2 Pressure Sensors and Measurement

8/18/2019 2 Pressure Sensors and Measurement

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Principle of Pressure Measurement

Basic Concept

Unit of measurement

Sensing Methods:

Pressure Transducers

Diaphragms

Bellows

Bourdon Tube

Capacitive Fibre-Optic

Resonant-Wire Devices

High-Pressure (Greater Than 7000Bar)

Pressure Reference Instru

Manometers

Dead-Weight Tester

Special Measurement DLow Pressures

Summary



Explain and differentiate absolute pressure, gauge predifferential pressure

Understand and explain the physical principal of mpressure measurement and the various types of trused to measure pressure

Understand the applications of pressure sensors



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Absolute pressure, : This is the difference between the pressure of tabsolute zero of pressure (perfect vacuum)

Gauge pressure, : This describes the difference between the press

atmospheric pressure. Absolute and gauge pressures are therefore expression:

where 0 is the reference pressure – commonly used reference presabsolute atmospheric pressure. The pressure under standard atmosphedefined as 1.01320 x 105 Pa absolute (where 1 Pa = 1 N/m2). This is equivale

101.32 kPa absolute

1 atm absolute

1.013 bar absolute

Differential pressure, : This term is used to describe the differenabsolute pressure values, such as the pressures at two different points with

(often between the two sides of a flow restrictor in a system measuring vol

= + 0



= + 0



Absolute pressure is measured

relative to a perfect vacuum

Gauge pressure is measured relativeto ambient pressure

Differential pressure is the differencein pressure between two points of

measurement

Note that the same sensor may beused for all three types; only thereference is different.



Static Pressure. Pressure, P, is defined as force, F, per unit are

P = F/A



Pressure can also be described in terms of the pressure exerted on a su

submerged in a column of fluid at depth h, as depicted in the figure. From

the pressure at any depth within a fluid of specific weight g can be written

ℎ = (ℎ0) + ℎ = 0 + ℎ

0 = pressure at an arbitrary

datum line at ℎ0

ℎ = measured depth relative to ℎ0 = ; fluid specific weight

= density of fluid



Static pressure is measured under steady-state or equilibriumbut most real-life applications deal with dynamic or changing p

For example, the measurement of blood pressure usually gisteady-state values of systolic and diastolic pressure.

There is much additional information in the shape of the blosignal, however, which is the reason for the monitors used insituations.

To measure changing pressures, the frequency response of the be considered. As a rough approximation, the sensor frequenshould be 5-10 × the highest frequency component in the press

Another issue is the remote measurement of pressure whcoupling medium is used. Care must be taken to purge all aircompressibility will corrupt the waveform.



Bernoulli's theorem states that for horizontal flow the following rholds:

PS = PO + PI

PS = stagnation (or total) pressure

PO = static pressure

PI = Impact pressure due to moving fluid

PI = ρVo ²/2

Where V0 = the velocity of the fluid

Hence we can measure the velocity if we know the pressure



Pressure Units

pascal

(Pa)

bar

(bar)

technical

atmosphere

(at)

atmosphere

(atm)

torr

(Torr)

1 Pa ≡ 1 N/m2 10−

5 1.0197×10−

5 9.8692×10−

6 7.5006×1

1 bar 100 ≡ 106 dyn/cm

2 1.0197 0.98692 750

1 at 98,066 0.980665 ≡ 1 kgf/cm2 0.96784 735

1 atm 101,325 1.01325 1.0332 ≡ 1 atm 7

1 torr 133 1.3332×10−3 1.3595×10−

3 1.3158×10−

3

≡ 1 To

≈ 1 mm

1 psi 6,894 68.948×10−3 70.307×10−3 68.046×10−3 51.7



Pressure is sensed by mechanical elements such asplates, shells, and tubes that are designed and

constructed to deflect when pressure is applied.This is the basic mechanism converting pressure to

physical movement.

Next, this movement must be transduced toobtain an electrical or other output.

Finally, signal conditioning may be needed,depending on the type of sensor and theapplication. Figure 8 illustrates the threefunctional blocks.

displacement

electric



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How it works?

Elastic-element pressure transducers

Applied pressure causes displacement of thediaphragm and this movement is measuredby a displacement transducer

Different versions of diaphragm sensors canmeasure both absolute pressure (up to 50

bar) and gauge pressure (up to 2000 bar). Adiaphragm can also be used to measuredifferential pressure (up to 2.5 bar) byapplying the two pressures to the two sidesof the diaphragm



Material

The diaphragm can be plastic, metal alloy, stainless steel, or ceram

Plastic diaphragms are the least expensive, but metal diaphragmaccuracy.

Stainless steel is normally used in high temperature oenvironments.

Ceramic diaphragms are resistant even to strong acids and alk

used when the operating environment is particularly harsh.The name aneroid gauge is sometimes used to describe this ty

when the diaphragm is metallic.



Applications:

Sanitary processes (food,pharmaceuticals, etc.) where allowing

process fluid to accumulate in thepressure port of the sensor wouldcompromise the purity of the fluid(such as milk getting into the pressureport of a pressure gauge and spoiling)

Very pure process fluids, where themetal surface of the pressure sensormight contaminate the fluid (such ascopper ions from brass leaching intoultra pure water.) An example of a diaphragm s

to protect a pressure sensor.



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How it works?

A capacitive pressure sensor is simply a

diaphragm-type device in which diaphragmdisplacement is determined by measuring thecapacitance change between the diaphragmand a metal plate that is close to it

Such devices are in common use and are

sometimes known as Baratron gauges It is also possible to fabricate capacitive

elements in a silicon chip and thus form verysmall microsensors.



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How it works?

Another elastic-element type of pressure sensor that operate oprinciples to the diaphragm pressure sensor

Pressure changes within the bellows, which are typically faseamless tube of either metal or metal alloy, produce translatithe end of the bellows that can be measured by capacitive, induc

potentiometric transducers

Can measure either absolute pressure (up to 2.5 bar) or gauge p150 bar). Double-bellows versions also exist that are designedifferential pressures of up to 30 bar





Applications:

Valves, valve actuators and valve assemblies rely on bellows techand provide feedback

With pressure, bellows can expand or contract to the stroke reqto implement proper seating or relay the position of the valve

From sub-sea to aerospace components, bellows proviperformance in valve systems in the most critical applications.



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How it works?

Also an elastic element type of pressure

transducerThree common shapes of Bourdon tubes are

shown in the figure; C-type, spiral type andhelical type

The maximum possible deflection of the free

end of the tube is proportional to the anglesubtended by the arc through which the tubeis bent



How it works?

As the fluid/gas pressure enters the bourdontube, it tries to be reformed and because of a

free tip available, this action causes the tip totravel in free space and the tube unwinds.The simultaneous actions of bending andtension due to the internal pressure make anon-linear movement of the free tip. This

travel is suitable guided and amplified for themeasurement of the internal pressure. Butthe main requirement of the device is thatwhenever the same pressure is applied, themovement of the tip should be the same andon withdrawal of the pressure the tip should

return to the initial point.



Type C-type Spiral and Helical typ

Resolution

and sensitivity

Less resolution as

the maximum

extended value for

the arc is somewhat

less than 360

Give much greater de

the free end for a give

applied pressure

Maximummeasurement

6000 bar 700 bar

Cost and

complexity

Low manufacturing

complexity and cost

High manufacturing c

and cost



Applications:

Bourdon tubes only have guaranteed

accuracy limits when measuringgaseous pressures.

Their use for accurate measurementof liquid pressures poses greatdifficulty unless the gauge can be

totally filled with liquid during bothcalibration and measurement, acondition that is very difficult to fulfillpractically



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How it works?

Also known as optical pressure sensors, provide an alternameasuring displacements in diaphragm and Bourdon tube presoptoelectronic means and enable the resulting sensors to have a

size compared with sensors in which displacement is measured byLight travels from a light source, down an optical fiber, reflect

diaphragm, and then travels back along a second fiber to a photois a characteristic relationship between the light reflected and ththe fiber ends to the diaphragm, thus making the amount o

dependent on the diaphragm displacement and hence the measu



Advantages of fiber-optic displacement sensor over Bourdon tube

Lower mass and size compared to Bourdon tube

Immune to electromagnetic noise

Disadvantages of fiber-optic displacement sensor:

Less measurement accuracy

Higher cost

Applications:

Limited to applications where their small size, low mass, and immelectromagnetic noise are particularly desired



How it works?

The refractive index of the fiber (and hence the

intensity of light transmitted) varies according tothe mechanical deformation of the fiber causedby pressure

Microbend sensors are typically used to measurethe small pressure changes generated in Vortexshedding flowmeters

When fiber-optic sensors are used in this flowmeasurement role, the alternative arrangementshown in the figure can be used, where a fiber-optic cable is merely stretched across the pipe



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How it works?

Wire is stretched across a chamber

containing fluid at unknown pressuresubjected to a magnetic field. The wireresonates at its natural frequency accordingto its tension, which varies with pressure

Pressure is calculated by measuring thefrequency of vibration of the wire

Particularly insensitive to ambient conditionchanges and can measure pressures between5 mbar and 2 bar



Advantages?

Good repeatability

StabilityAccuracy

High resolution

Strong output signal



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How it works?

Normally carried out electrically by monitoring the change of resistspecial materials

Materials having resistance pressure characteristics that are suisensitive include manganin and gold –chromium alloys

Unknown pressure is applied to one end of the bellows, which transmcoil. The magnitude of the applied pressure is then determined by mresistance

Pressures up to 30,000 bar can be measured by devices such as th

pressure sensor



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Passive instruments that give a visualindication of pressure values

3 types of manometers:

U-Tube Manometer

Well-Type Manometer (CisternManometer)

Inclined Manometer (Draft Gauge)

Applications: Extensive usage in precision

measurement of pressure, vacuum anddifferential pressure. These can also beused for flow measurement in plants andlabs where accuracy is essential



How it works?

Most common form of manometer – read pressure data up to2 bar

Applied pressure causes a displacement of liquid inside the U-

shaped glass tube, and output pressure reading P is made byobserving the difference, h, between the level of liquid in thetwo halves of the tube A and B, based on the equation:

= ℎ ; = the specific gravity of the fluid

If an unknown pressure is applied to side A, and side B is open

to the atmosphere, the output reading is gauge pressure Alternatively, if side B of the tube is sealed and evacuated, the

output reading is absolute pressure

The U-tube manometer also measures the differentialpressure, (p1 – p2), according to the expression (p1 – p2) = ℎ ,if two unknown pressures p1 and p2 are applied, respectively,

to sides A and B of the tube



How it works?

Almost similar to a U-tube manometer but one-half of

the tube is made very large so that it forms a wellLiquid level in only one tube has to be measured,

which makes the instrument much easier to use thanthe U-tube manometer

If an unknown pressure, p1, is applied to port A andport B is open to the atmosphere, the gauge pressureis given by:

= ℎ



How it works?Variation on the well-type manometer in which one leg of the tu

increase measurement sensitivity



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How it works?

Dead weight testers are a piston-cylinder typemeasuring device

As primary standards, they are the mostaccurate instruments for the calibration ofelectronic or mechanical pressure measuringinstruments

Dead weight testers can measure pressures ofup to 10,000 bar, attaining accuracies of

between 0.005% and 0.1% 1 - Handpump2 - Testing Pum

3 - Pressure Gau

4 - Calibration W

5 - Weight Supp

6 - Piston

7 - Cylinder

8 - Filling Conne



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How it works?

At low pressure, the kinematic theory of gases predicts a linear relationshipand thermal conductivity

Thus, measurement of thermal conductivity gives an indication of pressure

Thermistor gauge – same concept as thermocouple, but used to measure tem



How it works?

Has heated element that consists of four coiled tungsten wires connected i(a)

Two identical tubes are normally used, connected in a bridge circuit, as show

Current is passed through the tungsten element, which attains a certain temto the thermal conductivity of the gas

Resistance thermometer as the heated element to relate to pressure

Cover the pressure range 10-5 to 1 mbar



How it works?

Low-pressure fluid is compressed to a higher pressurethat is then read by manometer techniques

Can be visualized as a U-tube manometer that is sealed atone end

Often used as a standard against which other gauges arecalibrated

To operate the gauge, the piston is first withdrawn. Thiscauses the level of mercury in the lower part of the gauge

to fall below the level of junction J between the twotubes marked Y and Z in the gauge. Fluid at unknownpressure Pu is then introduced via the tube marked Z,from where it also flows into the tube of cross-sectionalarea A marked Y. Next, the piston is pushed in, moving themercury level up to block junction J



How it works?

Used for measuring very low pressures in the range10-10 to 1 mbar

Normally, they are only used in laboratory conditionsbecause their calibration is very sensitive to thecomposition of the gases in which they operate

Two forms: hot cathode and hot anode

Gas pressure is determined by measuring the current

flowing between an anode and a cathode within thevessel.

This current is proportional to the number of ions perunit volume, which in turn is proportional to the gaspressure



Pressure is also one of the most widely measured enginevariables, providing variety of control and safety system

Selection of a pressure sensor involves consideration omedium for compatibility with the materials used isensor, the type (gauge, absolute, differentiameasurement, the range, the type of electrical output, anaccuracy required

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2 Pressure Sensors and Measurement