gravity of liquid
16. Capacitance:
As level changes, capacitance in the sensor
circuit also changes. Bridge misbalance occurs
- between probe and process media
(conductive applications).
- between probe and vessel wall
(insulating applications).
This change is detected by the level switch's
internal circuitry - translated into a change in
the relay state of level switch.
17. Advantages / Applications:
Capable of operating at extreme temperature
& pressure.
No moving part.
Usually only a single tank penetration is
required.
18. Disadvantages / Limitations:
Problems for media with varying
dielectric.
Unsuitable to process media that
can coat the sensing element that
can reduce its reliability & accuracy.
- NOTE: even acids and caustics that don’t appear to coat
the sensing element
are so conductive that the thin film they leave can cause
serious error in
measurement.
19. Ultrasonic:
Works on the principle of sending a sound
wave from a peizo-electric transducer to the
contents of the vessel.
- Measures the time taken for the reflected sound
wave (echo) to return to transducer.
- Successful measurement depends on reflection
from process material in a straight line back to
transducer.
20. Advantages / Applications:
Unaffected by product density, conductivity /
dielectric constant.
No moving part; device is non-contact with
process media.
Single top vessel entry; leakage is less
probable.
21. Disadvantages / Limitations:
Return signal may be affected by
- powders / particles.
- heavy vapour.
- surface turbulance.
- ambience noise.
Inapplicable to vacuum & very-high-pressure
applications.
22. Radar:
Microwave signal emission will burst towards
the process media.
This burst is reflected by the surface and
detected by sensor.
Level is inferred from the time of flight
(transmission to reception) of the microwave
signal.
Microwave "echoes" are evaluated by sampling
and building up historical profile of the echoes.
23. Radar Types:
This design is suitable for
buffer & process tanks
This design is suitable for
requirement of high chemical
compatibility
Guided Type Regular Type
25. Guided-Type VS Regular-Type:
Guided-Type Regular-Type
Waveguide makes the signal less
vulnerable to distortion.
Cannot be used if agitator is
present inside the vessel.
Total non-contact solution for level
measurement.
Agitator has little effect on the signal.
26. Advantages / Applications:
Applicable to liquids & slurries application.
Unaffected by
- extreme pressure / temperature.
- surface turbulence / agitation.
- steam / vapor / condensation.
- dense foam / dust / encrustation.
No moving part.
Single top vessel entry; leakage is less probable.
Low maintenance & easy installation
27. Disadvantages / Limitations:
Inapplicable for interface-interface
measurement.
Vapor effect can weaken the radar signal,
thus reducing its accuracy.
28. Nuclear
Radioisotopes used for level measurement emit energy at a
fairly constant rate but in random bursts.
Gamma radiation is generally used.
Short wavelength and higher energy of gamma radiation
penetrates the vessel wall and process media.
A detector on the other side of the vessel measures the
radiation field strength and infers the level in the vessel.
Different radioisotopes are used, based on the penetrating
power needed to "see" the process within the vessel.
29. Advantages / Applications:
Used when all other measurement types are
unsuccessful
high pressure & temperature.
toxic & corrosive media.
Completely non-invasive; non-contact with process
media / atmosphere.
30. Disadvantages / Limitations:
Expensive:
-maintenance
- periodic survey
- disposal is usually handled by licensed, external
authorities.
31. Level switch:
Level switch is one type of level instrument that
performs.
point level measurement.
Level switch is used to indicate high or low level
alarm.
A level switch is usually installed on the vessel as
addition to
continuous-type level devices (for safeguarding
requirement).
32. Level switch:
- There are many types of level switches available:
Float Switch
Displacer Switch
Thermal Switch
Conductivity Switch
Measuring electrode
33. Advantages / Applications:
Cost-effective: Economical method for point
level measurement.
Compact: Easy mounting in small spaces.
No moving parts: No wear and tear.
34. Disadvantages / Limitations:
Level is not measured continuously.
Intrusive-type: There is direct-contact with
process fluid.
Its function as level alarm indicator can be
taken over by PLC.