# Pressure Instrumentation II-Control and Instrumentation-Lecture Slides, Slides for Electronic Measurement and Instrumentation. Agra University

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This lecture was delivered by Prof. Sonu Vamsi at Agra University. This lecture is part of lecture series for course Control and Instrumentation. Its main points are: Pressure, Instrumentation, Measurement, Force, Exerte...
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4/9/2012

1

Pressure Instrumentation

Javaid Khurshid

How do you work?

Pressure Measurement

Pressure is defined as force exerted on a unit

surface area.

• Mathematically, we have: P = F/A

where P=Pressure(Pa), F=Force(N) and A=Area(m2)

• Basic unit of pressure in SI units is Pascal (Pa).

– Pascal is defined as force of 1 Newton (N) per square

meter (m2). That is: 1 Pa = 1 N/m2

• Since the Pascal (Pa) is a very small unit (1

Pa = 1.45 x 10-4 PSI)

– it is more common to use units of kPa and MPa,

when we deal with the pressures in typical industrial

process applications.

Absolute, Gauge and Differential

Pressures • Pressure measurements can be stated as either

– gauge,

– absolute or differential.

Gauge pressure is the unit we encounter in everyday work (e.g. tire ratings are in gauge pressure).

• A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e. gauge pressure is in reference to atmospheric pressure – that pressure above atmospheric pressure).

• Gauge pressure is denoted by a (g) at the end of the pressure unit (e.g. kPa(g)).

Absolute pressure includes the effect of atmospheric pressure with the gauge pressure. It is denoted by an (a) at the end of the pressure unit (e.g. kPa(a).)

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An absolute pressure indicator would indicate

atmospheric pressure when completely bled down -

it would not indicate scale zero.

The relationship between absolute pressure and

gauge pressure is:

Absolute Pressure = Gauge Pressure + Atmospheric

Pressure

• The standard value of atmospheric pressure is

taken as the atmospheric pressure at sea level,

which is 101.3 kPa.

• Note a reading of less than 101.3 kPa(a) indicates

a vacuum condition.

– For example, a typical condenser pressure is 5 kPa(a) or a

vacuum of 96.3 kPa.

• Differential Pressure is the difference of two pressures.

• In order to produce a standard (4 - 20 mA) electronic

signal which represents the pressure in a process,

the pressure must be sensed and a physically

detectable motion or force in proportion to this

applied pressure must be developed.

• To sense the process, we use a pressure sensor.

Pressure Measurement Devices

• Some common sensors include:

– Bourdon Tubes

– Bellows

– Diaphragms

– Capsules

Bourdon Tube

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Reference: http://matec.org/ps/library3/secure/modules/101/LA3/M101LA3.html

Bellows

Diaphragms

When a force acts

against a thin

stretched diaphragm,

it causes a deflection

of the diaphragm with

its centre deflecting

the most.

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Pressure Capsules

• If it is fixed at the

air inlet, it can

expand like a

balloon.

• It expands in both

ways whereas in

Diaphragm it

expands in one

direction.

Pressure Capsules

DIFFERENTIAL PRESSURE DEVICES

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Differential Pressure Devices • Used to measure Differential Pressure (that

is, the difference between a high pressure input and a low pressure input) and hence called DP transmitters or DP cells.

• A differential pressure capsule is mounted inside a housing.

• One end of a force bar is connected to the capsule assembly so that the motion of the capsule can be transmitted out of the housing.

• A sealing mechanism is used where the force bar penetrates the housing.

• This seal also acts as the pivot point for the force bar.

A Typical DP Cell

• Provision is made in the housing for high

pressure fluid to be applied on one side of the

capsule and low pressure fluid on the other.

• Any difference in pressure will cause the capsule

to deflect and create motion for the force bar.

• The top end of the force bar is connected to an

electronic motion detector, which via an

electronic system, will produce a 4 - 20 mA

signal that is proportional to the force bar

movement

DP Capsule Construction • The DP capsule is formed by welding two metallic

(usually stainless steel) diaphragms together.

• To provide over-pressurization protection, a solid plate

with diaphragm matching convolutions is mounted in the

center of the capsule.

• Silicone oil is used to fill the cavity between the

diaphragm for even pressure transmission. Most DP

capsules can withstand static pressure of up to 14 Mpa

(2000 psi) on both sides of the capsule without any

damaging effect.

• However, the sensitive range for most DP capsules is quite

low. Typically they are sensitive up to only a few hundred

kPa of differential pressure.

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DP Transmitter DP Transmitter • A DP transmitter is used to measure the gas

pressure (in gauge scale) inside a vessel.

• In this case, the low pressure side of the

transmitter is vented to atmosphere, and the

high pressure side is connected to the

vessel through an isolating valve.

• The isolating valve facilitates the removal of the

transmitter

• The output of the DP transmitter is

proportional to the gauge pressure of the gas

in the tank, i.e., 4 mA when pressure is 20 kPa

and 20 mA when pressure is 30 kPa.

Strain Guage

• The strain gauge is a device that can be affixed to

the surface of an object to detect the force applied

to the object.

• One form of the strain gauge is a metal wire of

very small diameter that is attached to the surface

of a device being monitored.

• For a metal, the electrical resistance will increase

as the length of the metal increases or as the

cross sectional diameter decreases.

• When force is applied as indicated in Figure, the

overall length of the wire tends to increase while

the cross-sectional area decreases.

• The amount of increase in resistance is

proportional to the force that produced the change

in length and area. The output of the strain gauge

is a change in resistance that can be measured

by the input circuit of an amplifier.

• Strain gauges can be bonded to the surface of a

pressure capsule or to a force bar positioned by

the measuring element.

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Strain Gauge DP Cell Shown in figure is a

strain gauge that is

bonded to a force

beam inside the DP

capsule.

The change in the

process pressure

will cause a resistive

change in the strain

gauges, which is

then used to

produce a 4-20 mA

signal.

Capacitive pressure sensor

• Principle:

– A measuring diaphragm and a fixed plate electrode

consist a capacitor

– The measuring diaphragm moves relative to fixed

plates

– Changes in capacitance are detected by an oscillator

or bridge circuit

– For a plate capacitor we can write:

Capacitive differential-pressure

transducer • Measuring diaphragm

moves by pressure difference

• Two fixed electrodes are mounted on the glass insulation

• Measuring diaphragm Glass insulation Protective diaphragm

• Measured pressure acts through a protective diaphragm and silicone oil on the measuring diaphragm.

Impacts on Operating Environment

• Vibration

• Temperature

• Vapor Content

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Vibration

• The effect of vibration is obvious in the

inconsistency of measurements, but the more

dangerous result is the stress on the sensitive

membranes, diaphragms and linkages that can

cause the sensor to fail.

• Vibration can come from many sources.

– most common are the low level constant vibration of

an unbalanced pump impeller and the larger effects

of steam hammer.

– External vibration (loose support brackets and

insecure mounting) can have the same effect.

Temperature

• The temperature effects on pressure sensing will

occur in two main areas:

– The volumetric expansion of vapor is of course

temperature dependent.

– The second effect of temperature is not so apparent.

An operating temperature outside the rating of the

sensor will create significant error in the readings.

• The bourdon tube will indicate a higher reading

when exposed to higher temperatures and lower

readings when abnormally cold due to the

strength and elasticity of the metal tube.

Vapor Content • The content of the gas or fluid is usually controlled and

known.

• Since the purity of the substance whose pressure is being monitored is of importance - whether gaseous or fluid especially, if the device is used as a differential pressure device in measuring flow of a gas or fluid.

• Higher than normal density can force a higher dynamic reading depending on where the sensors are located and how they are used.

• Also, the vapor density or ambient air density can affect the static pressure sensor readings and DP cell readings.

• Usually, lower readings are a result of the lower available pressure of the substance.

• However, a DP sensor located in a hot and very humid room will tend to read high.

Failures and Abnormalities

• Over-pressure

• Blocked sensing lines

• Leaking sensing lines

• Loss of electrical power

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Over Pressure • All of the pressure sensors are designed to

operate over a rated pressure range.

• Plant operating systems rely on these pressure

sensors to maintain high accuracy over that given

range.

• Instrument readings and control functions derived

from these devices could place plant operations in

jeopardy if the equipment is subjected to over

pressure (over range) and subsequently

damaged.

• If a pressure sensor is over ranged, pressure is

applied to the point where it can no longer return

to its original shape, thus the indication would

• They are also however, the most prone to

fracture on over-pressuring. Even a small

fracture will cause them to read low and be less

responsive to pressure changes.

• Also, the linkages and internal movements of the

sensors often become distorted and can leave a

permanent offset in the measurement.

• Bourdon tubes are very robust and can handle

extremely high pressures although, when

exposed to over-pressure, they become slightly

• Very high over-pressuring will of course rupture

the tube.

• Diaphragms and bellows are usually the most

sensitive and fast-acting of all pressure sensors.

Faulty Sensing Lines

• Faulty sensing lines create inaccurate readings

and totally misrepresent the actual pressure.

• When the pressure lines become partially

blocked, the dynamic response of the sensor is

naturally reduced and it will have a slow

response to change in pressure.

• Depending on the severity of the blockage, the

sensor could even retain an incorrect zero or

low reading, long after the change in vessel

pressure.

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• A cracked or punctured sensing line has the

Sometimes, there can be detectable down-

swings of pressure followed by slow increases.

Loss of Loop Electrical Power

• As with any instrument that relies on AC power,

the output of the D/P transmitters will drop to

zero or become irrational with a loss of power

supply.

Assignments (Last Date: April 23)

1. Find Pressure Transducers to be used in Nuclear Industry? Find one to be used with their photos and information. (Check data sheet)

2. Each of you should give a write up on (include figures and animations for higher marks) 1. Bourdon Tubes

2. Bellows

3. Diaphragms

4. Strain gauge

5. Failure reasons of Pressure sensors

6. Pressure Transmitter (with circuit)

7. Capacitive differential pressure transducer

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