Pressure measurement.pptx

7/26/2019 Pressure measurement.pptx

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MALAVIYA NATIONAL INSTITUTE

OF TECHNOLOGY

PRESSURE MEASUREMENT

Submitted to: Submitted by:

Dr. Kailash Singh Shivangi Bansal 2013UCH1100Associate Professor Nimish Agarwal 2013UCH1212

Himanshu Sharma 2013UCH1091

Divij Chopra 2013UCH1238

Tarushi Parashar 2013UCH1460


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Pressure is the ratio of force to

the area over which that force is

distributed.

Pressure is force per unit area applied

in a direction perpendicular to the

surface of an object.

Introduction


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1 Pa Pascal = 1 N/m2 (The SI unit)

1 psi = 1 lb/in2(English unit) = 6,891

Pa

1 Bar = 105N/m2= 100 kPa 1 atm

1 Torr = 1 mm Hg = 133.3 Pa 1 kPa

1 atm = 101.3 kPa = 760 mm Hg =29.92 in Hg = 14.70 psi

Pressure Units and their Conversion


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How is pressure generated?

Collision of molecule with wall

Momentum is mass x velocity

Change of momentum is double

Collision is isothermal = perfectly elastic

Sum collisions over area to get force


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Absolute Pressure- The actual pressure at a givenposition is called the absolute pressure, and it is measured

relative to absolute vacuum (i.e., absolute zero pressure).

Gauge Pressure- Gauge pressure is the pressurerelative to the atmospheric pressure. In other words, how

much above or below is the pressure with respect to the

atmospheric pressure.

Types of Pressure


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Vacuum Pressure- Pressures below atmospheric pressureare called vacuum pressures and are measured by vacuum gages

that indicate the difference between the atmospheric pressure

and the absolute pressure.

Atmospheric Pressure- The atmospheric pressure is thepressure that an area experiences due to the force exerted by the

atmosphere.


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Pgauge= PabsPatm Gauge pressure

Pvac= PatmPabs Vacuum pressure

Pabs= Patm+ Pgauge Absolute pressure


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Moderate Pressure Sensors

Manometer

Bourdon Tube gauge

Bellow gaugeDiaphragm gauge

High Pressure Sensors

High wire pressure transducer

Low Pressure Sensors

McLeod Gauge Ultra Low Pressure Sensors

Knudsen gauge

Ionisation gauge

Classification of Measuring Devices


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MODERATE PRESSURE

MEASUREMENT

There are two types of devices for moderate

pressure measurement-

1. Manometersfor measuring static pressure

2. Elastic elementsfor measuring static and dynamic

pressure.


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Manometers-

A 'manometer'is an instrument that uses a column of liquid

to measure pressure, although the term is often used

nowadays to mean any pressure measuring instrument.

Important and desirable properties of the manometricfluids are:

High chemical stability

Low viscosity

Low capillary constantLow coefficient of thermal expansion

Low volatility

Low vapour pressure


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U- Tube

Manometer: A manometer is comprised

of a bulb containing a gasand a U-shaped tube.

The U-shaped tube is

partially filled with mercury.

The weight of the mercury

puts pressure on the gas.

If the U-tube is OPEN there

is also air pressure acting on

the gas.

The gas molecules put

pressure on the mercury.

PHg


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CONSTRUCTIONOF A U-TUBEMANOMETER

This manometer consists of a

U shaped tube in which the

manometeric liquid is filled.

The manometer is used tomeasure the pressure which is

unknown by the balancing

gravity force and acceleration

due to gravity, g = 9.81 m/sec2


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Inclined Tube Manometer

Well Type Manometer


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Elastic Transducers-

The main types of Elastic Transducers are-

Bourdon tubesdiaphragmscapsulesbellowsCorrugated diaphragmsHelical tube

All except diaphragms provide a fairly large displacementthat is useful in mechanical gauges and for electricalsensors that require a significant movement.


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Types of Measuring Devices-

Bourdon Pressure Gauge The Bourdon pressure

gauge uses the principlethat a flattened tube tendsto straighten or regain its

circular form in cross-section when pressurized.The strain of the materialof the tube is magnified byforming the tube into a Cshape or even a helix, suchthat the entire tube tendsto straighten out or uncoil,elastically, as it is

pressurized.


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LVDT TypePressure Transducer

Current is induced in a circuit when there is change in

magnetic flux. The change in flux can be obtained by

moving a high permeability magnetic core relative to

any circuit.


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Piezo-Electric

Transducer

This sensor consists of a piezoelectric crystal (made

from quartz) which functions as a force-sensitive

voltage source where the piezoelectric will be in

between two plates.

Pressure exerted on the crystal surface is

proportionate to the voltage produced by the crystal.

This sensor does not require any voltage supply.

This sensor is suitable for fast changing pressure

measurement.


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Capacitance Type Pressure Transducer

Change in distance between two plates of a capacitor results in

change in capacitance. Change in pressure results in deflection in diaphragm and hence

change in capacitance. this change is converted into electrical signal

and used in CTPT.


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R1and R3measures radial strain near the outer radius of the

diaphragm while R2and R4are bonded near the centre and

measures tangential strain.r- tangential stress

t - radial stress

t- diaphragm thickness

v-Poissons ratioP-pressure on the diaphragm

Diaphragm Type Strain GaugePressure Transducer


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p-pressure on the diaphragmE- Youngs Modulus of the diaphragm material

v- Poissons ratio

R- radius of diaphragm

t- thickness of diaphragm

From strength and elasticity of

materials, it is seen that deflection

y at a radius r of a circulardiaphragm clamped at its outer

periphery is given by-


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High Pressure Measurement

High wire pressure transducer

For pressures above 1000 atm, techniques based on the

electrical resistance change of a manganin or a gold

chrome wire with hydrostatic pressure, due to bulk

compression effect is used.

R- Resistance of wire

E- Youngs Modulus of wire

p- Pressure

- Density


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Low PressureMeasurement

A McLeod gaugeis a scientific instrument

used to measure very low pressures, down

to 10-6Torr.

It isolates a sample of gas and compresses

it in a modified mercury manometer until

the pressure is a few mm Hg.

Pi = ghA/(V-Ah)

V- Volume of capillary tube

A- Area of cross section of

capillary

McLeod gauge


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IONISATION GAUGE

The most convenientmethod of

measuring pressures below 0.1

Pa is to ionise the gas molecules,

collect the ions and measure theion current which is included in

ionization gauge.

Useful range: roughly 10-8-0.1 Pa

P = Ip/S Ig

PPressure

IgGrid Current

IpPlate Current

SSensitivity (Chemical

Dependant)


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KNUDSEN GAUGE

An instrument for measuring very

low pressures, which measures the

force of a gas on a cold plate

beside which there is an

electrically heated plate.

It is an absolute gauge for

measurement of pressure in range

10-8to 10-3 torr.

Pi = K F /((Ts /Tm)-1)Ts-Absolute temperature of stationary plate

Tm- Absolute temperature of movable plate

K-Elastic constant of gauge

F- Force


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REFERENCES

1. Nakra, B.C. and Chaudhary, K.K., Instrumentation,

Measurement and Analysis, 2nd ed., Tata McGraw

Hill, New Delhi, 2004.

2. Eckman, D. P., Industrial Instrumentation, Wiley

Eastern , 1978

3. http://en.wikipedia.org/wiki/Pressure_measurement


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Pressure measurement.pptx