2. Introduction 2
Petrochemical manufacturing units use many chemicals, solvents, steam and
other liquids, and it is frequently necessary to measure the level, either to
monitor the quantity of fluid in the container or to maintain system pressures
at the required value, or often simply to ensure that there is fluid in the vessel
and that it is not overflowing. The degree of accuracy required in each case
varies according to the reason for measurement.
Introduction
3. Table of Contents 3
INTRODUCTION
LEARNING
OBJECTIVES
LEVEL
MEASUREMENT
MAIN MENU
4. • One of the most common, and most useful, pressure measuring
instruments in industry is the differential pressure transmitter. This
device senses the difference in pressure between two ports and
outputs a signal representing that pressure in relation to a calibrated
range. Differential pressure transmitters may be based on any of the
previously discussed pressure-sensing technologies, so this section
discusses practical application rather than internal workings.
4
Pneumatic Transmitter
6. In a similar manner, an increasing pressure applied to the “high” port of a DP transmitter
will drive the output signal to a greater level (up), while an increasing pressure applied to
the “low” port of a DP transmitter will drive the output signal to lesser level (down)
6
Pneumatic Transmitter
8. The simplest of all the systems of level measurement
is a set of graduations on the wall of an open tank,
there the liquid actually covers the scale as its level
rises.
The dipstick is also very simple, the stick being
dipped periodically through an access hole into the
tank, and the immersion mark being read off
graduations on the stick. A useful variation of this is
the dip-tape.
Sight Glasses and Dip Sticks 8
Dip Stick
Dip Tape
Sight Glass and Dip Stick
9. Where these methods cannot be used, whether for safety or accessibility
reasons, the sight glass method is often used. Depending upon the
manometer principle, the transparent tube is placed in a convenient position
and is connected to the the tank, and graduated. Figure 1. refers.
Sight Glasses and Dip Sticks 9
Sight Glass and Dip Stick
11. Float and Displacer Devices 11
Theory
Although float and displacer devices are often similar in appearance
they have differing theories of operation. Float devices operate on the
Buoyancy Principle, as liquid level changes a (predominately) sealed
container will, providing its density is lower than that of the liquid, move
correspondingly.
Float and Displacer Device
12. Float and Displacer Devices
Float and Displacer Devices 12
Theory
Displacers work on the Archimedes Principle, when a body is immersed in a
fluid it loses weight equal to that of the fluid displaced. By detection of the
apparent weight of the immersed displacer, a level measurement can be
inferred. When the cross sectional area of the displacer and the density of the
liquid is constant, then a unit change in level will result in a reproducible unit
change in displacer weight.
13. Float and Displacer Devices
Float and Displacer Devices 13
There are numerous variations in the devices using the float as a sensing
element, but all are based on the fact that the float will follow the surface of
the liquid. The problem then becomes one of designing a mechanism to follow
the float. In general, there are two methods.
The first is to arrange to have the float position a lever. This lever motion is
then used to drive linkages and/or pneumatic mechanisms.
14. Float and Displacer Devices
Float and Displacer Devices 14
The second method is to follow the float with a cable which is usually a wire or a
flat metallic tape. The cable then serves as the input to pneumatic, electrical, or
mechanical mechanisms.
The lever method has the, advantage of simplicity, but it has the serious
disadvantage that the range of level measurement is very limited. This is because
the lever must be kept fairly short.
15. Float and Displacer Devices
The cable method does not have this disadvantage, the range
being limited only by cable lengths.
It however requires a fairly formidable take-up mechanism if the
measurement is displayed on recording devices.
Both methods share the same problem of a pressure-tight bearing
if the measurement is being made on a liquid within a pressure
vessel. For the lever device, the pressure-tight bearing is usually a
packing stuffing-box through which the lever shaft rotates, stuffing-
box friction is not too much of a problem because the float may be
made large to furnish the driving power required.
Float and Displacer Devices 15
16. Float and Displacer Devices
A pressure-tight bearing for float and cable devices presents an extremely
difficult problem. In general, the only practical solution is a gas-tight liquid seal
through which the cable passes. One acceptable method for avoiding a
pressure-tight bearing is to follow the float with a magnet that drives the
cable. The cable and magnet are inside a closed pipe immersed in the liquid.
The float is guided by this pipe and rides up and down the pipe as the level
changes. The magnet follows this float.
Float and Displacer Devices 16
17. Float and Displacer Methods
Displacer and Displacer Methods 17
Principle of the displacer
Spring Balance
Displacer
Principle of operation of Displacer
For the purposes of describing the operation, a
simplified system has been used. The actual
instrument is somewhat more complicated, though
in principle it is identical to the spring scale and
displacer..
19. Radiation Methods
Theory
Radiation level controls are used for point and continuous measurements,
typically where most other technologies are unsuccessful. The radioisotopes
used for level measurement emit energy at a fairly constant rate but in random
bursts. Gamma radiation, the source generally used for nucleonic level gauging
is similar to microwaves or even light (these are also electromagnetic radiation,
but of lower energy and longer wavelength). The 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.
Nuclear Methods 19
20. Radiation Methods
Nuclear Methods 20
Different radioisotopes such as Caesium Cs137 or
Cobolt 60 are used, based on the penetrating
power needed to "see" the process within the
vessel. With single point gauges the radiation
provides a simple on/off switching function,
whereas with continuous level measurement the
percentage of transmission decreases as the level
increases.
21. Radiation Methods
The equipment consists of three units; a control unit, a detector Unit and an
emitting radioactive source housed in a shielded container. Gamma radiation
from the radioactive source is directed through the vessel to the detector
which is mounted opposite the source on the plane where the level is to be
monitored. Gamma radiation reaching the detector produces a high voltage
discharge which is registered by the control unit.
Nuclear Methods 21
22. Radiation Methods
Nuclear Methods 22
Nucleonic proportional level detector
Source
Output Signal
to
Control Room
20`
Receiver
Sensitive Length of
detector
Detector Output
pulses/second
Vessel Level
24. Ultrasonic Methods
Theory
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. A successful measurement depends on
reflection from the process material in a straight line
back to the transducer.
Ultrasonic Methods 24
25. Ultrasonic Methods
Configuration
A measuring system consists of an ultrasonic transducer and an electronic unit.
Function
The transducer transmits ultrasonic pulses per second. These pulses are
reflected by the surface of the material being measured, and converted into
electrical pulses in the receiver.
Ultrasonic Methods 25
26. Hydrostatic Methods
Gauge methods
The simplest arrangement of an indirect method of level measurement is the
pressure gauge connected at the bottom or side of a uniform tank containing
liquid.
The rise or fall of the level causes changes in pressure, which are transmitted
to the gauge. The dial or scale of the instrument may be graduated in units of
length or volume etc.
Hydrostatic Methods 26
Pressure = (height of liquid) x (density of liquid) P = h x p
h
p
P
27. Hydrostatic Methods
If the nature of the liquid under measurement is such that it must not enter
the actual gauge, then a transmitting fluid (e.g. oil) must be used between the
liquid and the gauge mechanism.
Ther pressure variation in transmitting medium, due to change in the liquid
level is thus transmitted to a pressure receiver.
Pressure sensing element
A diaphragm, capsule or bellows may be used as the primary sensing
element.
Hydrostatic Methods 27
30. Load Cells
The reliability of electrical
equipment is accepted without
question in all branches of
engineering today, but doubt
sometimes exists as to the
robustness of electrical equipment
compared to its mechanical
counter-part. No reservations
apply to the load cell which
structurally is as robust as the
equipment in which it is installed.
Load Cells 30
31. Load Cells
Theory
All the best ideas are simple and the load cell is no
exception. Consisting essentially of a high tensile,
alloy-steel billet, its action is based on the
fundamental law that, within the limits of
elasticity of the material, a body suffers
deformation directly proportional to the applied
load. When the load is removed, the body reverts
to its original shape.
Load Cells 31
32. Load Cells
It will be obvious that, for steel, the actual change in dimensions under load
will be extremely small of the order of micrometers, and depending on the
design of the billet, the load may be anything from, say, 20 kg to a thousand
tones or more. To sense these small changes and to provide an electrical
output, use is made of the resistance strain gauge.
This is a highly sensitive, accurate and reliable means of sensing linear change
and has been perfected after many years of application in aircraft testing and
other fields.
Load Cells 32
33. Load Cells
When a thin resistance wire is stretched, its length increases and its cross-
sectional area is reduced. Both actions result in an increase in the electrical
resistance and this change can be detected and displayed by suitable electrical
or electronic equipment. The change in resistance can be made directly
proportional to the strain and this simplifies conversion to any desired weight
scale.
Construction and use
The strain gauges are cemented to the billet. In practice, two active and two
passive elements are used, connected in a bridge circuit.
Load Cells 33
35. Load Cells
The output voltage from the bridge is directly
proportional to the thrust and this voltage is
sensed by associated equipment connected to
the cell by a cable. This equipment displays
the information on an easily-read dial
calibrated directly in weight or records it on a
paper, chart or provides an output in analogue
or digital form for external processing.
The load cell may be completely enclosed to
provide protection against mechanical damage
and the adverse effects of dust, liquid or
gaseous pollution.
Load Cells 35
36. Capacitance Methods
Theory
The capacitance probe type element is simple in
design and reliable in its operation on installations
where there is sufficient difference in the dielectric
constants of the liquid being measured and the gas
or vapour above the liquid. The output of the probe
is a capacitance change which is proportional to the
level of the liquid being measured.
Although the probes described above are for use
with non-conducting (dielectric) fluids, others which
are coated with Teflon or other suitable plastics, are
available for use with conducting fluids.
Capacitance Methods 36
1
2
37. Capacitance Methods
A level measuring system of the capacitance type consists of two main parts:
1) The probe which changes the level variations into capacitance
variations.
2) The Electronic unit.
Function
As the level rise’s and material begins to cover the sensing element, the
capacitance within the circuit between the probe and the media (conductive
applications) or the probe and the vessel wall (insulating applications)
increases. This causes a bridge misbalance, the signal is demodulated
(rectified), amplified and the output is increased.
Capacitance Methods 37
2
38. Capacitance Methods
The capacitance for the basic capacitor arrangement
shown in the Figure and can be computed from the
equation:
C = E (K A/d) where:
C = capacitance in picofarads (pF)
E = a constant known as the absolute permittivity of
free space
K = relative dielectric constant of the insulating
material
A = effective area of the conductors
d = distance between the conductors
Capacitance Methods 38
Capacitance Methods- Basics.
All share the same principle of operation.
