This document provides an overview of pressure instrumentation and process control. It discusses various methods of pressure measurement including manometers, elastic pressure transducers like Bourdon tubes, diaphragms and bellows, as well as electrical pressure transducers. It also covers topics like measurement of vacuum using instruments like the McLeod gauge and thermal conductivity gauge. Maintaining and calibrating pressure measuring instruments is important for accurate process measurement and control.

1. Kathiriya Brijesh
Presentation On
Under Subject Of
Plastic Process Instrumentation
And
Process Control

2. Contents
 Introduction
 Pressure
 Method of Pressure Measurement
 Manometers
 Elastic Pressure Transducers
 Measurement of Vacuum
 Force-Balance Pressure Gauges
 Electrical Pressure Transducers
 Pressure Switches
 Calibration of pressure Measuring Instruments
 Maintenance
 Care of Instrument

3. INTRODUCTION
 Nearly all industrial processes use liquids,
gases or both. Controlling these process
requires the measurement and control of
liquid and gas pressures.
 Thus, pressure measurement is one of the
most important of all process measurement

4. Pressure
 Pressure means force per unit area, exerted by a
fluid on the surface of the container.
P=F/A
WHERE,
F - FORCE (in Newton)
A - AREA (in meter²)
 Pressure is of two types
STATIC PRESSURE
DYNAMIC PRESSURE

5.  Pressure is of two types-
1- STATIC PRESSURE
2- DYNAMIC PRESSURE
 STATIC PRESSURE- when the force in a system
under pressure is constant or static (i.e. unvarying),
the pressure is said to be static pressure.
 DYNAMIC PRESSURE- If the force is varying, on
the other hand, the pressure is said to be dynamic
pressure.

6.  Unit of Pressure
1 atm = 14.7 Psi at sea level
= 101.3 Kilo Pascal
= 760 mm of Hg
= 10.3 m of water
= 1013 mili bar
1 Pascal = 1N/m2
1 Bar = 100 Pascal

7. Method of Pressure Measurement
 Manometer method.
 Elastic pressure transducers.
 Pressure measurement by measuring
vacuum.
 Pressure measurement by balancing the
force produced on a known area by a
measured force.
 Electrical pressure transducers.

8.  Monometers
 A manometer is an instrument that uses a column of
liquid to measure pressure.
 The manometer utilizes the hydrostatic (standing
liquid) balance principle where in a pressure is
measured by the height of the liquid it will support.
 Manometer depends on 3 factors;
1. the Height of the column of the fluid [H]
2. The density of the fluid [ρ]
3. The gravitational constant [g] which equal
9.81m/s2
So the pressure in manometer = H x g

9.  There are many types of manometers, some of which
are as follows:
• U tube manometer.
• Well type manometer.
• Barometer.
• Inclined tube manometer.
• Micromanometer.
 Manometers measure the unknown pressure
by balancing against the gravitational force
of liquid heads

10.  U tube manometer.
• Simplest manometer.
• Used in the measurement of
liquid or gas pressure.
• Both legs have same area.
• Manometric fluid of known specific
gravity is used.
• Water and mercury are used as a
manometric fluid.
• Advantage of using these fluid is that
mass density of these fluid can be
obtained easily and they do not stick to
the tube.

11. • Since,
P = ρgh
h = (P₁ - P₂)/ρg
P₁ - P₂ = ρgh
Where,
ρ - mass density of fluid
g - gravity
P₁ - unknown pressure
P₂ - atmospheric pressure

12.  Well type manometer.
• One leg is a simple tube, other leg is a
large well.
• For small displacement of liquid level in
the well there will be a large change in
the height of simple tube.
• The well type manometer is widely used
because of inconvenience; the reading
of only a single leg is required in it.

13. • It consist of a very large-diameter vessel (well)
connected on one side to a very small-sized
tube. Thus the zero level moves very little
when pressure is applied.

14.  Barometer.
• Principle of working: If one
end is at zero absolute
pressure then “h” indicates
the absolute pressure.
• Well type absolute pressure
gauge.
• Its range is from zero
absolute to atmospheric
pressure.
• High vacuum are not
measured.

15.  Inclined tube manometer.
• It is slant manometer.
• The angle of measuring leg
is about 10⁰.
• Inclination is done to
improve the sensitivity.
• This manometer is used to
measure very small
pressure difference

16.  Micromanometer.
• One leg is well type and other
leg is inclined tube.
• Inclined leg consist magnifier.
• Initially both well and inclined
legs are at same pressure.
• Application of unknown
pressure causes meniscus to
move towards the reference
point.
• The difference between initial
& final reading gives change in
height.

17. Elastic Pressure Transducers
• The elastic pressure transducers are the mechanical
elements that are used for converting one form of
energy into the other form of energy that can be
measured easily.
• There are number of mechanical transducers, some of
the commonly used ones are described below:
1) Bourdon tube pressure transducers
2) Diaphragm pressure transducers
3) Bellows pressure transducers

18.  Bourdon tube pressure transducers
• A Bourdon gauge uses a coiled tube, which,
as it expands due to pressure increase
causes a rotation of an arm connected to
the tube. In 1849 the Bourdon tube
pressure gauge was patented in France by
Eugene Bourdon

19. Advantages:
• Low cost
• Simple construction
• Time-tested in applications
• Availability in a wide variety of ranges, including
very high ranges
• Adaptability to transducer designs for electronic
instruments
• High accuracy, especially in relation to cos
Disadvantages:
• Low spring gradient (i.e. below 50 psig)
• Susceptibility to shock and vibrations
Susceptibility to hysteresis

20.  Diaphragm pressure transducers
• A second type of aneroid gauge uses the deflection of a
flexible membrane that separates regions of different
pressure.
• The amount of deflection is repeatable for known pressures so
the pressure can be determined by using calibration.
• The deformation of a thin diaphragm is dependent on the
difference in pressure between its two faces.
• The reference face can be open to atmosphere to measure
gauge pressure, open to a second port to measure differential
pressure, or can be sealed against a vacuum or other fixed
reference pressure to measure absolute pressure. The
deformation can be measured using mechanical, optical or
capacitive techniques.
• Ceramic and metallic diaphragms are used.

21. • Diaphragm are widely used for pressure (gauge
pressure), particularly in very low ranges. They can
detect a pressure differential even in the range of 0
to 4mm.
• The diaphragm can be in the form of Flat,
Corrugated and Capsules the choice depends on the
strength and amount of deflection required.
 Two types of diaphragm are generally
used:
1) Metallic diaphragm gauge
2) Slack diaphragm gauge

22.  Metallic diaphragm gauge

23.  Slack diaphragm gauge

24. Advantages:
• Diaphragm Pressure Transducer cost is moderate.
• Diaphragm Pressure Transducer possesses high over
range characteristics.
• Diaphragm Pressure Transducers are adaptable to
absolute and differential pressure measurement.
• Diaphragm Pressure Transducer has good linearity.
• Diaphragm Pressure Transducer is small in size.
Disadvantages:
• Diaphragm Pressure Transducer lack good vibration and
shock resistance
• Diaphragm Pressure Transducers are difficult to repair.
• Diaphragm Pressure Transducer is limited to relatively
low pressures

25.  Bellows pressure transducers
• A bellows gauge contains an
elastic element that is a
convoluted unit that
expands and contracts
axially with change in
pressure.
• The pressure to be
measured can be applied to
the outside or inside of the
bellows however, in
practice, most bellows
measuring devices have the
pressure applied to the
outside of the bellows.

26. Advantages:
Disadvantages:
• Moderate cost
• Delivery of high force
• Adaptability for absolute and
differential pressure
• Good in the low to moderate
pressure range
• Ambient temperature compensation neede
• Unsuitable for high pressure
• Limited availability of metals and work hardening of
some of them
• Unsuitability of its zero and the stiffness (therefore it
is used in conjunction with (in parallel with) a
reliable spring of appreciably higher stiffness for
accurate characterization

27. Measurement of Vacuum
• “Pressures below atmosphere are generally
termed as low pressures or vacuum pressures.”
• “When the term vacuum is mentioned it means
that the gauge pressure is negative.”
• “However, atmospheric pressure serves as a
reference and absolute pressure is positive. Low
pressures are more difficult to measure than
medium pressures.”
• “Pressures above 1 Torr can easily be measured
by the direct measurement method, wherein the
force applied causes a displacement.”

28. • “Manometers, diaphragms, bellows, and Bourdon
tubes are some examples of the instruments used in
direct measurement of pressure.”
• “These devices are generally employed to measure
a pressure value of about 10 mmHg.”
• “For measuring pressures below 1 Torr, indirect or
inferential methods are often employed.”
• “In these methods, pressure is determined by
drawing indirect references to pressure-controlling
properties such as volume, thermal conductivity,
and ionization of gas.”
• “Some of the devices that fall under this category
include McLeod gauge, Pirani gauge, and ionization
gauge.

29.  Capsule Gauges
• Capsules are more sensitive than
bellow and bourdon tubes
• They are use for low pressure
measurements and also where
highest accuracy is required
• The capsule made of beryllium
copper, it uses two corrugated
diaphragms joined at the edges.
• The pressure range of the capsule
gauge is from 1 atmosphere(1000
m bar) to 0.5 m bar

30.  McLeod Gauge
• “It is employed as an absolute
standard of vacuum measurement
for pressures ranging from 10 to
10−4 Torr.”
• “A McLeod gauge, which is also
known as a compression gauge, is
used for vacuum measurement by
compressing the low-pressure gas
whose pressure is to be
measured.”
• “The trapped gas gets
compressed in a capillary tube.
Vacuum is measured by
measuring the height of a column
of mercury.”

31.  Thermal Conductivity Gage
• Thermal conductivity of a gas is independent of
pressure at normal pressure. But at low pressure,
thermal conductivity of a gas depends on pressure
(decreases with pressure)
• Heat loss from a heated conducting wire (or hot thin
metal surface) is dependent on thermal conductivity
of the surrounding gas. Thus, equilibrium
temperature of a heated conducting wire(or hot thin
metal surface)is a function of pressure heated
conducting wire (or hot thin metal surface) is a
function of pressure.
• There are two types of thermal conductivity gage
Thermocouple Gage
Pirani gage

32.  Thermocouple Gage:
• For a given gas and heating current,
the temperature assumed by the hot
surface depends on pressure.
Temperature of the surface is
measured by a thermocouple.
• Range: 10-4 to 1 Torr
• Measurement may be affected by
ambient temperature
• Surface of low emissivity is used to
reduce effect of radiation
• For a given gas and heating current,
the temperature assumed by the hot
surface depends on pressure.
Temperature of the surface is
measured by a thermocouple

33.  Pirani Gage:
• Uses same principle as
Thermocouple gage
• Function of heating and measuring
temperature bidiilltare com bined
in a single element
• Generally more accurate and
expensive than Thermocouple gage
• Range: 10-4to 1 Torr
• Gage has to be calibrated for
individual gas
• Temperature compensation is
provided

34.  Ionization Gage:
• An electron passing through a potential difference
will acquire a kinetic energy that is proportional to
the potential difference
• If an electron strikes a gas molecule, the electron
may knock out an electron from the gas molecule
leaving it positively charged
• Number of positive ions formed is dependent on
number of gas molecules per unit volume ->
pressure
• Ionization gage measure vacuum by measuring the
current produced by ionized gas molecules. The gas
molecules are ionized as a stream of electrons
collide with them.

35. • An electron passing through a potential
difference will acquire a kinetic energy
thatisproportionaltothepotentialdifferenceIfthi
senergyislargeandthe that is proportional to
the potential difference. If this energy is large
and the electron collides with a gas molecule,
the electron may knock out a secondary
electron from the gas molecule. Thus the gas
molecule will be a positively charged ion.
• Number of positive ions (ion current) depends
on electron current (no. of electrons emitted
by the cathode) and number of gas molecules.
For a given gas and a constant electron
current, ion current become a direct measure
of g, number of gas molecules per unit volume
that is pressure.

36. Force-Balance Pressure Gauges
• This kind of system is mostly linear.
• It is a continuous balancing system.
• They find application in calibration purposes.
• Pressure easily converted to force with introduction of surface
area.
• Dead weight piston gauge, ring balance and bell type pressure
gauge are its commonly used devices.

37.  Dead Weight Piston Gauge
• It is used in higher steady
pressure measurement.
• Also find usage in calibration
of bellows and diaphragms.
• The units of measurement are
force and area
• Accuracy < 0.1 %
• Range up to 300 psig.
• It consists of a very accurately
machined, bored and finished
piston which is inserted into a
close-fitting cylinder.

38. • The area of cross section of both the piston and cylinder
are known.
• A platform is provided at the top of the piston where
standard and accurate weights are placed.
• An oil reservoir with check valve is provided at the
bottom.
• The oil can be sucked by displacement pumps on its
upward stroke.
• For calibration, a known weight is first placed on the
platform and fluid pressure is applied on the other end
of the piston until enough force is developed to lift the
piston weight combination and the piston floats freely
within the cylinder between limit stops.

39.  Ring Balance Gauge
• For measurement of low differential pressure.
• It consists of a hollow ring of circular section,
partitioned at the upper part and partially filled
with liquid to form two pressure chambers.
• The ring is supported at the centre of a knife edge.
• Made up of aluminium alloy or plastic moulding.
• Force operating the instrument is generated by the
difference between the pressure on two sides of
partition.
• Cross section is large in case differential pressure
is large.

40. • The fluid under test are
led into the ring through
flexible connections.
• Placed such that length
and movement are
minimum.
• The ring balance is
controlled by a weight
which is at its lowest point
with same pressure on
both sides.
• It is the rotation of the
ring that indicates the
pressure difference.

41.  Bell Type Pressure Gauge
• Range for differential pressure is
between 0.06 Pa and 4 KPa.
• For static pressure, it is as high as 4
to 6 MPa.
• Force produced inside and outside
the bell is balanced against a weight
by compression of the spring.
• Types: Two : - Thick Wall and Thin
Wall.

42.  Thick Wall Bell Gauge
• Thick wall comprises a bell suspended with the
open end downwards in a sealed chamber, made
of cast iron, liquid being oil or mercury.
• Open end is covered by liquid as a seal creating
two chambers.
• Higher pressure acts on the inside and lower on
the outside of the bell.
• The bell rises until equilibrium is maintained
between upward force and weight.
• As bell rises, a small portion of it is immersed in
the sealing liquid so that the upward thrust on it,
due to buoyancy is reduced, with increase in
apparent weight.

43. • Since the pressure inside is
greater than outside, it will
cause the level of the liquid on
the outside to be greater than
the level on the inside, causing
the bell to rise. The travel of
the bell is proportional to the
differential pressure.
• The thickness, density and area
of cross section of the bell and
the density of the sealing liquid
are calculated based on the
range of pressure to be
measured.

44.  Thin Wall Bell Gauge
• In Thin Wall Bell Gauge, the bell is thinner
in construction and control is achieved by
means of a spring arrangement.
• Here high pressure is applied at the
outside and lower pressure is applied at
the inside.
• The force difference due to pressure
causes changes in the length of the spring
eventually changing the position of the
bell.
• The travel is proportional to the
differential pressure.
• Range is determined here, by the modulus
of elasticity of the spring and density of
the sealing liquid.

45. Electrical Pressure Transducers
• The electrical transducers is one which converts the
nonelectrical quantity into the equivalent electrical
quantity.
• Non-electrical quantity such as force, displacement,
stress, temperature.
• Electrical quantity such as current , voltage
• This is four type
o Strain Gauge Pressure Transducer
o Potentiometric Pressure Transducer
o Capacitive Pressure Transducer
o Reluctance Pressure Transducer
1. Linear Variable Differential Transformer
2. Servo Pressure Transducer
3. Piezoelectric Pressure Transducer

46.  Strain Gauge Pressure Transducer
• Passive resistance transducer.
• Resistance changes when compressed or stretched.
• Attached to pressure sensing device.
• There are four strain gauges connected to a bridge
circuit, for two resistance increases with increase of
pressure and for the remaining two, resistance
decreases with increase of pressure.
• Under no load condition, bridge remains at balance and
therefor no current flows in the galvanometer.
• With application of pressure the strain gauges stretch or
compress and the bridge becomes unbalanced, resulting
a current flow.
• The measuring the current, pressure may be calculated.

48. Advantages of Strain Gauge Pressure Transducer:
• Small and easy to install
• Considerably accurate
• Offers wide range of measurement (vacuum to 20000 psig)
• Good stability
• High output signal strength
• High over range capacity
• No moving parts
• Good stability against shock and vibration
• Fast speed of response
Disadvantages of Strain Gauge Pressure Transducer:
• Cost is high
• Electrical readout is necessary
• Require constant voltage supply
• Require temperature compensation

49.  Potentiometric Pressure Transducer
• Here a potentiometer is involved.
• A movable electrical contact, called wiper, slides
along the cylinder, touching the wire at one point on
each turn.
• The wiper position determines the resistance value
between wiper and wire end.
• A mechanical linkage from a bellow or a diaphragm
controls the position of the wiper.
• The wiper position determines the resistance which
eventually determines the value of pressure.

50. Advantages
Disadvantages
• Resistance easily
convertible to standard
voltage or current
• Simple in construction
and lesser in cost
• Easy to design
• Have finite resolution
• Wear and tear tend to be
a problem
• Noise due to make and
break of contact

51.  Capacitive Pressure Transducer
• Consists of a parallel plate capacitors coupled with a
diaphragm, usually metal and exposed to the process
pressure on one side and the reference pressure on
the other side.
• Electrodes are attached to the diaphragm and are
charged by a high frequency oscillator.
• The electrodes sense any movement of the diaphragm
and this changes the capacitance.
• The change of the capacitance is detected by an
attached circuit which then outputs a voltage
according to the pressure change.
• This type of sensor can be operated in the range of
2.5 Pa - 70MPa with a sensitivity of 0.07 MPa.

52. Working Principle

53. Advantages
Disadvantages
• Gives out rapid response to
changes in pressure
• It can withstand a lot of
vibration and shock
• It is extremely sensitive
• Offers a good frequency
response
• Metallic parts need insulation from
each other
• Performance affected by dirt and
other contaminants
• Temperature may lead to
inaccuracy

54.  Reluctance Pressure Transducer
• Here the reluctance as electrical property suffers
changes.
• There primarily exists three types.
1. Linear Variable Differential Transformer
(LVDT)
2. Servo Pressure Transducer
3. Piezo electric Pressure Transducer

55. 1. Linear Variable Differential Transformer
• The device consists of a primary winding (P) and two
secondary windings named S1 and S2.
• Both of them are wound on one cylindrical former, side by
side, and they have equal number of turns.
• Their arrangement is such that they maintain symmetry
with either side of the primary winding (P).
• A movable soft iron core is placed parallel to the axis of the
cylindrical former.
• An arm is connected to the other end of the soft iron core
and it moves according to the displacement produced.
• The pressure range is 250 Pa - 70 MPa with a sensitivity of
0.35 MPa.

56. Working Principle of LVDT
• AC voltage with a frequency of (50-400) Hz is supplied to
the primary winding. Thus, two voltages VS1 and VS2 are
obtained at the two secondary windings S1 and S2
respectively.
• The output voltage will be the difference between the two
voltages (VS1-VS2) as they are combined in series.
• Null Position – This is also called the central position as the
soft iron core will remain in the exact centre of the
former. Thus the linking magnetic flux produced in the two
secondary windings will be equal. The voltage induced
because of them will also be equal. Thus the resulting
voltage VS1-VS2 = 0.
• Right of Null Position – In this position, the linking flux at
the winding S2 has a value more than the linking flux at
the winding S1. Thus, the resulting voltage VS1-VS2 will be
in phase with VS2.

57. • Left of Null Position – In this position, the linking flux at the
winding S2 has a value less than the linking flux at the winding
S1. Thus, the resulting voltage VS1-VS2 will be in phase with
VS1.
• VS1-VS2 will depend on the right or left shift of the core from
the null position.
• The resulting voltage is in phase with the primary winding
voltage for the change of the arm in one direction, and is 180 °
out of phase for the change of the arm position in the other
direction.
• The magnitude and displacement can be easily calculated or
plotted by calculating the magnitude and phase of the resulting
voltage.
• The LVDT is connected to a diaphragm or bellow and with
changes of pressure, the position of the LVDT changes,
producing current output from where the pressure difference
can be evaluated.

59. Advantages of LVDT:
• It possesses high sensitivity
• Very rugged in construction and therefore
tolerant towards shock and vibration
• Stable and easy to align
• Offers infinite resolution
• Low hysteresis, hence repeatable
Disadvantages of LVDT:
• Relatively large core displacement
• Sensitive to stray magnetic fields
• Affected by temperature

60. 2. Servo Pressure Transducer
• Also called a force balance pressure transducer.
• It produces an electrical signal proportional to the
pressure.
• It finds huge application in industrial differential
pressure measurements. An increase in pressure P1
over P2 flexes the diaphragm and moves the short end
of the force beam.
• The force beam pivots and the long end moves a
magnetic material in the reluctive detector.
• There the signal is converted from A.C to D.C power
and amplified via an amplifier.
• An induction motor is activated by the amplifier
which brings the force beam back to its original
position.

61. • The range of this type of pressure transducer is below
500 psi. It is unresponsive to high frequency pressure
oscillations.

62. 3.Piezo Electric Pressure Transducer
• Piezoelectric characteristics of certain crystalline materials
are used. Electricity is generated when pressure is applied.
• Some of these materials are Barium Titanate Sintered
powder, quartz, tourmaline, Rochelle salt.
• The main advantages of these crystals are that they have
high mechanical and thermal state capability, capability of
withstanding high order of strain, low leakage, and good
frequency response.
• Each crystal has three sets of axes – Optical axes, three
electrical axes OX1, OX2, and OX3 with 120 degree with
each other, and three mechanical axes OY1, OY2 and OY3
also at 120 degree with each other. The mechanical axes
will be at right angles to the electrical axes. Some of the
parameters that decide the nature of the crystal for the
application are

63. 1. Angle at which the wafer is cut
from natural quartz crystal
2. Plate thickness
3. Dimension of the plate
4. Means of mounting
Working Principle
• A force, when applied on the quartz crystal, produces
electric charges on the crystal surface.
• The charge thus produced can be called as piezoelectricity.
• Piezo electricity can be defined as the electrical
polarization produced by mechanical strain on certain class
of crystals.
• The rate of charge produced will be proportional to the
rate of change of force applied as input.
• As the charge produced is very small, a charge amplifier is
needed so as to produce an output voltage big enough to
be measured.

64. • The figure shows a conventional
piezoelectric transducer with a
piezoelectric crystal inserted
between a solid base and the
force summing member.
• If a force is applied on the
pressure port, the same force
will fall on the force summing
member. Thus a potential
difference will be generated on
the crystal due to its property.
• The voltage produced will be
proportional to the magnitude of
the applied force.

65. Advantages of Piezo electric Pressure Transducer
• Very high frequency response.
• Self-generating, so no need of external source.
• Simple to use as they have small dimensions and large
measuring range.
• It has a large dielectric constant. The crystal axis is
selectable by orienting the direction of orientation.
Disadvantages of Piezo electric Pressure Transducer
• It is not suitable for measurement in static condition.
• Since the device operates with the small electric charge,
they need high impedance cable for electrical interface.
• The output may vary according to the temperature
variation of the crystal. The relative humidity rises
above 85% or falls below 35%, its output will be affected.
If so, it has to be coated with wax or polymer material.

66. Pressure Switches
• Pressure switches are devices that are
configured to sense a change in pressure and
respond in a specified manner.
• A pressure switch turns an electric circuit ‘ON’
or ‘OFF’ at a preset pressure.
• The pressure is the set point of the switch
• Pressure is use in some form of control
• The pressure switch is usually a micro switch or
mercury switch.
• The pressure is fed to the inside of a bellows
which carries a contact plate
• The contact plate touches contact point

67. • The pressure switch can be modified so as to make a low
pressure contact in addition to high pressure contact.
• Most switches contain two sets of contacts, one normally
open and the other normally closed

68. Uses of Pressure Switches
• Pressure switch is uses as limiting the pressure
• The pressure switch is used to operate a safety valve
which vents steam when the pressure exceeds the
upper limit.
• Another important uses of the pressure switch is in the
computer panel.
• In the computer panel, blowers’ are used for cooling
purposes.
• Whenever the blower fails due to any reason, a
pressure switch is actuated which cuts off the power
supply of the panel.
• The computer panel components are protected from
the high temperature which can occur due to failure of
the blower.

69. Calibration of pressure Measuring
Instruments
• It is a process of adjusting instruments’ output to match a
range of pressure with better accuracy and precision.
• It includes zero, span and linearity adjustment.
• It is thereafter tested in measuring some components
whose values are accurately known.
• Low for pressure (up to 3 inches of water gauge),
micrometer or inclined gauge is used.
• For the range up to 144 inches of water gauge, water
manometer and mercury manometers are used.
• In the range till 73 inches of mercury pressure, mercury
manometer is used. For ranges beyond that, dead weight
piston gauge is used.

70.  Calibration of Pressure Transmitters
• Following steps are involved to calibrate pressure
transmitters in the range from 25 to 125 psig for output
signal of 4 to 20 mA.
1. Transmitter allowed to reach normal
temperature.
2. Dead weight piston gauge is used to supply 25 psig to
instrument.
3. Zero is adjusted for an output of exactly 4 mA.
4. Next a pressure of 125 psig is supplied, and span is
adjusted at 20 mA.
5. 75 psig (half scale) pressure is applied and output is
checked at 12 mA.
6. These steps need iteration as zero and span may
throw off against each other.

71. Maintenance
• Highly important to long life and good accuracy and precision
of instruments.
• Periodic maintenance is done by the following methods:
1. Visual inspection for any leak or damage in pipes
and wiring.
2. Blowdown and venting in case there is clogging in
the pipe or channel.
3. Blowdown is used to remove dirt and foreign
particles.
4. Venting is used to prevent gas build ups in the pipe
to stop faulty pressure reading.
5. Cleaning and lubricating keep the mechanical
components smooth and stops misalignment.

72. Care of Instrument
• Following steps are to be taken for pressure measuring
instruments.
• Liquid filled manometers need levelling,
maintaining a reasonable constant temperature.
• Proper allowances must be provided for pressure
where liquids enter the manometer.
• Pressure gauges should be mounted correctly,
protected from heat, corrosion and vibration.
• Zero setting should be periodically.
• Recalibration should be done quarter yearly.
• Instruments having mercury should be cleaned and
refilled periodically based on the expected
accuracy.