Module - Pps

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MODULE - PPS

MODULE – PPSPROCESS PRESSURE SENSORS

OBJECTIVE:

Without the use if references, you must be able to:

a. Describe the operation of various pressure sensing devices.

b. Sketch and label the pressure sensing devices.

c. State any limitations under which the pressure sensing devices may

operate.

At the end of the module you will be required to write a criterion test in which all

of the questions must be answered correctly before proceeding to the next

module.

RESOURCES

1. Your Training Officer.

2. Module PPS

3. Pressure sensor models

Instrumentation : Document No. 10 of 28

Created : 01 February 2003 Revision No: 01Last Approved : March 2003 First Published : March 2003

Revised : Owner: Learnership Dept.



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MODULE – PPS

PROCESS PRESSURE SENSORS

INTRODUCTION

The following devices utilize the principle of pressure measurement by balancing

the force produced on a known area against the stress in an elastic medium. It is

important to know how and when to use these devices under different working

conditions encountered in the working environment, with the view to preventing

failures due to faulty application.

This module will introduce you to the various pressure sensing devices and make

you aware of their limitations.

PRINCIPLE OF OPERATION

The basic principle of operation, is that the pressure is measured by balancing

the force produced on a known area against the stress in an elastic medium.

The following devices are typical examples of such pressure sensing devices.

a. THE BOURDON TUBE (C – TYPE)

b. THE SPIRAL BOURDON TUBE

c. THE HELICAL BOURDON TUBE

d. THE BELLOWS

e. DIAPHRAGMS






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THE BOURDON TUBE (C – TYPE)

FIGURE – 1

C TYPE BOURDON TUBE GAUGE WITH QAUDRANT AND PINION

In its simplest form, the Bourdon tube consists of a tube of oval section, bent in a

circular arc. One end of the tube is sealed and attached by a light ling system to

the mechanism, which operates the pointer. The other end is fixed and is open

for the application of the pressure, which is to be measured. The internal

pressure tends to change the section of the tube from oval to circular, and this

tends to straighten out the tube. The resulting movement of the free end of the

tube causes the pointer to move over the scale.

The tube is made from a variety of materials and in a variety of material

thickness.

The material selected depends on the nature of the fluid to be measured and the

thickness of the material depends upon the pressure range to be measured. The

actual dimension of the tube will determine the force available to drive the pointer






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mechanism and this force should be large enough to overcome any frictional

force that might be present. The construction of the standard concentric

pressure sensor of “C” type bourdon tube is shown in figure – 1. The motion of

the free end of the tube is communicated by means of a connecting link to the

lower end of the pivoted quadrant. The upper end of the quadrant consists of a

toothed segment, which engages with the teeth of a central pinion, which rotates

the pointer. The play between the quadrant and the pinion is taken up by a fine

phosphor bronze hairspring.

READING THE TEST GAUGE

The dial situated on the front of the gauge shown in the following figure, is

marked in divisions from 0 to 250 kPa.

The error of parallax is made when the person reading the gauge is not directly in

line with the gauge. I.e. the reading is taken looking from either one of the sides.

To avoid this error, you must take the reading by looking directly in line with the

gauge and at eye level, as shown in the following figure.






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NAGNIFYING LINKAGE

FIGURE – 2

The effect of a magnifying linkage on a pen arm. The arm is pivoted at A and the

pen being at B. At the end C, the pen arm is linked to the Bourdon tube. Any

deflection of the Bourdon tube is magnified at B by the ratio a. To take some

practical figures, if the Bourdon tube deflection over the instrument range is

6mm, b is 150 mm a is 10 mm, the movement of B will be:

150mm x 6mm

10mm = 90mm

The angle that the pen arm traces out in moving from one extreme of the chart to

the other is about 38º.

The angle that the pen arm traces out in moving from one extreme of the chart to

the other is about 38º.






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This angle is suitable for recording instrument pens. For indicating instruments

of the circular type, the angle that the pen moves through can be as much as

300º. It is clear that the ordinary linkage will become too involved to produce this

large angular movement and it is customary in such a case to use a sector gear

and pinion movement as shown in figure – 1. It should be noted that apart from

the magnifying action of the gear and pinion, the sector itself by virtue of its pivot

position could also magnify.

THE SPIRAL BOURDON TUBE

FIGURE – 3

The amount of movement of the free end of a Bourdon tube varies inversely with

the wall thickness and depends upon the cross upon the cross sectional form of

the tube. It also varies directly with the angle subtended by the arc through

which the tube is bent. In a tube having an arc of 180º, the movement of its free

end will be twice that of a similar tube having an arc or 90º. The movement of

the free end of the tube may be increased without changing its wall thickness by

increasing the arc or the tube.






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When the arc through which the tube is bend reaches 360º, its length can be

increased further in two ways. The tube can then either be bent in the form of a

spiral, or in the form of a helix. By increasing the number of turns in the spiral of

helix, an enlarged movement is obtained and the need for further magnifying

mechanisms is avoided. In this way the need for the quadrant and pinion is

eliminated, and with them the backlash which tends to occur when they become

worn owing to continued use or to the presence of vibration.

The spiral form of the Bourdon tube is typically used for lower pressure

measurements. The movement of the free end of the tube is transmitted to a pen

arm or pointer through a flexible metal connecting strip, which joins the free end

to the pointer shaft. This enables the end of the spiral to move freely in a radial

direction as the spiral expands. In this type of element, the spiral is made from

chrome-molybolenium steel tubing. All joints and closures are welded and the

element is heat treated to remove any stress, which may have been set in the

material. This ensures uniform properties in the tube. The junction between the

spiral and the connecting tube is made by means of a special compression fitting.






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PROCESS PRESURE SENSORS

SELF TEST –1

Question –1

Name three pressure sensing elements of devices incorporating the principle of

pressure measurement by balancing the force produced on a known area against

the stress in an elastic medium.

Question –2

Sketch and label a Bourdon tube gauge.






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Question – 3

With the aid of sketches, you must show how a pressure gauge should be read.

Question – 4

Sketch and label the magnifying linkage and also indicate the magnifying ratio.






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Question – 5

Sketch a spiral Bourdon Tube.

Question – 6

State why a Spiral Bourdon tube is used, give two reasons.






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Question – 7

Is a quadrant and pinion arrangement used with a spiral Bourdon tube? Motivate

your answer.

LEARNER

TRAINING OFFICER

DATE






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THE HELICAL BOURDON TUBE

FIGURE – 4

For higher pressures, the tube is wound in the form of a helix. The material used

for the seamless tube from which the helix is wound is determined by the nature

of the fluid being measured and the range of the instrument. This is the same

practice as for the “C” type and the spiral type Bourdon tubes. Where the natureof the fluid allows it, a special Bronze alloy is used for ranges between 1 barg

and 40 barg. Beryllium-copper is used for ranges between 40 and 700 barg.

Chrome-molybolenium steel is used for ranges between 7 bar and 300 bar

particularly in the presence of ammonia.

NOTE: 1 Bar = 100kPa

Barg = Bar gauge pressure

THE BELLOWS

The bellows is made from a tube sealed at one end and is produced by forming

the bellows under high internal fluid pressure in a collapsible die. The die consist






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of a series of plates equal in number to the number of convolutions in the bellows

and spaced equidistantly and surrounding the tube.

THE BELLOWS

FIGURE - 5

The internal pressure causes the tube to flow between the plates as the tube

collapses endwise to form the bellows in one continuous operation.

Since the internal pressure is several hundred-thousand bar, the bellows is

destroyed in the making if the tube alloy has any imperfections. These bellows

are produced in a variety of forms in brass and alumbro, an alloy similar to brass

with the addition of about 2% aluminium which increases its resistance to

corrosion and makes it suitable for use in salt air or sea water.

The flexibility of a bellows, is defined as the change in length when a pressure of

1N/M² is applied, is proportional to the number of convolutions and inversely

proportional to the wall thickness and modulus of elasticity of the bellows

material.

The “spring rate” or compression modulus of a bellows is the load in Newtons

which when applied to the bellows at its free end will compress it by 1mm. It






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varies directly with the modulus of elasticity of the material of the bellows and

with the cube of the wall thickness.

It is inversely proportional to the number of convolutions and to the square of the

outside diameter of the bellows. In order to increase the spring rate of the

bellows in certain uses, the force tending to compress the bellows may be

opposed by a spring contained in the bellows. By varying the spring rate of the

internal spring, the calibration of the instrument may be varied.

Figure - 6 shows the way which bellows units may be used to measure

differential pressures, absolute or gauge pressures.

Differential Pressure Absolute Pressure Gauge Pressure

Figure – 6

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PROCESS PRESSURE SENSOR






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SELF TEST – 2

Question - 1

Sketch a helical Bourdon tube.

Question – 2

Give the main reason why you would possibly use a helix instead of a spiral

Bourdon tube.






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Question – 3

Draw the sketch of a typical bellows

Question – 4

Define the flexibility of a bellows.

Question – 5

How will you increase the spring rate of a bellows?






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Question – 6

Sketch and label two bellows units used for measuring differential and gauge

pressure.

LEARNER

TRAINING OFFICER

DATE






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THE DIAPHRAGM

STIFF DIAPHRAGM TYPE

FIGURE – 7

A typical stiff diaphragm type pressure gauge is the Schaffer model depicted in

figure – 7. It consist of a hardened and tempered stainless steel corrugated

diaphragm of about 65mm in diameter held between the two flanges. Pressure is

applied to the under side of the chamber shown and movement of the centre of

the diaphragm is transmitted through a ball and socket joint and a high

magnification link to a pointer. The upper flange is flat to prevent further

movement of the diaphragm when the pointer has reached the end of the scale.

The instrument is particularly suitable for low range pressure measurements in

the order of 0 – 1 bar.

SLACK DIAPHRAGM TYPE

This type of instrument has application in the measurement of pressure of the

order of a few centimeters of water pressure. Basically the device consist of an

annular diaphragm made from such materials as leather, nylon, oiled silk etc.

with a center plate. It can be made to act as a single pressure or differentialInstrumentation : Document No. 10 of 28





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pressure-measuring device. The effective area of the diaphragm may be made

relatively large, so that small pressures to the order of 12,5mm WG may be

measured.

FIGURE – 8

In figure –8 the elements of a circular scale instrument are shown with the

diaphragm deflecting against a spring. Overload protection may be introduced

by providing limiting stops against which the central plate of the diaphragm

assembly may come to rest.

METALLIC CAPSULE OR BELLOWS TYPE

A number of corrugated diaphragms may be soldered together in the manner as

shown in figure – 9 to form a capsule or diaphragm stack. A pressure difference

between the inside and outside causes the stack to contract (this unit is based on

compression rather on expansion) The deflection of the stack unit is coupled to a

suitable movement, it may for example be made to rotate an instrument pointer






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over a circular scale. As shown in the diagram, the interior of the stack is open to

atmosphere and the instrument thus measures gauge pressure.

METALLIC CAPSULE OR BELLOWS TYPE GAUGE

FIGURE – 9

ELECTRONIC PRESSURE SENSORS

Modern pressure measuring instruments are more incline to use electronic

measuring sensors rather than the more mechanical counterparts as discussed

up to now in this module. Pressure transducers are an advanced form of

pressure sensor element. The smallest possible component defined by the

measuring principle is the pressure sensor. It is the pressure sensor that






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changes the physical variable pressure to an electrical value that can be

measured electronically.

Typical outputs from such electronic pressure sensors lie between 10mV and

100mV depending on the sensor principle employed.

A pressure transducer is the next level of sophistication. In a pressure

transducer, the sensor element is encased into a unit, which has process

connections and places the sensor element into contact with the process medium

of which the pressure is to be measured. The pressure transducer may also

include electronic converters to amplify and change the small signals obtained

from the pressure sensor element to the standard 4mA to 20mA instrumentation

signals.

ELECTRONIC PRESSURE SENSOR TYPES

Although there are a number of technologies employed in the manufacture of

electronic pressure sensing devices, the main types are as follows:

• CERAMIC PIEZORESISTIVE SENSOR

• CAPACITIVE MEASURING CELL

• RESONATING WIRE CELL

CERAMIC PIEZORESISTIVE SENSOR

This type of sensor produces a voltage when pressure is applied to the Ceramic

Piezeresistive measuring sensor. As stated previously, this type of sensor

typically emits a voltage ranging between 10mV and 100mV. This technology is

very stable and is fast becoming the preferred technology to be employed in






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modern pressure measuring devices. The following sketch depicts a typical

Piezoresistive sensor element. See figure – 10.

FIGURE – 10

THE CAPACITIVE MEASURING CELL

The capacitive measuring cell is a device in which a very small capacitor is

attached to a diaphragm, which is in contact with the process medium. As the

pressure that is applied to the diaphragm causes the diaphragm to move, this

movement is applied to a very small capacitor, causing the capacitance value to

change. This change is capacitance is then measured, amplified and converted

into a standard 4mA to 20mA instrumentation signal.

THE RESONATING WIRE CELL

The resonating wire cell consists of an evacuated cell with a diaphragm. Inside

the cell and attached to the diaphragm is a thin wire similar to the string of a

guitar. This wire resonates at a certain frequency. As pressure is applied to the






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diaphragm, the tension in the resonating wire is changed. This change in the

tension of the wire, causes it to resonate at a different frequency. This principle

is similar to a musician changing the sound emitted from a guitar string by

changing the tension of the guitar string. The changing frequency is measured

and converted to the standard instrumentation signal.

FACTORS AFFECTING ELECTRONIC SENSORS

The accuracy of electronic pressure sensors is affected by the following factors:

The supply voltage: AS these instruments normally he to be powered from a

power supply source, any fluctuations in the voltage supplied to the sensor will

have and adverse effect on the reading obtained from the sensor. These

transducers thus have internal stabilizing components to compensate for any

power supply instability.

The other factor, which has an adverse influence on the readings obtained from

electronic sensors is ambient and process temperatures. These electronic

pressure transducers must therefore be provided with sophisticated temperaturecompensating circuits in order to provide accurate and reliable readings.






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SELFTEST – 3

Question –1

Sketch a typical stiff diaphragm type pressure gauge.

Question -2

Sketch a typical metallic type pressure gauge.






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Question – 3

Name the main types of electronic pressure sensor employed in modern

pressure measuring instruments.

Question – 4

Describe the factors that have an adverse effect on the readings of electronic

pressure sensors.

LEARNER

TRAINING OFFICER

DATE

PPS






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CRITERION TEST

LEARNER

AGREEMENT NUMBER

LEARNER SIGNATURE

ASSESSOR SIGNATURE

DATE

RESULT COMPETENT / NOT YET COMPETENT

Question –1

Name the pressure sending elements or devices incorporating the principle of

pressure measurement by balancing the force produced on a known area against

the stress in a elastic medium.

Question – 2

With the aid of sketches, you must show how a pressure gauge should read.

Question – 3

Give two reasons why a spiral Bourdon tube is used for a particular application,

instead of a C-type Bourdon tube?






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Question –4

Sketch and label two bellows units used for measuring differential and gauge

pressure.

Question – 5

Name the main types of electronic pressure sensors employed in modern

pressure measuring instruments.

Question –6

Describe the factors that have an adverse effect on the readings of electronic

pressure sensors.






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Instrumentation : Document No 10 of 28

Module - Pps

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