Chapter 4(Equipment and Accessories)

8/13/2019 Chapter 4(Equipment and Accessories)

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Equipment andAccessories

(M. Anees Akhtar)


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Vacuum Boxes Vacuum Boxes are used for Bubble Leak testing in the

field

Vacuum boxes are available for rounded surfaces,

corner seams and vertical seams. The box should be able to withstand an external pressure

of 100 kPa.

Flexible gaskets are provided to seal the enclosure to thetest surface when pressure is applied to the vacuum box.

The vacuum box should be of convenient size such as 15cm by 75 cm (6 inches wide by 30 inches long).

Each box contains a window in its top opposite the openbottom.

Suitable connections, valves, lighting and gauges arethe accessories of the vacuum boxes.


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Vacuum Boxes


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Vacuum Boxes


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Desirable features of Vacuum Boxes Desirable features for a vacuum box are. Ability to readily admit natural or artificial light Close proximity of viewing window to the surface of the

weldment being inspected. Capability for easy initial seating when starting evacuation,

and good sealing properties to hold the vacuum.

Equipped with a highly portable, readily availableevacuation device with sufficiently high flow to be able toovercome a certain amount of seal leakage and rapidlyevacuate the box to the required pressure differential.

Easily read dial gauge Quick acting valve for either shutting off the evacuation

device or controlling the level of pressure differential. A vacuum box for bubble leak testing should be

checked for workability before performing leak testing todetermine that the condition of the box components issuch that it is capable of attaining a higher pressuredifferential than is required for the test.


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Application of Foaming Solutions following are the methods listed in order of

decreasing preference

Spray application using a pump-type garden spray

can with a fine orifice.

Spray application using a plastic squeeze bottle or anoil squirt can

Brush application using a short handle painters brush,

1 to 3 cm in width.


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Gaskets The gasket system used with a vacuum box is

critical to the ease with which the box can behandled and sealed to the test surface so as to be

able to hold a vacuum. The gasket must be shaped so that the initial area

of the gasket contact with the test surface is smallto make seating of the vacuum box easier.

Neoprene gaskets of 20 to 40 durometer providegood flexibility.


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Dial Gauges for Vacuum BoxA pressure or vacuum dial gauge must be readily

visible to the operator controlling the pressure withinthe vacuum box during leak testing.

Pressure indicating gauges used in testing shouldpreferably have dial graduations covering a rangeof about double the intended maximum pressure.

In no case should the range be less than one and

one half not more than four times that pressure.All gauges used shall be calibrated against a

standard dead weight tester a calibrated mastergauge, or a mercury column, and recalibrated atleast once each 12 months.


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Pirani Guages


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Pirani Guages Pirani gauges employ a wheatstone bridge circuit,which serve to heat a filament and to balance itsresistance against a standard resistor sealed off in highvacuum.

A change of pressure causes a change of filamenttemperature and, consequently, of the filamentresistance, thus unbalancing the bridge.

The pressure can then be measured in terms of theunbalanced voltage. Alternatively, the power required

to maintain the filament temperature at a constant levelis a measure of pressure. The temperature in this case iskept constant by means of a feedback circuit.

produce fairly accurate absolute pressure readings fromatmosphere to 0.1 or 0.01 Pa (1.0 or 0.1 mtorr).


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Hot Cathode Ionization Guage The high filament temperature (1500 C to 2200 C)causes electrons to escape and become free electrons.

The free electrons create gas ions as they acceleratetowards positively charged grid.

They are absorbed by the grid; forming electron currentcalled emission current.

Gas ions are attracted to negatively charged ioncollector where they pick up electrons and becomeneutral molecule. This neutralizing electron current iscalled ion current.

The magnitude of the ion current is an indicationof pressure.

A conventional hot cathode ionization gauge measurespressures in the range of 10-3to 10-10mbar.


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Hot Cathode Ionization

Gauge(BayardAlpert Ionization

gauge)


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Cold Cathode Ionization Gauge

(Penning Gauge) Measuring Range

10-210-7mbar The cold cathode ionization gauge is basically a direct current (dc) glow

discharge tube . The tube has two metallic electrodes(an anode and a cathode), one at

each end of the tube. A dc voltage is applied in series with a large current limiting resistor. The

resistor ensures discharge stability and prevents excessive current flow. To strike a discharge, an initial supply of electrons is needed from the

cathode. Field emission occurs when the cathode is held at a high voltage and

electrons are freed. Once electrons are released from the cathode, they accelerate towards

the anode creating positive gas ions by impact ionization. The positive

ions accelerate towards the cathode. The energy from positive ionsimpacting the cathode releases an additional supply of secondaryelectrons sufficient in quantity to sustain the discharge process. Thequantity of electrons released by ion impacts on the cathode dependson the number of ions produced in the discharge.

The number of ions produced depends on the gas pressure in the tube. Measuring the ion current at the cathode is an indication of pressure.


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cold Cathode Ionization Gauge

(Penning Gauge)


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Radioactive Ionization Gauge

The ions are produced by radiations (usually alphaparticles) emitted from a radioactive source.


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Evacuation system for Vacuum Box

Leak TestingA small box mounted air ejector connected to a

compressed air supply.

A small portable vacuum pump.

The vacuum box, when placed over theexamination area, should be evacuated at least35 kPa pressure differential with respect toatmospheric pressure.

The 35 kPa pressure differential can be verified bythe dial gauge.


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Lighting Good lighting is necessary for the safety and reliability of

the test results.

Two general sources of lighting:

Artificial Lighting

For continuous shift operation, artificial light is essential.

For other types of operation, it must be relied upon from 20to 50% of the total working hours, excluding overtime work ornight work.

General Lighting

General lighting is the base or minimum amount of light

required. The uniform distribution of light to produce equivalent seeing

conditions throughout an interior.

Localized general lighting sources are arranged 3 meter ormore above the work to provide sufficient light for generalsafety and for ordinary visual needs.


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Absolute Pressure Dial Guages Aneroid Capsules Upto 350 kPa

Bourdon Tube PressureGauge Above 350 kPa

The ordinary dialpressure gauge hastypical accuracies inthe range of 0.25 to

0.33% of full scaleindication whenrecently calibrated.


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Barometers

Can be used to measureabsolute pressure.

The height of mercurycolumn indicatesatmospheric pressure.

These pressuremeasurements are absolute,because the pressure insidethe sealed end of the tubeis always zero.

There is only vacuum abovethe mercury column, sothere is no contribution dueto air pressure.


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Instruments for Pressure

Measurement Types of pressure gauges that provide visible

indications during leak testing include;

Absolute pressure dial gauges.

Aneroid barometers.

Ordinary dial gaugesindicating pressure relative to

ambient atmospheric pressure as well as water.

Manometer , UTube mercury manometers and

mercury column barometer.


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Instruments for Temperature

MeasurementDew point sensor

The dew pointis the temperature below whichthe water vapor in a volume of humid air at a

constant barometric pressure will condense into liquidwater.

Vapor pressure due to moisture adds to the totalpressure measured by the pressure measurementinstruments.

It works on the principle of balancing the moisturecontent in the element with the moisture content ofthe surrounding atmosphere.

Used to test volume subject to pressure changeleakage rate measurement tests.


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Vacuum Pumps

Vacuum pumps are classified into two

groups Gas transfer pumps

Capture pumps


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Low and Medium Vacuum

Pumps Rotary Vane Pump


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High and Ultra High Vacuum

PumpsDiffusion Pump


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High and Ultra High Vacuum

Pumps Turbomolecular Pump


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Vacuum ValvesValves are required to isolate either part or all

of the pumping group from the system which isbeing evacuated or are necessary to control

the pumping sequence.Vacuum valves must;

be free from leakage.

Offer minimum flow resistance and

Contain materials that do not outgas.


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Vacuum Valves

Diaphragm Valves Gate Valve

Closed

Open


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Vacuum Connector O Rings

The O ring are made of an elastomer materialwhich is trapped and compressed between two

surfaces to form the seal. Demountable joints for vacuum in the range

from atmospheric pressure down to 10-7mbarnormally utilize O ring for the sealing method.

Grooved flanges are used to issue the leak

tight system and less compression suchgrooves should meet the followingrequirements.


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Vacuum Connector O Rings

The O ring are made of an elastomer material

which is trapped and compressed between two

surfaces to form the seal. Demountable joints for vacuum in the range

from atmospheric pressure down to 10-7mbar

normally utilize O ring for the sealing method.

Grooved flanges are used to issue the leaktight system and less compression.


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Vacuum Connector Groovs in the flanges should meet the following

requirements. The cross-section of the groove should be of the order of 10

percent greater than the cross section of the O ring, toallow for a metal to metal flange contact withoutovercompressing the O ring.

To provide positive retention of the gasket, when the seal isnot made.

To provide the minimum volume of trapped gas.

To allow reuse of the O ring after the seal is broken. In thisrespect, overcompression or grooves with sharp corner oracute angles are of bad design as this leads to permanent

deformation or cutting of the O ring. To permit easy flange machining end to wide machine

tolerance.

Orings generally provide better sealing than flat gaskets.


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Vacuum Connector Hinged Clamps

These clamps are

used for quickrelease

applications


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Mass Spectrometer Common functions of commercially available

mass spectrometer leak detector instruments

are: Pumping tracer gas samples from leaks in test

objects into the vacuum of the instrument.

The ionization of gas sample molecules by

electron impact.

The sorting and identification of positive ions

according to their mass-to-charge ratios.


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Mass Spectrometer Tube


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Sniffer It is a simple tracer gas

collection device

It is connected to leak

detector with a pumping

device. The pump provides the

suction necessary to

draw tracer gas samples

to the sniffer inlet through

the connecting hose andinto the sensing element

of the leak detector.


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Compound mastics for sealing to vacuum

Apiezon Type

(hydrocarbon)Silicone type Fomblin

Good lubricant Yes No Yes

Vapour pressure (mbar) Low Approx. 10-6 Low

Temperature suitability 125C max. -40 to 200C -02 to 200C

Cost Moderate Low High

Chemical stability Moderate Good Very good


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Tuning and calibration Tuninga helium leak detector is a procedure

of adjusting the electronic controls of theunit to achieve maximum signal strength,highest reading on the leak rate meter whileadmitting a known flow of helium to the leak

detector.Calibrationof the instrument merely

correlates the output signal with a knownflow of helium standard calibration leak.

Calibration is usually accomplished byadjusting the gain, or amplifier feed back,until the leak rate meter reading agrees withthe value stamped on the standard leak.


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Calibrated Leak Standards

Reservoir Calibrated Leaks

Reservoir leaks contain their own tracer gas

supply.

NonReservoir calibrated Leaks

Non-reservoir leaks to which tracer gas is

added during testing.


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Classification of Calibrated

Leaks

LEAKS

Reservoir

Permeation Capillary

Fixed ValueVariable

Value

Porous Plug

Non -Reservoir

Capillary

Fixed ValueVariable

Value

Porous Plug


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Properties of calibrated LeaksAn ideal calibrated leak should have the

following properties:

The leakage rate should be constant, and should

remain unaffected by ambient conditions. The calibration should be accurate.

The physical size should be convenient.

The calibrated leak should not be too delicate or

fragile. The calibrated leak should have its own gas

supply.

Chapter 4(Equipment and Accessories)

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