Pressure Measurement

PRESSURE MEASUREMENT pressure is not a fundamental quantity .

But derived from force and area.

The basic standard for pressures ranging from medium about 10-1 mm hg to several hundred thousand pounds /sq. inch.

Elastic pressure transducer's

Are use for measurement of very high pressures up to 700MN/M2

The action of these mechanical transducers depends up on the displacement caused by pressure.

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PRESSURE MEASUREMENT .Elastic pressure transducer's


Bourdon tube

The bourdon tubes find application because of their simple design an low cost.

There are three type of Bourdon elementals

(1) C Type

The tube which is oval in section is formed in to an arc of 250˚.

One end of the tube is sealed and is attached by a light link to a mechanism which operates a pointer.

The other end is fixed to a socket where the pressure to be measure is applied.


Bourdon tube

(1) C Type

The internal pressure tends to change the section of the tube.

The movement of the tip is ideally proportional to the pressure applied.

The tip is connected to a gear and pinion arrangement through a mechanical link , which amplifies the displacement of tip and converts in to deflection of the pointer.


Bourdon tube

(1) C Type


Bourdon tube

(1) C Type


Bourdon tube

(1) Spiral type

PRESSURE MEASUREMENT Bourdon tube

(1) Spiral type

An increased displacement of the free end can be obtained by the number of turns in the spiral there by avoiding the need for further magnification.

The gear sector and pinion arrangement is not used ,

The absence of gear sector and pinion arrangement eliminates the backlash.

When the pressure is applied to the spiral , it tends to uncoil producing a relatively. long movement of the tip.

The accuracy of the spiral tube elements is higher than C type elements.


(1) Helical type

Coil is wound in the form of a helix.

The displacement of the tip of a helical element is larger than that of the spiral element.


(1) Helical type

A central shaft is installed within a helical element and the pointer is driven from this shaft by connecting links.

Materials used for Bourdon tubes

Brass, alloy steel, stainless steel, phosphor bronze, etc

Phosphor bronze is used for in low pressure applications where the atmosphere is non-corrosive.

Bellows

A series of circular parts which are joined together in such a way that they are expand or contracted axially by changes in pressure.

PRESSURE MEASUREMENT

Bellows

A series of circular parts which are joined together in such a way that they are expand or contracted axially by changes in pressure.

The metal used in construction of bellows must be thin enough to be flexible, and have high resistance to fatigue failure.

Material commonly used are brass, bronze, alloys of nickel

and copper, steel etc.


Bellows

Displacement of bellows element is given by


Bellows

The advantages of bellows include their rugged construction, moderate price, usefulness for measurement of low, medium and high pressures.

Diaphragms

The pressure to be measured is applied to the diaphram,causing it to deflect, the deflection being proportional to the applied pressure.

The movement of the diaphragm depends on its thickness and dia.

PRESSURE MEASUREMENT Diaphragms

The pressure to be measured is applied to the diaphram,causing it to deflect, the deflection being proportional to the applied pressure.

The movement of the diaphragm depends on its thickness and dia.


The maximum deflection ,dm and the deflection at any radius dr are given by


In some cases a diagram element consists of two or more capsules connected together with each capsule deflecting on the application of pressure.

Material used for diagrams include phosphor bronze, stainless steels, beryllium copper, nickel etc.

PRESSURE MEASUREMENTPiezoelectric transducersPiezoelectric transducers produce an emf when they

are deformed by a displacement.

The pressure to be measured is converted in to a displacement through use of force summing devices there by causing a deformation of the piezoelectric crystal.

The emf is an indicative of pressure applied.

They are most useful where pressure changes occur frequently.

Quartz is the most commonly used crystal for piezoelectric transducers.

PRESSURE MEASUREMENT Piezoelectric transducers

Advantages……….

Suitable for dynamic measurements.

They are small in size.

Negligible phase shift between input and output.

Dis advantages

They are not suitable for static measurements

Sensitivity to temp. changes

They have a high output impedance which leads to loading effects.

PRESSURE MEASUREMENT Low pressure(vacuum) measurements

The vacuum pressure gauges are used for measurement of pressure below that of atmosphere and this pressure is called vacuum.

Two commonly used units of vacuum measurement are the torr and the micrometer.

One torr is equivalent to 1mm Hg at standard conditions.

One micrometer is 10 -3 torr.

McLEOD Gauge

it is essentially manometer and use mercury as the manometric fluid.

The principle of working of a McLeod gauge is the compression of a sample of the low pressure gas.

PRESSURE MEASUREMENT McLEOD Gauge

Plunger is withdrawn , lowering the level of mercury below the opening thereby admitting the gas at the unknown pressure “P”.

Let the volume of the gas admitted in to the measuring capillary and the bulb be “V”.

The bulb and the measuring are at the same pressure as vacuum source “p”.

Plunger is pushed in , the mercury level goes up

Further motion of the plunger causes compression of this sample and the motion of the plunger is continued until the level of the mercury is at zero mark in the reference capillary.



Unknown pressure P={h2 A t } /{ V-A t h}

“h” is the height of the mercury in reference capillary ,m

At is the cross- section of measuring capillary tube, m 2

Advantages of McLEOD Gauge

The unknown pressure is calculated in terms of the physical dimensions of the gauge .Therefore , this gauge can be used for calibration of other vacuum measuring gauges.

Can be use d for measurement of pressure below 0.1x 10 -3 torr.


It doesn’t give continuous output.

Gauge is sensitive to condensed vapours that may present in the sample of the gas.

There are limitations to the lowest pressure that can be measured.

Measurement of high pressure(Bridgeman gauge)

Pressure up o about 70000Mpa can be measured with strain gauge cells or bourdon tubes.

Bridgeman gauge employs fine wire of Manganin or Gold chrome.

The wire is wound in the form of a coil enclosed in a pressure container filled with kerosene oil.

PRESSURE MEASUREMENT Measurement of high pressure(Bridgeman gauge)

PRESSURE MEASUREMENT Measurement of high pressure pressure(Bridgeman gauge)

The coil is enclosed in a flexible kerosene filled bellows which transmit the pressure under measurement to the coil .

The change in the resistance with change in pressure is measured with Wheatstone bridge.

Change in value of resistance, ΔR=R1 b ΔP

b- is the pressure co-efficient of resistance.

Ionization Gauges

An electron passing through an elecric field in a gas produced by two electrodes acquires a kinetic energy which is proportional to the potential difference between two electrodes.

PRESSURE MEASUREMENT Ionization Gauges

When this electrons strikes a gas molecule and when this acquired energy is large enough , there is definite probability that the electron will drive an electron out of the molecule leaving it a positively charged ion.

The driven out electron is called a secondary electron.

Electron is edited from a cathode.

Some of these electron strike gas molecule's and drive out secondary electrons/.

Ion current is a direct measure of the pressure of gas.

type

Spiral type

Helical type

C TYPE

3 MN/m2

Arc of 250

The degree of linearity depends upon the quality of gauge from oval to circular

Accuracy + 1%

SPIRAL TYPEThe length has increased

Magnification factor not required

It tends to uncoil producing relatively long movement of tip

+ 0.5%

HELICAL TYPERatio as high as 1:10

+ 0.5% to + 1%

MATERIALS USED

Phosphor bronze-atmosphere is non-corrosive

Stainless steel ,monel- corossive atmosphere

BELLOW

A series of circular parts, resembling the folds in accordion

Material flexible, ductile easy to fabricate, high resistance to the fatigue failure

Brass, bronze, alloy of copper and nickel ,steel , monel

Absolute pressure

Gauge pressure

Differential pressure

DOUBLE BELLOW DIFFRENTIAL GAUGE

Liquids having having low-coefficient of thermal expansion

Ethyl glycol

+ 0.5%

DIAPHRAGMS

DIAPHRAGM ELEMENT USING THREE CAPSULES

POTENTIOMETER PRESSURE TRANSDUSER

BONDED FOIL STRAIN GAUGE

Documents

Pressure Measurement