Pressure_Measurement.ppt

PRESSURE MEASUREMENT
Pressure is the force exerted per unit area
Pressure is the action of one force against another force. Pressure is
force applied to, or distributed over, a surface. The pressure P of a
force F distributed over an area A is defined as P = F/A
TOTAL VACUUM - 0 PSIA
PRESSURE
ABSOLUTE
GAUGE
COMPOUND
BAROMETRIC RANGE
ATMOSPHERIC PRESSURE
NOM. 14.7 PSIA
3#

PRESSURE MEASUREMENT
TERMS
Absolute Pressure
Measured above total vacuum or zero absolute. Zero absolute represents total
lack of pressure.
Atmospheric Pressure
The pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea
level is 14.696 psia. The value of atmospheric pressure decreases with
increasing altitude.
Barometric Pressure
Same as atmospheric pressure.
Gauge Pressure
The pressure above atmospheric pressure. Represents positive difference
between measured pressure and existing atmospheric pressure. Can be
converted to absolute by adding actual atmospheric pressure value.
Differential Pressure
The difference in magnitude between some pressure value and some reference
pressure. In a sense, absolute pressure could be considered as a differential
pressure with total vacuum or zero absolute as the reference. Likewise, gauge 4#

PRESSURE INSTRUMENTS
5#
Types of Pressure Instruments
Pressure Gauges (Vacuum, Compound, Absolute, Gauge)
Differential Pressure Gauge
Pressure Switch (Vacuum, Absolute, Gauge)
Differential Pressure Switch
Pressure Transmitter (Vacuum, Absolute, Gauge)
Differential Pressure Transmitter
PRESSURE
GAUGE
PRESSURE
SWITCH
DIFFERENTIAL
PRESSURE
TRANSMITTER

5 November 2022 PMI REVISION 00 6
TERMINOLOGY
Span: the range of measured variable that a sensor can measure.
Least-count: the smallest difference of measured variable that can
be detected by a sensor.
Readability: the closeness with which the scale of a sensor can be
read in analog output.
A sensor with a 30-cm scale would have a higher readability than a
sensor with a I5-cm scale and the same span. In digital output,
readability will be the relative size of the letters.

TERMINOLOGY
Sensitivity: the change in
output of the sensor with the
unit change in input variable to
be measured; e.g., if a I-mV
recorder has a 5-cm scale
length, its sensitivity would be
5 cm/ mV.

SENSOR ACCURACY
Accuracy: the deviation of the output of a sensor from a known
measured input. Accuracy is usually expressed as a percentage of
full scale reading
e.g., a 100-kPa pressure transducer having an accuracy of 1 %
would be accurate within: 1 kPa over the entire range.

SENSOR ACCURACY
Measured variable:  absolute
measured value
Percentage of full-scale(FS) reading
Percentage of instrument span
Percentage of actual reading
Precision: the ability of a sensor to reproduce a certain
output with a given accuracy.

SENSOR ACCURACY
The difference between precision and accuracy:
Consider the measurement of a known temperature of
100°C with a certain transducer.
Five observations are recorded, and the indicated values
are 103, 105, 103, 105, and 103°C. These values show
that the accuracy of the transducer is 5% (5°C),

CONTD
and its precision is ±1%, since the
maximum deviation from the
average reading 104°C is only 1°C.
The transducer can be calibrated
and then can measure temperature
within ± 1°C. Thus accuracy can be
improved by calibration up to, but
not beyond, the precision of the
transducer

CONTD
Threshold: If the input of a sensor is very gradually increased from
zero, there will be some minimum value below which no output can
be detected.
This minimum value is the threshold of the sensor.
Resolution: the input increment that gives some small but definite
numerical change in the sensor output.
Thus resolution is the smallest measurable change, while threshold
is the smallest measurable input.

CONTD
Hysteresis: A sensor exhibits hysteresis when there is a
difference in readings depending on whether the values
of the measured variable are approached from above or
below.
Linearity: the maximum deviation of any calibration point
from the linear relationship. This may be expressed as a
percent of the actual reading or as a percent of the full
scale reading.

CONTD
Error: a deviation of the measured value from the true
value. Errors are divided into three broad classes-gross,
systemic, and random.
The gross errors arc mostly human errors such as
incorrect reading, adjustment, and application of
instruments.
Loading effects, due to improper circuit impedance, also
give this type of error.

CONTD
Systemic errors are related to the functioning of the
instruments and their mechanical or electrical structure
and calibration errors, and to the effect of environment
on the equipment performance.
These errors related to the instrument can be avoided or
reduced by proper calibration of the instrument, and by
selecting a suitable instrument

CONTD
Environmental errors are reduced by using the
instrument at the recommended conditions of
temperature, relative humidity, pressure, etc.
Random errors are due to unknown causes. They can be
reduced by proper design of the instrument, and by
taking more readings and using statistical methods to
correct the observations.

PRESSURE SENSOR
The basic pressure sensing
element
A: C-shaped Bourdon tube
B: a helical Bourdon tube
C: flat diaphragm
D: convoluted diaphragm
E: capsule
F: a set of bellows

BOURDON TUBE
A sealed tube that deflects in response to applied
pressure.
Provide a fairly large displacement (except diaphragms)
Useful in mechanical gauges and for electrical sensors
that require a significant movement.

POTENTIOMETRIC PRESSURE
SENSORS
Use a Bourdon tube, capsule, or
bellows to drive a wiper arm on
a resistive element.
Inexpensive, but subject to
repeatability and hysteresis
errors

INDUCTIVE PRESSURE
SENSORS
Linear Variable Differential
Transformer pressure sensor
drives a moving core that varies
the inductive coupling between
the transformer primary and
secondary

CAPACITIVE PRESSURE
SENSORS Typically use a thin diaphragm as one
plate of a capacitor. Applied pressure
causes the diaphragm to deflect and
the capacitance to change.
The change in capacitance may be used
to control the frequency of an oscillator
or to vary the coupling of an AC signal
through a network.

PIEZOELECTRIC PRESSURE
SENSORS
Bi-directional transducers
capable of converting stress
into an electric potential and
vice versa.
Consist of metallized quartz
or ceramic materials.
Dynamic effect, providing an
output only when the input is
changing

STRAINED GAUGE SENSORS
Used a metal diaphragm
with strain gauges bonded
to it.
A strain gauge measures the
strain in a material subjected
to applied stress
Signal due to deformation of
the material is small, on the
order of 0.1% of the base
resistance.

SEMICONDUCTOR STRAIN
GAUGES
Widely used, both bonded and integrated into a silicon
diaphragm, because larger (one order of magnitude) response
than metallic strain gauge.
Piezoresistive effect: Change of resistance, when the crystal
lattice structure of silicon is deformed by applied stress.

A silicon bar can be bonded
to a diaphragm to yield a
strain gauge sensor with a
relatively high output.
IC processing is used to form the
piezoresistors on the surface of a
silicon wafer to fabricate an
integrated piezoresistive pressure
sensor

MANOMETERA simple pressure standard
May be used for gauge, differential, and
absolute measurements with a suitable
reference.
Useful mainly for lower pressure work
because the height of the column of
mercury will otherwise become unwieldy.
The difference in column heights gives
the pressure reading

PRESSURE GAUGE
PRESSURE GAUGES:
A Pressure Gauge is used for measuring
the pressure of a gas or liquid.
A Vacuum Gauge is used to measure the
pressure in a vacuum.
A Compound Gauge is used for measuring
both Vacuum and Pressure.
Pressure Gauges are used for Indication
only.
27#

PRESSURE GAUGE
29#
Measuring Principle
Bourdon tube measuring element is made of a thin-walled C-shape tube or
spirally wound helical or coiled tube. When pressure is applied to the measuring
system through the pressure port (socket), the pressure causes the Bourdon
tube to straighten itself, thus causing the tip to move. The motion of the tip is
transmitted via the link to the movement which converts the linear motion of the
bourdon tube to a rotational motion that in turn causes the pointer to indicate the
measured pressure.
Coiled Bourdon
Helical Bourdon
“C” Type Bourdon

DIFFERENTIAL PRESSURE
GAUGE
Measuring Principle:
Differential pressure gauges have
two inlet ports, each connected to
one of the volumes whose pressure
is to be monitored.
In cases where either input can be
higher or lower than the other, a bi-
directional differential range should
be used.
30#

DIFFERENTIAL PRESSURE
GAUGE
Unidirectional and Bidirectional DP Gauges
31#

PRESSURE GAUGE -
ACCESSORIES
32#
Pulsation Damper (Adjustable Snubber)
Threads on to a gauge socket and provides a restriction by means of
a pin, which may be placed in either of five different sized holes, and
thus allows the user to vary the amount of dampening to suit
requirements. The pulsating pressure moves the pin up and down,
providing self cleaning action.
Safety Glass Front
Safety Glass is normally used to prevent the glass shattering in the event of the
bourdon tube rupturing.
Liquid Filled Gauge
The liquid filling is used to dampen any vibration/pulsation in the bourdon, either
silicone oil or glycerin is used.
Snubber
Used for dampening and filtering and reducing the damaging effects of
pulsation on a gauge. The snubber has a metal disc available in standard
grades of porosity.

PRESSURE GAUGE -
ACCESSORIES
33#
COIL PIPE
PIG TAIL
Pressure Limit Valve
Protects pressure instruments against surges and
pulsations. Provides automatic positive protection and
accurate, repeatable performance. Automatic pressure
shut-off, built in snubber enhances instrument protecting
performance.
Siphon Tubes
Used to dissipate heat by trapping condensed liquid to
keep high temperature steam or condensing vapor
from damaging the pressure gauge.

DIAPHRAGM SEALS
 Diaphragm seals, also known as chemical seals, isolate pressure
measuring instruments from the process media. The system pressure
is transmitted to a fill fluid in the upper housing of the diaphragm seal,
and from there to the pressure-measuring instrument itself. The use of
diaphragm seals should be discussed with and approved by the Client.
Diaphragm sealed gauges should be considered for:
•Process fluids that would clog the pressure elements.
•Process fluids that are toxic, corrosive, slurried and viscous.
•Process fluids that could crystallize or polymerize.
•Materials capable of withstanding the process fluids that are not
available as a pressure element, such as high temperature.
•Process fluid that might freeze due to change in ambient temperature
and damage the element.
•Hydrocarbon services having a Reid vapor pressure (RVP) of 18 psig
and over. (RVP is the absolute vapor pressure exerted by a liquid at
100°F. The higher this value, the more volatile the sample and the
more readily it will evaporate).
•Auto-ignitable hydrocarbon services.
34#

PRESSURE GAUGE
SELECTION GUIDELINE
36#
When selecting a Pressure Gauge, care should be
given to a number of parameters which have an effect
on the gauge’s accuracy, safety, and cost.
• Accuracy required
• Dial size
• Operating pressure range
• Chemical compatibility with gauge construction
materials
• Operating temperature range
• Vibration, pulsation, and shock
• Pressure fluid composition
• Mounting requirement

PRESSURE GAUGE RANGES
Since the accuracy of most pressure gauges is better
in the middle portion of a gauge, you should always
select a gauge with a range that is about double your
normal anticipated pressure.
The maximum operating pressure should not exceed
80% of the full pressure range of the gauge.
Standard pressure ranges are measured in PSI, Bar,
Pa or kPa and most of the gauges have dual
PSI/metric scales.
Very low pressure gauges have scales that measure
in Inches of H2O, mm H2O, and Inches of Hg.
Vacuum gauges have scales in inches of mercury,
while compound gauges have scales that measure in
both vacuum and pressure.
37#

PRESSURE GAUGE
INSTALLATION
38#
Gas Service
Liquid Service
• Top connection preferred for gas installations & side connection preferred
for liquid installations.
• The pressure gauge can be connected to the pipe by individual block and
bleed valves or a two way manifold.

PRESSURE SWITCH
Measuring Principle:
The device contains a micro switch,
connected to a mechanical lever and set
pressure spring. The contacts get actuated
when process pressure reaches the set
pressure of the spring.
It can be used for alarming or interlocking
purposes, on actuation.
It can be used for high / high-high or low /
low-low actuation of pressure in the process
. The set range can be adjusted within the
switch range.
The sensing element may be a Diaphragm or
a piston
39#

PRESSURE SWITCH
Pressure/Vacuum Switch - A device that senses a change
in pressure/vacuum and opens or closes an electrical
circuit when the set point is reached.
Pressure switches serve to energize or de-energize
electrical circuits as a function of whether the process
pressure is normal or abnormal.
The electric contacts can be configured as single pole
double throw (SPDT), in which case the switch is provided
with one normally closed (NC) and one normally open
(NO) contact.
Alternately, the switch can be configured as double pole
double throw (DPDT), in which case two SPDT switches
are furnished, each of which can operate a separate
electric circuit. 40#

PRESSURE SWITCH
The switch housings can meet any of the NEMA standards from
Type 1 (General Purpose) to Type 7 (Explosion Proof), or Type 12
(Dust Proof) or Type 4 (Water Proof).
Pressure switches are also available in hermetically sealed
enclosures.
Gold plated contacts are available for reliability .
Pressure Switches are not as commonly used today, since they
contain mechanical moving parts and moving parts are
significantly more likely to fail than transmitters and the failures
can go undetected.
41#

PRESSURE TRANSMITTER
A Pressure Transmitter is used where indication and/or
record of pressure is required at a location not adjacent
to the primary element.
A Pressure Transmitter is used for both indication and
control of a process.
A Pressure Transmitter is used where overall high
performance is mandatory.
Both Electronic and Pneumatic Transmitters are used.
These can be either Gauge, Absolute or Differential
Pressure Transmitters.
43#

TRANSMITTER MEASURING
PRINCIPLE
44#
• The diagram shows an electronic differential
pressure sensor. This particular type utilizes a
two-wire capacitance technique.
• Another common measuring technique is a
strain gauge.
• Process pressure is transmitted through
isolating diaphragms and silicone oil fill fluid to
a sensing diaphragm.
• The sensing diaphragm is a stretched spring
element that deflects in response to the
differential pressure across it.
• The displacement of the sensing diaphragm is
proportional to the differential pressure.
• The position of the sensing diaphragm is
detected by capacitor plates on both sides of
the sensing diaphragm.
• The differential capacitance between the
sensing diaphragm and the capacitor plates is
converted electronically to a 4–20 mA or 1-5
VDC signal.
• For a gauge pressure transmitter, the low
pressure side is referenced to atmospheric
pressure.

Typical Outputs
 4 to 20 milliamp (mA). analog signal
 Smart HART digital signal (superimposed on
analog signal)
 Fieldbus digital signal
 3 to15 psi pneumatic signal
45#

DIAPHRAGM SEAL SYSTEM
A diaphragm seal system consists of a pressure transmitter, diaphragm seals, a
fill fluid, and either a direct mount or capillary style connection.
During operation, the thin, flexible diaphragm and fill fluid separate the pressure
sensitive element of the transmitter from the process medium. The capillary
tubing or direct mount flange connects the diaphragm to the transmitter.
When process pressure is applied, the diaphragm transfers the measured
pressure through the filled system and capillary tubing to the transmitter
element.
This transferred pressure displaces the sensing diaphragm in the pressure-
sensitive element of the transmitter.
The displacement is proportional to the process pressure and is electronically
converted to an appropriate current, voltage, or digital HART output signal.
46#

DIAPHRAGM SEAL SYSTEM
47#
•WHY USE DIAPHRAGM SEALS?
Diaphragm Seal systems provide a reliable process pressure measurement and
prevent the process medium from contacting the transmitter diaphragm.
Transmitter/ Diaphragm Seal systems shall be used for:
• For process fluid that would clog the pressure elements.
• For process fluids that are toxic, corrosive, slurry and viscous.
• For process fluids that could freeze or solidify.
• For process temperatures outside the normal operating range and cannot
be brought to those limits by impulse piping.
• For process that needs frequent cleaning.
• For processes that need replacement of wet legs, to reduce
maintenance.

SELECTION GUIDELINE
48#
When selecting a pressure transmitter care should be
given to a number of parameters which have an effect
on transmitter accuracy, safety, and utility.
• Accuracy required
• Power supply
• Operating pressure range
• Operating temperature range
• Body Material
• Pressure fluid composition and Internal parts
• Mounting requirement
• Process connection size

INSTALLATION
49#
• Mounting above tap is typical for gas service and mounting below
tap typical for liquid and steam services.
• Direct mount is possible for low temperature services.

DIAPHRAGM SEAL
INSTALLATION
50#

Selection of Pressure Instruments: Rules of Thumb:
Application: Understand your application. Examine the
particulars of your application. Is it necessary to know if
the pressure is negative or positive? Do you need to
know the difference in pressure between two points?
Answering these questions about your application will go
a long way in helping select the right pressure
transmitter.
Wetted Parts: Selecting the transmitter with wetted parts
that are compatible with the medium to be measured
helps to ensure a long-lasting measurement solution.
51#

Accuracy: From an accuracy point of view, the range of a transmitter
should be low (normal operating pressure at around the middle of the
range), so that error, usually a percentage of full scale, is minimized. On
the other hand, one must always consider the consequences of
overpressure damage due to operating errors, faulty design, or failure
to isolate the instrument during pressure-testing and start-up.
Therefore, it is important to specify not only the required range, but
also the amount of overpressure protection needed.
Output Required: Pressure transmitters can send the process pressure
of interest using an analog pneumatic (3-15 psig), analog electronic (4-
20 mA dc), or digital electronic signal.
52#

Protection: Do you need special protection from the
elements? Many applications require special protection,
such as, corrosive environment, or an outdoor
environment. Pressure transmitters are available in
various NEMA ratings or can be assembled in special
NEMA rated housings that help protect them from harsh
environments.
53#

QUESTIONS
54#GY
Any Questions???]nb bvc

Pressure_Measurement.ppt

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