Level measurement instrument

1. Unit-6 Level measurement

5. 1.INTRODUCTION
Level Measurement is regarded by many as one of the oldest forms of
instrumentation, and perhaps the most commonly used in everyday life.
Although level measurement is usually thought of in terms of liquid levels,
many level measuring instruments also lend themselves to the measurement of
solids.
An often cited example of early level measurement is when our ancestors
discovered the need to gauge the depth of water when attempting to safely cross
streams and rivers. They "invented" the simple dip-stick; a stick prodded into
the water in front of them that would allow them to gauge the depth of the river
and let them decide if it was safe to take another step forward. Dipstick level
measurement is still used daily by millions of people, for example to check the
oil level in their car engine.
When we get in our car to go on a drive you often look at your gas gauge, which
is a type of a level sensor. Industries use several different level sensor types to
detect their products.
On a simpler note level is measured daily in thousands of kitchens throughout
the world by cooks using a graduated scale etched on the side of a measuring
jug. Measuring the level in the jug lets them know what volume of liquid they
are about to add to their recipe.
In the process industries there are two methods of level measurement, namely:
Direct Level Measurement, and Indirect Level Measurement. Sometimes these
are referred to as Mechanical Level Measurement, and Inferential Level
Measurement.
Liquid level measurement is ubiquitous throughout industrial fluid processing
operations. Whether via direct or inferential means, the fluid level data point is
an integral part of operational and safety plans for an installation.
2. IMPORTANCE OF LEVEL MEASUREMENT
Effective level measurement plays a critical role in a broad range of industries
like ethanol production, oil and gas production, pulp and paper mills,
wastewater management, mining, and biochemical plants. It enables
organizations to determine the precise amount of liquids, solids or gases within
their equipment, therefore ensuring safety and optimal performance.

6. 3.Direct Level Measurement (Mechanical Level Measurement)
Direct level measurement is generally simple, and almost always an
economical solution. Though it does come with limitations, most notably its
inability to easily provide remote indication and/or control. The basis for this
type of fluid level measurement is the measurement of level from a datum line
e.g. in the examples above the data line is the river bed, and the base of the
measuring jug. Typical methods of Direct Level Measurement include:
- Dipsticks, and Lead Lines. These measure the wetted length of a stick or
"string" submerged in the liquid,
- Sight Glass (also known as Level Glass, or reflex sight glass). These give
visual indication of liquid level through a transparent window. The window can
either be part of a vessel or tank wall (e.g. the graduations on the measuring jug
in your kitchen), or the gauge can be external to the vessel in which case it is
bolted on to the outside of the vessel.
Float Gauges. These use the principle of buoyancy. Here a buoyant element
floats on the surface of a liquid and changes position as the liquid level varies.
Mechanical or magnetic methods are used to communicate the measured liquid
level to a remote indicator
4. Indirect Method of Level Measurement
Indirect methods of level measurement, or inferred methods as they are
sometimes called do not measure level directly but infer the level from
measurement or detection of a physical property of the liquid to be measured.
Examples of inferential level measurement include:
- Hydrostatic Head. One of the most common liquid level measuring
techniques used in the process industries relies on measuring pressure produced
by a head of liquid. The level can be easily calculated from liquid density,
measured pressure, and vessel pressure.
- Time of Flight level detection. This principle relies on measuring the time
taken for a signal to travel from its point of origin to the liquid surface and back
to a detector. Knowing the location of source and detector, and speed of signal
in the gas above the liquid allows level to be inferred. Typical sources of signal
include ultrasonics, and microwave. Vessel shape, vessel internals and
conditions of the gas above the liquid can limit the use of this method.

7. - Capacitance. The liquid and gas above it act as the dielectric in a capacitor.
As the level varies the capacitance of the dielectric varies due to more or less
liquid between the plates. Knowing the location of the plates and the
capacitance of the liquid and gas allows the level to be calculated. This method
requires the liquid to have good capacitive properties.
- Conductance. This method relies on the liquid conducting an electrical signal
between a probe and receiver. Obviously, the conductive properties of the liquid
may limit use of this method.
- Nucleonic. A beam of radiation is transmitted across a vessel and the strength
of the beam is measured on the other side. The liquid to be measured absorbs
radiation therefore the higher the level the less radiation is detected on the other
side. Advantages include that the source and detector can be "strapped" to the
outside of the vessel i.e. they are non intrusive. Nucleonic level measurement
techniques also lend themselves to interface measurement i.e. can be used
to measure the level of an interface between two liquids e.g. oil and water.
Downsides include the regulatory requirements around use of nucleonic
sources, i.e. licensing, transport etc. and the need to use specialist vendors for
installation and maintenance rather than in-house technicians.

8. 1.Dip Tape Level Measurement
A dip tape is a manual user-operated liquid level measurement device, which
consists normally of some form of calibrated tape, for example, meters, feet,
yards, with a weight attached to the end.
The difference between the two recorded values is then noted. Using
dimensional and capacity information known about the tank, the height of the
tank which is empty, the ullage, can be calculated and be expressed as a
percentage of the overall tank capacity.
2.Dip Sticks and Lead Lines: Flexible lines fitted with end weights called
chains or lead lines have been used for centuries by seafarinng men to gauge the
depth of water under their ships. Steel tape having plump bob-like weights and

9. stored conveniently in a reel are still used extensively for measuring level in
fuel oil bunkers and petroleum storage tanks.
Though crude as this methods seems, it is accurate to about 0.1% with ranges up
to about 20 feet.
3.SIGHT GLASS / GAUGE GLASS
A sight glass (also called a gauge glass) is another method
of liquid level measurement.
It is used for the continuous indication of liquid level
within, tank or vessel.

12. CONSTRUCTION & WORKING
• A sight glass instrument consists of a graduated tube of toughened glass which
is connected to the interior of the tank at the bottom in which the water level is
required.
• Figure shows a simple sight glass for an open tank in which the liquid level in
the sight glass matches the level of liquid in the tank, As the Ievel of liquid in
the tank rises and falls, the level in the sight glass also rises and falls
accordingly.
• Thus, by measuring the level in the sight glass, the level of liquid in the tank
is measured.
• In sight glass, it is not necessary to use the same liquid as in the tank.
• Any other desired liquid also can be used.

13. Advantages
 Direct reading is possible
 Special designs are available for use up to 316°C and 10000 psi.
 Glassless designs are available in numerous materials for corrosion
resistance.
Disadvantages
 It is read only where the tank is located, which is not always convenient.
 Since glasses are located on the outside of the tanks, the liquid in sight glass
may freeze in cold weather even though the liquid inside the tank does not, and
thus, it may cause error in the reading.
 Heavy viscous liquids or liquid containing materials which fall out of solution
and clog the tube cannot be measured satisfactorily by a sight glass.
 Overlapping gauges are needed for long level spans
 Accuracy and readability depend on the cleanliness of glass and fluid.
4. FLOAT OPERATED LEVEL MEASUREMENT
GAUGES
• Float operated level measurement gauges is used to measure liquid levels in a
tank in which a float rests on the surface of liquid and follows the changing
level of liquid due to buoyancy.
• The movement of the float is transmitted to a pointer through a suitable
mechanism which indicates the level on a calibrated scale.
• Various types of floats are used such as hollow metal spheres, cylindrical-
shaped floats and disc-shaped floats.

15. CONSTRUCTION & WORKING

16. • Figure shows the simplest form of float operated mechanism for the
continuous liquid level measurement.
• In this case, the movement of the float is transmitted to the pointer by
stainless steel or phosphor-bronze flexible cable wound around a pulley, and the
pointer indicates liquid level in the tank.
• The float is made of corrosion resisting material (such as stainless steel) and
rests on liquid level surface between two grids to avoid error due to turbulence.
• With this type of instrument, liquid level from ½ ft. (152 mm) to 60, ft. (1.52
m) can be easily measured.
Advantages
 It is possible to read the liquid levels in a tank from the ground level even if
the tank is kept below the ground level.
 Low cost, reliable design.
 Operates over a large temperature range
 Choice of corrosion- resistant materials to make these
Disadvantages
 Normally limited to moderate pressures
 They are tailored to tank geometry
Pneumaticlevel measurement
For pneumatic level measurements using the bubbling-through method, the
sensor, without having direct water contact , measures the counter pressure
created by an air compressor. This high precision level measurement is suitable
for large basins and storage tanks as well as barrages of up to 135 meters.

17. Advantages:
 High-precision level measurement up to 135 m
 Very precise calculation of the storage volume
 Sensor without direct water contact
 Suitable for small and medium volumes
Bubbler level gauge
In a Bubbler level gauge, a bubbler tube is used to measure and indicate
level. The Bubbler type level indicator is also known as purge type of liquid
level meter.
In this technique of level measurement, the air pressure in the pneumatic
pipeline is adjusted and maintained slightly greater than the hydrostatic
pressure at the lower end of the bubbler tube.
Air is forced to send through the bubbler tube. The bubbles come out of the
bottom of the liquid level. The greater the height of the liquid in the tank, the

18. more pressure it needs to push the air out. The bubbler system starts to
operate when a gas is passed through the bubbler tube.
The back pressure from the bubbles is measured and converted into level
value.
There is just enough gas pressure for the bubbles to appear when the liquid is
at the maximum level in the vessel. For maximum level of liquid, there will a
be a maximum back pressure, and the pressure gauge reads it.
At zero level the back pressure will be zero and the gauge or transmitter will
read zero. As the level increases the pressure reading on the pressure gauge
or transmitter increases.