Introduction to fundamentals of instrument & control

1. INTRODUCTION to FUNDAMENTALS
Of Measurement & Control
By Eng. Essam Marzouk
1

2. Introduction to the Fundamentals of Measurement and Control
Objectives and Abstract
Unit Introduction to Fundamentals of Measurement and
Control.
Scope this is an introductory unit, its purpose is to provide
The foundation for a progressive understanding of
Fundamentals of Measurement and Control of any
Process.
Prerequisite None.
Unit Objective On completion of this unit, the student will be able to
Discuss the fundamentals of Measurements and
Control in terms of both main process’s parameters
With its measuring units and instruments.
Specific Objectives Student will be able to:
1. Describe the concept of the main parameters
controlling any process.
2. Understand how these parameters are measured and
the unit system.
3. Understand and discuss the basic requirement of
controlling a process (Different types of controlling
loops).
4. Describe how these parameters are reflecting on the
performance of a process.
5. Understand the main concept of the main controlling
loop’s elements.
Unit Outlines/Contents
a. Introduction to process control
b. Units and Dimensions.
• SI.
• fps
• cgs.
• MKS.
c. Process’s Parameters
• Pressure.
• Level.
• Flow.
• Temperature.
d. App
2

3. e. lications to above Parameters.
• Measuring/Sensing Pressure.
• Measuring/Sensing Level.
• Measuring/ Flow.
• Measuring Temperatures.
f. Controlling Loops
• Feed Forward.
• Feed Back.
• Cascade.
• Split control.
g. Process Controllers
• On-Off Control Action.
• Proportional Control Action.
• Rest (Integral) Control Action.
• Rate (derivative) Control Action.
• Proportional Plus Integral.
3

4. Introduction to Process Control
Since the very early days, once the man had stepped his feet on the earth planet, he had been trying
to adopt himself to interact and live with the wild environment surrounding him. His need to live
and protect himself has been urging him to develop and improve his life style.
As god shaded him with the mind wealth, he had been chosen to be the master among the livings.
He had started to watch, understand and looking for satisfying answers to many amazing and
mysterious things happening around him; to be able to fulfill his previous target of safe and
comfortable life.
He realized the earliest simplest –but complicated- golden fact; that to do any thing, he has to act
or in a different way to gain; he has to lose /give. This means that to gain certain output out of
any thing; he has to put some inputs. However as he has being traveling in his journey of
development, he added to his knowledge that such inputs should be treated in a way to guarantee
the required specified output.
Recently this could be expressed, as these inputs should be controlled to fruit the desired output.
Process Control
It has been settled that any process should be controlled to get a certain output which
could be tolerated to a limit depends on the type of the output, its purpose and the
nature of the process adopted to produce such output.
Consequently the objective of any control system is to maintain a balance between the
supply (Inputs) and demands (output) of the process over a period of time. In another
words, it is to balance Energy/Materials consumed to get something out of the process.
OUTPUT
INPUTS
PROCESS
From the above figure, to get a specified output; the inputs should be controlled through
understanding the properties of them which could be Weight, Pressure, Volume/Level
and Flow –for materials- or Heat/Temperature, Voltage/Current and Potential/Intensity of
fields –for energy-.
Such parameters have its own features and effects on the process, which could be
reflected into two different ways to the process. The first way, if the behavior of the
process is studied within the time boundary under the effect of any of the above
parameters; it is called Dynamic characteristics; otherwise it will be Static one.
In this unit the features of these parameters will be studied, HOW they could be
described, evaluated/measured, WHAT are their effect on a process, WHEN they have to
be controlled, WHO/WHAT will respond to the changes happen to them.
It could be a good starting to begin with the definitions of the physical parameters, their
measuring units, and stating the basic main relations linking them together.
Moreover, a reasonable brief will be given to each of the following:
PRESSURE, LEVEL, FLOW and TEMPERATURE.
4

5. Part I
Units & Dimensions
The United Kingdom has now adopted the system of metric units known as the System
International d’ Units, abbreviated to SI. It has six basic units, which are defined as:
Length: meter (m)
Mass: kilogram (kg)
Time: second (s or sec)
Electric current: ampere (A)
Absolute temperature: Kelvin (k)
Luminous intensity: candela (cd)
All other units are derived from these fundamental units, for example:
Force = mass x acceleration
This means that the units of force are the units of the resultant product of both mass
and acceleration. Larger or smaller units are formed by adding a prefix to the basic unit.
Ex: Multiples Sub- multiples
Deca da = x 10 deci d = x 10-1
Hecto h = x 102 centi c = x 10-2
Kilo k = x 103 milli m = x 10-3
mega M = x 106 micro µ = x 10-6
giga G = x 109 nano n = x 10-9
tera T = x 1012 pico p = x 10-12
femto f = x 10-15
atto a = x 10-18
metric tonne = 103 kg = 2205 lb.
The prefixes shown in italics are not preferred but they are still in common use.
While SI units are the preferred system, there are other systems, which remain of
importance in various parts of the world and in particular fields of activity. The Foot-
pound-second system (fps), the centimeter-gramme-second system (cgs) and the meter-
kilogram-second system (MKS). These two systems are coherent to each other and
based on the Newton’s second law, and appear in two forms; the Absolute and
Technical system. In the absolute system; the unit of mass is a fundamental unit while
the force is derivative one. In technical system is the opposite, refer to the given below
table:
Fps Cgs MKSQuantity
Absolute Technical Absolute Technical Technical
Length
Time
Mass
Force/Weight
ft ft
sec sec
lb-mass slug
pound lb-force
cm cm
sec sec
g--mass 981 g
dyne g--force
M
sec
9.81 kg
kg-force
Dimensions
The units chosen for measurement do not affect the quantity measured.
1 kg of water = 2.2046 lb of water
Sometimes it is convenient to use the fundamental dimension of mass, length, and time
(M, L, T respectively).
The given table is giving the Dimensions and units of common quantities
5

6. Quantity Defining equation Dimensions Unit Symbol
Geometrical
Angle Arc/Radius M0L0T0 Radian rad
Length L meter m
Area Length2 L2 Square meter m2
Volume Area x Length L3 Cubic meter m3
Strain Extension/length L0 A ratio
Kinematic
Time T Second Sec
Linear Velocity Distance/Time LT-1 meter/sec ms-1
Angular Velocity Angle/Time T-1 radians/sec rads-1
Linear Acceleration Linear velocity/Time LT-2 meter/sec
square
m-2
Angular Acceleration Angular Velocity/Time T-2 radians/sec
square
rad s-2
Rate of discharge Volume per Time L3T-1 cubic
meter/sec
m3s-1
Dynamic
Mass Force/Acceleration M Kilogram Kg
Force Mass x Acceleration MLT-2 Newton=kg-
m/sec2
N=kgms-
2
Weight Force MLT-2 Newton N
Density Mass/Volume ML-3 Kg/meter3 Kgm-3 ρ
Specific weight Weight/Volume ML-2T-2 Newton N
Specific gravity Density/Density of
Water
M0L0T0 Ratio
Pressure Force/Area ML-1T-2 N/meter2 =
pascal
Nm-2 =
pa
Stress Force/Area ML-1T-2 N / meter2 Nm-2
Elastic modulus Stress/Strain ML-1T-2 N / meter2 Nm-2
Impulse Force x Time MLT-1 Newton.Sec Ns
Moment of Inertia Mass x Length2 ML2 Kg.meter2 Kgm2
Linear momentum Mass x Linear Velocity MLT-1 Kg.meter/sec Kgms-1
Angular momentum Moment of Inertia x
Angular Velocity
ML2T-1 Kg.meter2 / sec Kgm2s-1
Work, Energy Force x Distance ML2T-2 Newton meter
=Joule
Nm = J
Power Work x Time ML2T-³ Joule / sec =
watt
JS-1 =W
Moment Force x Distance ML2T-2 Newton meter Nm
Dynamic viscosity Shear stress/velocity ML-1T-1 Kg/m.sec = 10
poise
Kgm-1s-1
Kinamatic viscosity Dynamic
viscosity/density
L2T-1 Meter2 / sec m2s-1
Surface tension Energy/area MT-2 Newton/meter
= kg/sec2
Nm-1 =
kgs-2
6

7. Part II
Pressure
Contents
SUBJECT Page
Introduction 8
What is Pressure 8
Gas Pressure 8
Liquid Pressure 9
Atmospheric Pressure 9
Pressure Sensitive Devices 10
Pressure Sensitive Error 10
U Tube Manometers 11
Bourdon Tubes 12
Dead Weight Tester 13
Bellows Elements 14
Electric Capacitance Sensor 14
Piezoelectric Pressure Transducer 15
Resistance Strain Gauge 16
Pressure Switches 16
Applications & Devices 17
Pressure Regulators
Flapper & Nozzle Systems
Pneumatic Relay
Transducers 21
Pressure to Current Converter
Current to Pressure Converter
Pressure Transmitters 23
Vector Servo Recorder 25
More Tips 27
7

8. Introduction
A major portion of all industrial measurement relates in some way to pressure in
its several forms. Flow for example is often measured by the determent of the
pressure difference between two points in a system. In a Bourden system, the
change in the pressure value inside a Bourden tube is used to produce a
mechanical deflection (motion) of a pointer on a scale or a pen on a chart.
Pressure can also be used to measure temperature in a filled system through
changes produced by expansion or contraction in a sensing element. Therefore
and for these reasons, it is important to understand the meaning of pressure, its
variation, its reflection to a process, who it could be sensed how it could be
treated in a way/form to be able to interact with a process.
What is Pressure?
5 kg’s
BA
Fig. “1”
Pressure is a term used to describe in a quantifying way the amount of force
applied to the unit of area of a surface.
In the above figure A and B are two equivalent weights exerting a down pushing
down forces to two studs to go through the shown surface. Notice that each has a
different tip’s shape. Now let us ask our self to what depth each will go and
why?
∴ Pressure = Force / Area
= Newton / m2
Pascal US
= Pounds / inches2
psi metric
Pressure Conversion Table
CONVERSION
Unit
psi. Inches
water
Inches
mercury
Standard
atmosphere
Bar pascal
1 psi. 1 27.68 2.0360 0.0680 0.0689 6895
1" water 0.0361 1 0.0734 0.00246 0.00248 249
1" mercury 0.4911 13.595 1 0.03342 0.0338 3378
1 Atmosphere 14.696 406.8 29.921 1 1.0133 101325
1 bar 14.503 401.36 29.53 0.9869 1 100000
1 pascal 14.5x10-3
40.2x10-
4
29.6x10-3
9.87x10-³
1x10-5
1
Gas Pressure
Gases have pressure because of the bombardment of gas’s molecules with the
walls of its container. Heat has its effect to the gas pressure as gases have no
coherence/attraction force between its individual molecules. Consequently the
molecules tend to fill the space of the container moreover applying heat will
8

9. agitate the molecules and push it to bombard the internal sides of the container
which means exerting a pressure.
∴
2
2
1
1
T
P
T
P
= at constant volume (pressure temp. law)
i.e. P α T this is used to measure temperature
(Expansion & Contraction).
This leads to conclude that the pressure exerted by a
fixed volume of gas depends on the temperature of the
gas. (ssv rupture disc of the wellheads).
Liquid Pressure
A liquid is a state of matter wherein there is a limited attraction force between its
molecules ( but higher than that of the gas). Therefore if a liquid is poured into a container, it will fill
the container to a uniform level while the gas will fill the entire container.
Cohesion It is the attraction force between molecules of the same material
Adhesion It is the same as above but between molecules of different materials.
The pressure at a point in a liquid is directly proportional to the depth of that point.
Ex: pressure at point “a” equal to the weight of the liquid above it divided by the area.
P = liquid weight above “a”/ surface area
= ==
area
umedesityxvol
A
w
A
xv.ρ ρ
h .a
h
A
hA
.
..
ρ=
ρ
= Fig. “2”
Where A is the area of the surface,
V is the volume of liquid above point “a”,
h is the depth of “a” and
ρ is the density of the liquid.
Liquid has aside pressure called the head (i.e. pressure at certain depth in a liquid is
equal as it is at the same level).
Surface Tension It is the resultant force acting on the surface of a liquid due to the
Attraction force between both molecules of the liquid and the
air.
Capillary Action It is the rise of a liquid surface inside a capillary tube due to the
attraction force between molecules of both liquid and walls of the
tube. It will continue till the weight of the liquid column balances
t
Atmospheric Pressure
It is the weight of the air column above the sea level; it is equal to 14.7 psi or 29.92
inches or 76 cm of mercury, 101.33 kpa. It is called bar.
Gauge Pressure It is the measured pressure value taking in consideration the
atmospheric pressure as a reference.
Absolute Pressure It is the measured pressure value, which eliminates the value of
The atmospheric pressure (its reference is a real zero).
Vacuum It is a gauge pressure less than the atmospheric value. It is
9

10. measured in inches or millimeters of mercury (Hg).
This point changes with atmospheric
Conditions and altitude
Fig. “3” 0
This dimension is a barometric pressure value measured at the sea
level, it is equal to 1 bar/29.92"Hg.
this point does not change and has absolute Zero pressure
Pressure Sensitive Devices
Now, it is required to know how pressure could be sensed or measured?
The most common pressure-sensitive device is the Bourdon element. In addition
to it, several other important devices are:
1. Limp Diaphragm.
2. Bellows.
3. Diaphragm.
4. Manometer.
The given table shows these types and their recommended applied ranges:
Pressure Span Device
Low 5 lb/in2
(34.5 kPa) or less Limp diaphragm, Manometer
Medium 5-25 lb/in2
(34.5 – 172.4
kPa)
Bellows, Manometer, Diaphragm in
conjunction with a balancing force
circuit, Bourdon element – if extreme
sensitivity is not needed-.
High 25 lb/in2
(172.4kPa) & up Bourdon Element.
Sensing elements of Diaphragm or Manometer are widely used to measure
differential pressure. Diaphragms need a force feedback circuit. Manometers
could be arranged to record and/or indicate an electric or pneumatic signal.
Pressure Sensitive Element Errors
The above-mentioned sensing devices have some errors due to the following:
Variation of Modulus of Elasticity
It is the linear relation (theoretically) between stress applied to any material and its strain. This
modulus of elasticity varies with time (this is known as aging) with copper base alloys, the
change is usually a slight increase, while with some other alloys it is increasing (max. up to 5.0%).
This aging is due to release of internal stresses arising during the manufacture phase and may be
removed by low temp. heat treatment.
Elastic Drift and Recover
Every time a pressure is applied to a sensing device; a minute drift –in a form of an increase- in
the direction of the pressure applied, will be produced. To recover this drift, the element should
be in a reset state for at least 24 hr’s.
Zero Shift
It is the change of the neutral position from which there is no recovery. It is generated due to the
release of the internal stress of the substance by cold working.
Hysteresis
It is the difference in reading at any given stress measured between an increase and decrease of
stress in a short period of time.
10

11. UTUBE MANOMETER
Sealing
Unknown Pressure
Normal U
1- If this cap is sealing; a vacuum will be
created and absolute pressure will be read.
2- If it is opened to atmosphere; a gauge
pressure will be there.
3- If both ends (limps) are connected to
unknown pressures; it will measure a
differential absolute pressure.
Fig. “4”
The second type of manometers is called Well Manometer
Fig. “4”
It is used to measure very low pressure in inches of
water.
The Unknown pressure = =
BA
H
/ A
BH.
, where
A & B are the areas of the two limps.
Level in tube “B” stays substantially constant than
The normal U tube, as the force required keeping
the Level up in tube “B” is much less than that
required for tube “A”.
Fig. “5”
This type is called
INCLIND manometer
It gives more accurate
low pressure in an easier
way of observation due
to the longer tube used
to measure,
11

12. B
nt
nd pinion type. In order to convert the deflection of the tube tip into rotary
ied the tube tends to straighten. The movement is very nearly
nd
ero are provided.
of a longer tube wound
irally to give three or four wraps. It is anchored at its open inner end and the
teristics to the spiral one (it gives more deflection). It could be
connected directly to the pointer as its deflection produces a rotary motion.
(Figure “8”).
ourdon Tube Gauges
The bourdon tube is the most common of all the pressure gauges. It relies on the
simple mechanical principle, which needs few moving parts. It consists primarily
of an elliptical cross-section tube bent into a “C” shape. One end of the tube is
anchored to the pressure connection, while the other is sealed and some form of
linkage is attached. This linkage is usually of the rack and pinion or quadra
a
pointer movement. A hairspring is fitted to eliminate backlash in the linkage.
As pressure is appl
linear with pressure, therefore facilities to adjust the linearity, span/range a
z
-Zero is normally adjusted by re-positioning the pointer- Refer to figure “6”.
For certain applications greater sensitivity is required and two other forms of
tube may be used. The flat spiral Bourdon tube consists
sp
pointer is attached by a link to the outer end (Fig.“7”)
Another arrangement of the Bourdon tube is the helically wound type. It has
similar charac
12

13. DEAD WEIGHT TESTER
It is basically a hydraulic pump, used mainly as a standard reference device.
Fig. “9”
The device as it is clear from figure “9” has a reservoir filled with hydraulic oil
wh e valve to both floating and “A” pistons. On dragging the
“A”
1.
3.
the
eight floats freely.
ich flows through th
piston out; the cylinder “A” will be filled with oil.
To calibrate a gauge:
2. Fix this gauge as shown.
Close the valve
4. Put known standard weights on top of the floating piston and equal to
required calibrating value (range).
5. Use the hand wheel to start pumping oil to the floating piston till the
carried w
The applied weight will exert a force F which produces a pressure P, where
P=
A
F
A is the cross section area of the floating
e full range,
e oil leakage.
o expand the range of the device the piston and its chamber (cylinder) could be
changed to provide a typical range of “6000” psi or 407 bar.
This device could be used as well to compare two gauges by replacing the
floating piston and its chamber with a standard gauge.
piston/cylinder.
Normally the error in such arrangement is within 0.05% of th
provided that thee piston and chamber are made of low thermal expansion
material and machined to a low tolerance to minimize th
T
13

14. BELLOWS ELEMENTS
The given below two figures 10(a) & 10(b) show a device which is used to sense
pressure.
Fig. 10(b)Fig. 10(a)
Electric Capacitance Sensors
Another type of sensors, which do not adopt theories of pneumatic effects and
nce
to
ity of the dielectric has its effect.
As it is seen in figure “11” which is self explanatory, the process pressures for
both sides HP & LP affect the position of the sensing diaphragm which
consequently affects the capacitor’s value
actions, these types are acting in the domain of electricity and electronics.
As it is clear from the main heading title, our current type uses the capacita
effect…. how?
Before going through, we have to remember what is the capacitance effect?
If two electrically conducting plates are charged, placed close together and
separated by an insulating medium; then a capacitance effect will be formed.
This effect is directly proportional to the area of the plates and inversely
distance between them. Moreover the qual
14

15. Fig. “11”
PIEZOELECTRIC PRESSURE TRANSMITTERS
It is particular types of mono-crystalline, such as Quartz, Tourmaline and
barium. Such materials build electric charges on a certain faces of their crystals
when submitted to mechanical forces. The change of the charge is proportional
to the applied force.
+ _
+ Q + _ _Q
+ _
Fig. “12”
The piezoelectric effect is a reversible phenomenon. If an electric potential is
connected to certain boundaries of a crystal, a corresponding change of shape
will result. This effect is highly stable and precise (therefore it is used in
fabricating quartz clock time control).
For the above these types of crystals are used as a pressure-sensing element to be
used in the field of control.
They could be used for measuring very fast pressure fluctuation of a range of
below 1 mbar up to 10kbar (frequency variation is 100 kHz). This wide range of
pressure has an accuracy of 0.1% to 10% of the full range. In addition their
performance is very stable at a wide temperature range – 200 0
C to 400 0
C.
For all have been mentioned up; they could be used to measure the explosion
pressure in engines.
15

16. Resistance Strain Gauge
It is simply consists of a resistance strain gauges which are cemented to a billet
made of high tensile alloy of steel. On acting a tension to the billet within its
elastic limit, it will expand. This expansion will make the resistor wire expands
also – its length will increases while its cross section will decreases- this means an
increase in its resistivety. The variation of the resistance value could be sensed by
connecting a Whetstone bridge circuit bridge, which gives an output voltage
proportional to the tensile stress.
This magic way is used in many applications such as load cell, and in some
pressure transmitters.
Fig. “13”
Pressure Switches
Fig. “14”
1. Any of the pressure sensitive elements may be used as the primary
element in a pressure switch.
16

17. 2. The sensitive element activates the micro switch to make or break at the
same pressure (No Hysteresis / deferential), the only difference is in the
contact arrangement as shown in figure “11”.
3. The operating pressure is adjusted by varying the spring force.
VARIABLE DEFFERENTIAL PRESSURE SWITCHES
This given arrangement is used to
overcome the effect of arcing of the
switch’s contact due to any pressure
fluctuation.
Spring “A” is used to adjust the max.
Pressure range. While spring “B” is
used to adjust the band of pressure
fluctuation.
The stiffness of “B” should
subtracted from “A”, therefore their
action are opposing each other.
Fig. “15”
APPLICATIONS & DEVICES
Control & Transmission
PRESSURE REGULATOR
17

18. Fig. “16”
Theory of Operation
• Air passes through the ceramic filter (50 : 60 micron) and applied to the
underside of the valve.
• It will be held in closing position by the light spring.
• On acting on the rang spring by the outer handle; a force will be applied
to the light spring pushing it down by the plate fixed to the plug of the
valve. This will allow air to come out.
• During the air passing some of it will pass through a small hole to balance
the diaphragm with the action of the range spring.
FLAPPER & NOZZEL SYSTEM
Practically it is desired to measure a pressure of certain process, not only that
and transmit this signal to a control room where operators are there or use this
signal to act on a final controlling element like a positioner of a valve.
To do this safely and easy the flapper and nozzle system has to be used.
But before starting to explain this system it is essential to add herein that this
system is producing a proportional signal expressing the actual process reading.
Fig.
“17”
18

19. This system is used to convert a weak movement of a measuring/sensing element
into a 3 to 15 psi output signal.
This is verified by linking a flapper to the measuring device, which positions it at
a distance; from a nozzle; proportional to the process reading.
Air will bleed through the nozzle at a rate proportional to the flapper distance,
which reflects the process reading.
To minimize the time lag of such system and make more quicker (sensitive), there
is a very important consideration, which is to reduce the pressure of the nozzle
and limit it to the range of 2 : 4 psi. this is achieved by minimizing the nozzle
cross-section and makes the restriction section smaller than the nozzle. This
pressure should be fed to a pneumatic relay, which converts it into a 3 : 115 psi
output signal. Therefore, this relay could be considered as an amplifier.
Nozzle Pressure
Flapper Travel
Fig. “18’
PNEUMATIC RELAY
Fig. “19” Foxboro Pneumatic Relay Model-11
19

20. The shown diagram is a pneumatic relay (Continuous bleeding relay). It acts like
the electronic amplifier. It receive thee weak process signal ( 2:4 psi) and convert
it to higher-level signal ( 3:15 psi).
Principle of Operation
The process signal is feed to the inlet of the nozzle to pass through its restrictor
to come out to the relay acting above its diaphragm, which will push down the
ball valve against the spring to allow the air supply to pass through the internal
relay chamber.
The vent is used to control the air supply pressure, as if it increases; it will push
the diaphragm upward, which will pull the ball valve to close the valve. This will
decrease the pressure in the chamber (due to bleeding). At that moment the
effect of the nozzle output will increase and push down again to open the valve
and so on.
There are many different models like the given below.
Fig. “20” Foxboro Model 40G
Another type is used to overcome the weak signal of the control system and its
output could be used to drive directly a controlling device. This given below
device boosters the output and assure the failsafe principle.
20

21. Now we are ready to step forward to a new concept, which is how to convert
from pneumatic to electrical signal and vice versa.
Transducers
The importance of this concept comes from the fact that nowadays most of
centralized controlling systems are based on using electric signals. Meanwhile we
still need pneumatic signals. As a common example the control room wants to
send a command signal to a control valve, which recognize only pneumatic
signals. Therefore the electric signal sent by the control room should be
converted to pneumatic one while if the valve wants to send any of its data; it
should be received at the control room side as an electric and so on.
Before starting, it is important to define what is the meaning of a transducer; “It
is a device used to covert a signal from one form to another.
Pressure to Electric Transducer
P / I Converter
The principle of operation of this device will be:
1. On a certain pressure the moving coil will in the center and the resultant
of the differential e.m.f. Will zero.
2. If the input pressure increases, the moving coil will goes down and the
induced e.m.f. in the lower secondary coil will be greater than that of the
upper secondary coil.
3. In case of a pressure drop; the upper secondary coil will have higher
induced e.m.f.
This e.m.f. Could be used to provide either current or voltage.
There are many other examples of transducers/converters, to suit many different
applications, but all of them are using the same main concept of operation, such
as variable capacitance, variable resistance coupled to devices like bourdon tubes
or DP cells,….etc. Refer to figure “20”
21

22. Fig. “22”
J.W.S. Current to Pressure Converter
Fig. “23”
It is a force balance converter used to convert 4 : 20 mA signal directly to a
pneumatic one of 3 : 15 psi. it uses a booster relay of a rate up to 10 SCFPM.
When the flapper closes the nozzle; on activating the coil; it creates a back
pressure on the servo diaphragm of the booster relay.
22

23. To transmit data from field to a control room, what is called transmitter should
be used .
Such device could be either pneumatic or electronics.
Pneumatic Pressure Transmitters
Although the golden days of such transmitters have been passed, but some of
them are still fighting to be used in some places and for certain applications.
It is still preferable to start such subject by understanding how they are
operating as any one of them combines many important controlling items.
Therefore it is useful to know how they could work together.
Before going ahead on studying an example of such transmitters, let us know
how its loop could be ( our chosen type will be Foxboro type “11” ):
Main air supply 100 psi
20 psi
Isolating Valve 3:15 psi
4
3
/
4
1
"ss
4
1
"ss
Air Pressure
regulator
Controlle
r
Receiver
Pressure
Transmitter
Drain Valve
Fig. “24”
Now let us go to know How it works
23

24. Fig. “25”
1. Pressure of the process is applied to the diaphragm capsule.
2. The movement of the diaphragm affects the force bore, which is pivoted
by the diaphragm seal.
3. The force bare moves the flexure connector, which pulls the flapper to or
from the nozzle.
4. The back pressure of the nozzle is amplified by the continuous bleeding of
the relay (which is more stable than the non bleeding one).
5. To stabilize the system and balance it, a feed back signal is connected via
a below to oppose the force bare movement.
Electronic Pressure Transmitter
here are three main types according the capsule’s technique used:
Capacitive (Rosemount).
Strain Gauge (Honeywell).
Vibrating Wire (Foxboro).
The output of any of the above types is converted to a standard 4 : 20 mA.
Each of these transmitters has a zero and span adjustments (range).
To protect plant
From any supply’s Fault Normally 24 vdc
+
--
Transmitter
Electric Series Loop
Power
SupplySafety
Barrier
RB
Rc
Controller
Tx
Safety
Barrier
RA
24

25. Generally, you can consider the transmitter behaves as a resistance (RT) in the
circuit. This resistance of a value RT changes with the value of process’s
pressure.
The current passing in this loop (I) is calculated as:
I =
RcRbRt ++ 2
Voltage
Notice that the zero of each pneumatic and electric transmitter is not actual
zeros, as:
In electric it is 4 mA.
In pneumatic it is 3 psi.
Principle of Operation
This instrument operates by applying the force balance principle. To know how
this could act here, let us go through the following:
1. Any increase in the process pressure will make the input bellow to expand
pushing the baffle.
2. This will decrease the distance between the nozzle and the baffle.
Consequently, a back pressure will be reflected to the positioning below (
its pressure will increase).
3. This action will push down the position lever, to make the pen arm raises
up around the suspension pivot.
4. This will act on the range spring which will moves the baffle away from
the nozzle…and so on.
25

26. Tips
Why on closing a nozzle, a back pressure is generated?
Near the nozzle outlet there is a concentric orifice, when the nozzle is uncapped
(by increasing the baffle distance away); a venture effect causes air to be sucked
out of the positioning below through the nozzle line. This line creates a vacuum
in the bellow using the 20 psi air supply.
i.e. this means on shutting off the air supply; the pen will points to position “0”
and pressure will be in the positioning bellow will be Zero.
26

27. Here in this section some more topics related to the practical side will be
discussed.
SETTING UP AN INSTRUMENT
An instrument must be set up to read as accurately as possible. This is usually
carried out to three features. They are the Zero, Mid-Scale, and Full scale.
Zero
This is the first action to set up an instrument. It could be done either
by a variable controlling facility or by moving the pointer’s position.
Range (Span, or Multiplication)
To read correctly at any value of the full range (0 : 100 %). let us say at 80 %.
For example refer to the above drawing, if “d” is the distance moved by the
bourdon tip, then the segment “b’ moves upward to “a”, this will push the
pointer to change its position. The segment arm should be adjusted to give the
proper range (it acts as an amplifier).
Linearity (Box Up)
The linearity adjustment is used to set the instrument reading correct at 50%
point.
This the best adjusted by applying 50 % value and varying the linearity control
until everything that moves looks symmetrical. This means that a change of + 50
% cause an angle which should be the same of – 50 % but in the opposite
direction.
Parameter Dependence
It is not an easy design to vary any of the above three parameters without
varying the other two. Therefore, to set an instrument accurately; first set the
Zero, then Range, then linearity. Keep repeating the previous three steps till
getting an accuracy of the required order.
27

28. CALIBRATION
The terminology “calibration” means to correct the actual reading of an instrument to as required
within the specified specs of it.
To carry out a successful calibration, the following should be taken care:
Check visually the instrument and be sure that there is no leakage or loose
connection.
Notice its reading.
Be sure that there is no choking anywhere in the impulse lines of the
instruments.
Make the setup procedure explained earlier.
Use a standard classified instruments either to inject well know values or to read
such values.
Then link these standard tools to your instrument.
Start injecting known values and check response of the instrument has to be
calibrated.
Make a table to record the actual reading against the true value.
Identify the correction factor which will be correction = True ±
indicated
Use this value to be as a guide to readjust your instrument.
Installation of a Pressure Gauge on a Steam/Gas Line
28

29. The tapping point for a pressure gauge may be drilled either on top or side of a
line whatever is convenient. This is to avoid any choking of the boss. This
choking could be due to sludge or scales.
Isolation valve is used to remove the gauge while the plant is running.
The vent valve is used to depressurize and drain the gauge to be removed safely.
Siphon is used to trap any condensate.
Never try to screw or unscrew the gauge by hand, but two spanners should be
used for that purpose to avoid deformation to the bourdon tube.
Fisher Rosemount Models Used In BAPETCO (Obaiyed field)
Model 3051CD – Differential Pressure Transmitter
It measures the differential pressure with a range between 0.5 in H2O and 2000 psi at an
accuracy of 0.075% with a range ability 100 : 1.
Model 3051CG – Gauge Pressure Transmitter
It measures the gauge pressure with a range between 2.5 in H2O and 2000 psi it uses Rosemount
Capacitance Cell.
Model 3051CA – Absolute Pressure Transmitter
It measures the gauge pressure with a range between 0.167 and 4000 psi it uses Piezoresistive
Silicon Sensor.
Model 3051T – Gauge and Absolute Pressure Transmitter
It measures the gauge pressure with a range between 2000 and 10000 psi it uses a signal isolator
design and microprocessor-based electronics.
Calibration Procedure for the 3051 series
29

30. 30