2. DISCLAIMER
•These powerpoint slides act only as a tool in
delivering lectures to the students.
•The materials presented in these slides are not
comprehensive as most of the materials are
explained to the students verbally with the guide of
these PowerPoint slides, smartboard and ELET241
reference book.
•Hence, the students are reminded that the main
reference for ELET241 is Instrumentation and
Process Control by Franklyn W. Kirk text book
2
3. LEVEL MEASURMENT
• Level can be defined as the height of the surface of liquid from a certain reference
point or datum.
• The datum can be the bottom of the tank or a point higher than the bottom of the
tank or vessel.
• Level measurement may be of two types
• Point Level Measurement
• Continuous level Measurement
4. LEVEL MEASURMENT
•The measurement and control of liquid level is essential in a process plant,
where a variety of liquids are handled in continuous process.
•The accurate measurement and control of level is important for:
1. Product quality.
2. Continuity of process.
3. Inventory control.
4. Plant safety.
5. Environmental protection. e.g., to prevent overflow of hazardous materials to
drains.
5. Point Level Measurements
• Point level measurement is a method of level measurement where the only
concern is whether the amount of material is within the desired limits.
• The measured value is commonly used to sound alarms or to determine when
to activate or deactivate pumps or other material handling equipment.
• A sensing element is installed at the selected level position.
• Point level measurement can be used to prevent overflows when filling a tank,
to avoid running a pump dry when emptying a tank, or to sound an alarm when
a surge tank is above or below a normal level.
6. Point Level Measurements
• If high- and low-level operation is required, one sensor is required
for each level point.
• This ensures that the level remains within the limits set by the
positions of the sensors.
7. Continuous Level Measurements
•Continuous level measurement is a method of tracking the change of
level over a range of values.
•This is commonly used for inventory tracking and for determining when
to add or remove material from containers.
•For example, the level of a material in a tank must be known in order to
maintain a safe level when transferring material, or the water level in a
boiler must be known at all times to prevent a low-water condition that
can lead to boiler damage or an explosion.
9. 1. Gauge Glasses/Sight Glass
• Gauge glass is a continuous level measuring instrument that consists of a glass tube
connected above and below the liquid level in a tank and that allows the liquid level to be
observed visually.
• The liquid level in the glass tube is the same as the level in the tank.
• Gauge glasses are usually installed with shutoff valves and a drain valve used for purposes
of maintenance, repair, and replacement.
• Scale on the glass help to read the level with a certain value (for example, between 0% and
100%).
10. Types of Gauge Glass
• Flat glass are used in industry for a wide range of level applications.
There are two basic designs: Reflex and Transparent
Reflex Glass - has a single vision slot on one side of the chamber in which light can enter to
determine liquid level.
The reflex-type design is chosen for non-viscous, colorless liquid.
Transparent Gauge Glasses.- is a gauge glass that uses flat glasses enclosed in metal bodies and
covers to protect against breakage when used in boilers or high-pressure vessels. it indicates the
level of a liquid by permits direct observation using Clear glass mounted on the front and back of
the gauge.
The Armored/Transparent gage is used for colored, viscous, and corrosive liquid.
11.
12. Floats for Level Measurement
•Some types of level instruments depend on the buoyancy of an object to
measure level.
•A floating object on the surface of a liquid ,determines the level of that liquid.
•Various types of floats and displacers are commonly used to measure level.
13. Ball Float
• A float is a point level measuring
instrument consisting of a hollow ball that
floats on top of a liquid in a tank and is
attached to the instrument.
• The float is connected by a lever to an
ON/OFF switch actuated by the
movement of the float.
• Floats can be used to indicate tank levels,
actuate alarms or shutdown switches, or
even mechanically control valves.
• The switch can start a pump when the
float is at one position and stop the pump
at the other position.
14. Tape Float
• A tape float is a continuous level measuring instrument
consisting of a floating object connected by a chain,
rope, or wire to a counterweight, which is the level
pointer.
• A scale fastened to the outside of the tank shows the
reversed tank level with 100% being at the bottom and
0% being at the top.
• When the float is at the bottom of the tank, the tank is
empty and the counterweight is at the top.
• When the float is at the top of the tank, the indicator
pointer is at the bottom of the scale.
• As level goes up, the float moves up and counter
weight along with the pointer moves down, and vice
versa.
• This type of gauge is normally used on storage tanks
for inventory purpose
15. Level Measurement through Pressure measurement
• There are many applications where it is more convenient to measure the pressure
at the bottom of a tank than to measure the actual location of the top of the liquid.
• This is the most commonly used method for sealed tanks.
• Tanks may be sealed to prevent the escape of volatile or toxic fluids.
16. HYDROSTATIC PRESSURE LEVEL SENSORS
• This method uses the measurement of pressure exerted by the liquid.
• it provides the means of determining liquid level in a storage vessel. As long as the liquid in the
tank has a constant density, variations in pressure are caused only by variations in level.
• A pressure gauge connected at the bottom of the tank will measure pressures that are directly
proportional to the weight of the column of water liquid above it and, thus, proportional to
the depth.
• Therefore, the pressure indicated by the instrument is directly proportional to the height of
the column of liquid above the gauge.
17. Exercise#1
•A tank holding 13 feet of liquid exerts an hydrostatic pressure on the
gauge mounted at the bottom. Calculate the magnitude of this
hydrostatic pressure, in units of PSI?
18. Differential Pressure Transmitter
•The differential pressure (dP) transmitter converts the pressure of the
liquid level into a standardized signal. Normally, a 3-to-15-psi
pneumatic signal or 4-to-20-ma Electronic signal depends on the type
of the transmitter.
•This device senses the difference in pressure between two ports and
outputs a signal representing that pressure in relation to a calibrated
range.
20. Differential Pressure Transmitter
•The most common sensing element used by modern DP transmitters is
the diaphragm.
•One side of this diaphragm receives fluid pressure from the “high”
port, while the other receives fluid pressure from the “low” port.
•Any difference of pressure between the two ports causes the
diaphragm to flex from its normal resting (center) position.
• This flexing is then translated into a standard output signal (e.g. 3-15
PSI or 4-20 ma)
23. Level Measurement by Differential Pressure Transmitter
•OPEN TANKS
The high side of the dP cell is connected to the bottom of the tank, and the low side of the dP cell is open to the
atmosphere. Since the tank is open to the atmosphere, the pressure of the liquid on the high side of the dP
transmitter is directly related to the level in the tank.
24. Exercise#2
• A storage tank contains water with density of 62.4 Ib/ft3. A pneumatic pressure transmitter
located at the bottom measure water level by hydrostatic pressure (head).
1. Construct the full corresponding calibration range table based on 25% reading of the span of this pressure
transmitter in order to properly translate the range of vessel level (0 to 216 in) into an output signal of (3 to 15)
PSI.(Km=0.03606 psi/in)
2. Determine the Transmitter output signal (PSI) at 144 in of level.
3. Determine the Water level at 5.9 PSI signal output.
29. Differential Pressure Transmitter
Closed Tanks
• If the tank is pressurized(closed), a differential pressure (d/p) instrument is required with the
high-pressure connection at the lowest level and the low-pressure connection in the vapor space
above the liquid. The pressure in the high-pressure line is the sum of the tank pressure and the
hydrostatic pressure due to the liquid height. Since a differential pressure transmitter responds only
to differences in pressure between “High” and “Low” sides, it will naturally subtract the gas
pressure to yield a measurement based only on hydrostatic pressure
30. Differential Pressure Transmitter
Compensating Leg(dry leg)
the tank is closed, and there is a“dry leg” between the instrument connection near the top of
the tank and the low (Lo) pressure port on the dP cell. A “dry leg” is defined as the length of
instrument piping between the process and a pressure transmitter that is filled with air or
another noncondensing gas.
4ma= Lmin(inches) * S.G = 0 * 0.95= 0 inH2O
20ma=Lmax(inches) * S.G = 100 *0.95=95 inH20
For Gas Pressure
Pgas= L* S.G= 120*1=120 inH20
31. Differential Pressure Transmitter
Compensating Leg(Wet leg)
• the tank is closed, and there is a “wet leg” between the top instrument tap and the low-pressure (LP) side of the dP
cell. A “wet leg” is defined as the length of instrument piping between the process and a pressure transmitter that is
filled with a compatible fluid and kept at a constant height.
4ma= (Lmin(inches) * S.G ) – (120*1.2)= 0 -144= -144 inH2O
20ma=(Lmax(inches) * S.G )-(120*1.2) = 95 -144=-49 inH2O
32. Differential Pressure Transmitter
•For dry leg applications,
• The dry leg must be dry. Condensation creates measurement error
•For a wet leg Applications,
• The density of the wet leg fluid should be maintained
• The height of the wet leg fluid must be constant.
• High temperature change will cause a change in the density of the fill fluid.
33. Bubbler System
•A bubbler is a level measuring instrument consisting of a tube extending to the
bottom of a vessel; a pressure gauge, single-leg manometer, transmitter, or
recorder; a purge flowmeter or sight feed bubble; and a pressure regulator.
•A gas is slowly fed into the bubbler system until the pressure is equal to the
hydrostatic head of the liquid in the tank.
•At that point, the gas flow bubbles out of the end of the bubble tube.
35. SONIC AND ULTRASONIC LEVEL SENSORS
• In applications when it is not acceptable for the level measuring instrument to
contact the process material, The generation and detection of sound waves is
another common method used to detect level. sonic or ultrasonic device may be
used.
• These devices measure the distance from a reference point in the vessel to the
level interface, using sonic or ultrasonic (sound) waves.
• measurement is made by measuring the elapsed time between emission and
reception of a signal reflected from a surface in a vessel.
• The amount of sound wave energy reflected from a surface depends on the
process material .
• Both liquids and solids are highly reflective, although solids are affected by
surface absorbency, material thickness, and stiffness.
• Sonic devices operate at frequencies of about 10 kHz, whereas ultrasonic
devices operate in the 20-40 kHz range.
36. SONIC AND ULTRASONIC LEVEL SENSORS
Ultrasonic level sensors measure the time it takes sound waves to travel through material. The velocity of a sound wave is a
function of the type of wave being transmitted .When a sound wave strikes a solid medium, such as a wall or a liquid surface,
only a small amount of the sound energy penetrates the barrier; a large percentage of the wave is reflected. The reflected
sound wave is called an echo.
Distance=speed of sound*(time of /2)
37. • The sound waves are produced by the generator and transmitter circuit, and the
transducer sends out the sound waves. These sound waves are reflected by the
material or the level being measured. A transducer senses the reflected waves and
converts the sound wave it received into an electrical signal. This signal is amplified
by a receiver/amplifier circuit and sent to a wave shaping circuit. A timing
generator is used to synchronize the functions in the measurement system. The
instrument measures the time that elapses between the transmitted burst and the
echo signal. This elapsed time is proportional to the distance between the
transducers and the object being sensed. You can easily calibrate the instrument to
measure fluid or material level in a process vessel.
Block diagram of ultrasonic level-measurement system
38. •A wide variety of instruments and sensors use basic electrical principles to
measure and detect level. The three common electrical-type level-
measuring devices are:
• Capacitance Probes
•Conductivity Probes
•Resistance Probes
Level Measurement by Electrical Sensors
39. Capacitance Probes
• A change in the characteristics of the material between the plates will cause a change
in dielectric constant, which is often larger and more easily measured than changes in
other properties.
• This makes the capacitance probes suitable for use to detect the level of material in
vessels because changes in process level change the dielectric constant.
• The capacitance change is directly proportional to the level of the liquid.
• As the dielectric Material Constant increases, the capacitance is also increasing
40. Capacitance Probes
• The electric size in farads of a capacitor is dependent on its physical dimensions and on
the type of material (dielectric) between the capacitor plates. The equation for a parallel
plate capacitor is given by the following:
Where
A = the area of the plates
d = the distance between plates
K = the dielectric constant, as listed in Table
41. Conductivity Probes
• Conductivity probes operate on the principle that most liquids conduct electricity.
• An electrode is shown above the liquid level on the left side of Figure . The circuit is
therefore open, and no current is flowing through the level probe electrodes to
energize it. When the liquid level rises, it establishes a conductive path at the
electrode, closing the low-level switch. When the level probe is activated, it closes dry
contacts in the instrument. This contact closure can be used to operate electric relays,
pumps, solenoid valves, or other equipment
42. Resistance Probes
•In these devices, resistive material is spirally wound around a steel tape.
•This type of probe is mounted vertically from top to bottom on a process tank.
• The pressure of the fluid in the tank causes the resistive tape to be
short-circuited, thus changing the total resistance of the measuring tape.
• This resistance is measured by an electronic circuit and is directly related to
the liquid level in the tank.
