1) INTRODUCTION

Helium Leak Testing is a sensitive, reliable and most widely used method of leak

detection in manufacturing industries. The typical users are the manufacturers of

electronic tubes and other devices with critical operating conditions. Mostly the heavy

and light fabricated tubes, pressure vessels and assemblies are tested using Helium leak

test. The growth of Helium leak detection technique was accelerated in the late 1950’s by

the space exploration program. Nearly, everything in space vehicles from hermetically

sealed electronic components to fuel tanks, transfer lines and valves need to be leak free

or nearly so. The commercial market for appliance and other consumer goods spurred on

by the development of the space age began to expand in the 1960’s. These and other

requirements challenged leak detector manufacturers. Helium leak detectors appeared on

production floors testing, beverages can ends, pressure transducers, torque converters,

pressure vessels and heat exchangers. In some applications the unit were automated to the

point where the need of operators was virtually nil.  Biotech companies use helium leak

detectors to helium leak test implant able medical devices such as pacemakers to insure

that the outer packages are protected from bodily fluids and to protect patients from

possible contamination from leaking batteries and other materials. Automobile

manufacturers use helium leak testing technology to test items such as air bag initiators,

radiators and air conditioning units. Semiconductor fobs are littered with helium leak

detectors used to leak test process equipment. 

Although mass spectrometers can be traced as far back as the turn of the century the

helium leak detector mass spectrometer was developed during the Manhattan Project. The

gaseous diffusion method used to enrich uranium-235 required leak tightness far beyond

available means. Project scientists were able to improve upon current mass spectrometer

designs and eventually developed what is still the basic formula for most helium leak

detectors.  Helium leak testing is used throughout industry to locate leaks in even the

most complex pressure, vacuum and hermetic systems and enclosures. The following are

just a few examples of the most common uses of helium leak testing:  

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1) Steam Turbine and Condenser Air in leakage Helium Leak Testing - Quickly

locate even the smallest vacuum leaks on the low-pressure side of any steam turbine or

condenser. Helium leak testing is widely recognized as the superior alternative to the

ultrasonic, smoke, and our favorite, the shaving cream method of vacuum leak testing

steam turbine condensers.

2) Chemical and Plastics Production - Oxygen and other contaminants entering

through leaks in flanges or broken welds can effect many manufacturing processes and

result in lower quality and efficiency or totally unusable product. Helium leak testing of

multi floor vacuum distillation towers, reactors and associated plumbing and pumps is

easily accomplished utilizing helium leak testing.

3) Heat Exchangers - Helium leak testing large tubular heat exchangers can be

expensive and time consuming. With the helium leak testing method thousands of tubes

can be scanned in a short period of time when compared to some of the other methods of

examination. Re-testing after plugging or other repairs are performed can be

accomplished immediately.

4) Underground Pipelines: - With helium leak testing we can locate leaks in

underground pipes to within a few feet keeping excavation to a minimum. Due to the

versatility of the helium leak testing method virtually any pressure or vacuum system can

be tested with a high

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2) FUNDAMENTALS OF LEAK DETECTION

1) Leak detection Defined:

A leak may be defined as an unintended crack, hole or porosity in a containing wall that

allows the admission or escape of fluid or gas. The basic function of leak detection is the

location and measurements of leaks in sealed products, which must contain or exclude

fluids.

2) The Need for Leak Detection:

Even with today’s complex technology, it is for all practical purposes, impossible to

manufacture a sealed enclosure or system that can be guaranteed to be completely leaking

proof without first being tested. The fundamental question in leak detection is; what is the

maximum acceptable leak rate consistent with reasonable performance life of the

product?

Anyone who manufactures or uses closed vessels needs leak detection. A partial list of

typical users includes:

a) Any industrial pressure vessel manufacturer

b) Manufacturers using tubular elements, such as refrigeration

equipment manufacturers, chemical plants etc.

c) Vacuum chamber manufacturers

In our case coolant carrying tube pipes of STIB Project were to be fabricated at the ends

with flanges and so required the Helium leak testing for its leak proof operation.

What type of leakage should these products avoid? For some, it is leakage, which will

damage the product or impair the process; for other, it is the loss of material vital to the

product or process.

There are four general classes of leak detection:

A) Hermetic Enclosures (Admission of material through leaks)

These are tested to prevent entrance of contaminants or loss of fluid that would affect

performance of the enclosed unit. Examples: electronic devices, integrated circuits, sealed

relays, motors, ring pull tab can ends, and multi-pin feed through.

B) Hermetic Systems

These are tested to prevent loss of operating fluid or gas within the system. Examples:

Torque converters, hydraulic systems and refrigeration systems.

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C) Evacuated Enclosures

These are tested to prevent too-rapid deterioration of vacuum with age. Examples:

electron tubes, TV picture tubes, bellows sensing elements, full-panel opening can ends,

etc.

D) Vacuum Systems

These are tested to minimize in leakage and allow attainment of better vacuum or higher

gas removal ability at any given vacuum (absolute pressure).

3) The sources of leak:

Imperfect joints or seals by which various parts are assembled to form the finished

products most commonly cause leaks in newly manufactured products. Still another class

of leaks consists not of holes or cracks in the usual sense. Instead the molecular structure

of the containing wall itself is arrayed in such a way as to permit gas diffusion through

the wall.

4) Leak Measurement terminology

Most leaks involve gas flow, and the most widely used methods of leak detection depend

upon observation of gas flow. Leaks are measured in units of gas flow per unit time. To

specify a quantity of gas, its pressure as well as its volume must be specified. Pressure is

customarily specified in “Torr” – a unit of measurement used in barometric

measurements. This gas volume per unit time is typically specified in “standard cubic

centimeters per second.” This practical limit of low-pressure measurement with a mercury

barometer is about 1 Torr.

1 Torr = pressure of 1 machine Hg.

Leak Rates can be defined in 2 ways: -

A) In terms of application, such as-

1 Oz. Of refrigerant R-12 in 2 years at 70 Psi or

65 cc of oil per year at 0 psig and 1400oF, or

B) In terms of the leak method detection method used, such as-

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2 bubbles/sec (1/8” diameter) when pressurized to 40 psi (Bubble method)

1.8 X 10-7 std cc / sec of helium at 1 atm. (Helium method)

2 Psi pressure decay in 5 minutes at 60 psi (Pressure decay method)

The generally accepted unit of leak rate for leak detection is that cc/sec, because it

contains the units of flow rate, namely MASS/TIME. The term “std cc/sec ” is an

abbreviated form of “cubic centimeters of gas (at standard Temperature and Pressure) per

seconds.”

In vacuum work, where pumping speeds are measured in liters/sec and pressure in Torr,

the term “Torr-liters/sec” is widely used.

1 std cc/sec = 0.76 Torr-Liters/sec.

Conversions:

1 std cc sec 0.76 torr-liter/sec

1 torr-liter sec 1.3 std cc/sec

1 std cc/sec 9.7 x 10-4 micron cubic feet per hour

1 mcf/h Practically 10-5 std cc/sec

5) The size of leaks that Affect Product Life:

The maximum acceptable leak rate for a given product depends upon the nature of the

product. Since the cost of leak detection increases as the specified leak rate decreases.

Some examples of products leak specification:

Product or System Leak Rate Specification Comment

Chemical Process Equipment 10-1 to 1 atm cc/sec High Process flow

Rates

Torque converter 10-3 to 10-4 atm cc/sec Retention of liquid

Pacemaker 10-9 atm cc/sec Long life implanted in

body

Although industrial leak rate specification range from 10-9 atm cc/sec to 1 atm cc/sec, the

majority of products have leak rate specification laying in a narrow range, from 10 -7 atm

cc/sec to 10-1 atm cc/sec. The upper part of this range is covered by bubble testing down

to 10-4 std cc/sec. Other methods overlap the bubble method and extend well below its

lower limit. The helium method can detect leaks smaller by a factor of 10,00,000. Leaks

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larger than 10-1 atm cc/sec can usually be spotted visually. 5.0x10-8 Std cc/sec He, means

that .00000005 cubic centimeters of helium will leak every second given a constant

pressure differential across the leak of one atmosphere. Also used is atm cc/sec.

Std cc /sec He Time to leak 1 cc of He Typical Products Remarks

1x10-2 100 seconds Oil from engine Protect engine

1x10-3 16 minutes Water tank Prevent water loss

1x10-4 3 hours Storage tankPrevent loss of product. Protect

environment

1x10-5 26 hoursBeverage can top C02 Retention

1x10-6 2 weeks

1x10-7 4 monthsIC packages Package integrity

1x10-8 3 years

1x10-9 30 years

Implant able

Medical Devices

Prevent malfunction due to

contamination. Long life

1x10-10 320 years

1x10-11 3,200 years

1x10-12 320,000 years

3) HELIUM AS A TRACER GAS

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Since a mass spectrometer may be tuned to virtually any mass, the choice of a tracer gas

is limitless. However, the particular application of leak testing and desired characteristic

for a tracer gas narrow this choice. A tracer gas for leak testing should have the following

characteristic:

It should be non-toxic.

It should diffuse readily through minute leaks

It should be inert

It should be present in not more than trace quantities in the atmosphere.

It should be relatively inexpensive.

The lighter gases have the highest diffusion rates and will therefore give the best

sensitivity. Only hydrogen (mass H2=2) is lighter than helium (mass He=4). Of the inert

gases, helium is buy far the lightest and the concentration of helium is only 5 parts per

million in the atmosphere.

Helium Vs Air-

If helium is used as a tracer gas but air leaks into or out of the product, what is the

relationship between the two?

The mass spectrometer leak detector is capable of indicating leak rates only in the tracer

gas to which it is tuned and calibrated (nearly always helium). It cannot indicate leak rates

in air. There is however, an approximate relationship that is derived from the kinetic

theory of gases.

Kinetic theory of gases predicts that the relative flow rate of two gases through an ideal

leak under molecular flow conditions (roughly, below one millionth of atmospheric

pressure) will be inversely proportional to the square root of the average molecular

weight. Since the molecular weight of air about 29 and helium is 4, helium will flow 2.7

times as fast as air through this leak. The implication is that the minimum detectable leak

in terms of air is ½.7 of that for helium.

4) LEAK LOCATION AND MEASUREMENT

Leak location:

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Leak location is the testing approach used to find the precise location of individual leak.

Leak location is carried out by means of two techniques, termed the detector-probe mode

and tracer-probe mode.

In the detector-probe mode the test piece is filled with tracer gas and the exterior is

scanned with a probe that is attached to the inlet of the leak detector. The probe

continuously admits (or “sniff”) some of the air directly in front of the test piece. This air

is inducted to the analytical portion of the leak detector, where any of the tracer gas that

may be leaking from the test piece is detected.

In the tracer-probe mode, the leak detector is used to evacuate the interior of the test

piece, and a probe is used to discretely spray test gas on suspected leak sites. Any leaks

are evidenced when the tracer gas flows through the evacuated test piece and is detected

by the leak detector.

Leak measurement:

Leak measurement is the approach used to actually measure the total (ideally) or partial

(as compromise) leakage of a device or system.

The two standards leak measurement techniques are known as the inside-out mode and

the outside-in mode. With either of these approaches, the tracer gas may be allowed to

accumulate before detection, or may be detected continuously. Generally continuous

detection yields a faster test with adequate sensitivity; however, circumstances sometimes

require the accumulation of tracer gas prior to analysis.

5) METHODS OF LEAK DETECTION

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There is a wide range of methods available for finding and measuring leaks in products or

assemblies. The more commonly used methods are summarized below. Leaks are special

types of defects, which can have a major importance in systems where they have

influence on safety and performance. Many objects will have a reduced reliability if they

contain leaks.

Leak testing is a non-destructive examination method that is used for detection and

localization of leaks and for measurement of leakage in systems or objects that is under

vacuum or pressure.

Before a leak test examination is performed it is necessary to determine if the

examination is to ascertain whether leaks are present or not, overall leak detection, or if

the examination is to determine the location of a leak, localizing leak detection. In some

cases an examination for overall leak detection is performed first, and if leaks are

detected, the localizing method is applied for pinpointing of the leak. This is however not

always required nor possible.

Secondly it is necessary to determine the leak rate that can be tolerated, as no objects are

100% tight. That is the requirements to tightness of the object. If for example the object

have to be watertight, a leak rate below 10-4 mbar l/s will be sufficient. But if the object

for example is to be used in the chemical industry the requirements can be a leak rate

below 10-6 mbar l / s.

In leak testing a pressure difference between the outer and the inner side of the object to

be examined is produced. Subsequently the amount of gas or liquid that is passing

through a leak is measured.

In principle two methods are applied for leak testing and localization of leaks, the

"Vacuum method" and the "Overpressure method".

At the "Vacuum method" the object to be examined for leaks is evacuated and sprayed

from the outside with a search gas, in this case Helium. The gas enters through any leaks

present in the object and is detected by a sensor connected to the leak test instrument.

At the "Overpressure method" the object to be examined for leaks is filled with a search

gas, Helium, under slight overpressure. The search gas escapes through any leaks present

to the outside and is detected by a detector probe. This detector probe is in most cases

called a "sniffer" acting as a gas-sampling probe.

For both methods specially developed leak detectors are available.

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The object under test should, if possible, be tested according to its final mode of use, i.e.

if it is used under vacuum, the vacuum method should be applied, if it is finally

pressurized, the overpressure method should be adopted.

Examples of testing with the two methods are illustrated below. The Vacuum method is

illustrated with the Hood Test and the Tracer Probe Test and the Overpressure method

with the Hood test, the Bombing test and the Detector Probe or Sniffer test.

1) Vacuum method - Hood test

The Hood Test is an overall leak test. The evacuated test object is covered with a (plastic)

hood. The space between the test object and the hood is filled with Helium from the

search gas reservoir so the total outer surface of the test object is exposed to the Helium

search gas. The helium enters through all leaks present at the evacuated test object and

thus the detector connected to it. The detector now indicates the total leak rate.

The hood test can for example be used for examination of small

vessels.

Fig 2: Helium Leak Test, Vacuum Method, Hood Test

2) Vacuum method - Tracer probe test

At the tracer probe test the same set-up as used for the hood test is applied, but without

the hood. A spray gun is used to spray a fine jet of helium search gas at areas suspected of

leaking. Again the helium enters through leaks present at the evacuated test object and the

detector connected to it. The detector indicates if a leak is present at the area the helium

search gas is exposed to.

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The tracer probe test can for example be used on seals, flange

connections, weld seams etc.

Fig 3: Helium Leak Test, Vacuum Method, Tracer probe test

3) Overpressure method - Hood test

As in the vacuum hood test the overpressure hood test is an overall leak test. A test set-up

similar to the vacuum test is used. A vacuum chamber is used for the hood, which can be

evacuated by a auxiliary pump and to which the leak detector is connected. The leak

detector indicates the helium search gas that is escaping through leaks in the test object.

The use of a helium leak detector allows detection of extremely small leaks and is

suitable for automatic leak detection in industrial equipment.

The hood test can for example be used for examination of vessels, heat

exchangers etc.

Fig 4: Helium Leak Test, Overpressure Method, Hood test.

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4) Overpressure method - Bombing test

The bombing test is a pressure/vacuum method of leak detection used

for testing of hermetically sealed components containing a cavity,

which can be gas-filled or evacuated. The object to be tested gets in a

pressure chamber exposed to the helium search gas. During an

exposure time of up to several hours at a high helium pressure, the

helium will penetrate through any leaks present in the test object. This

is the part referred to as the "bombing". After the pressurization or

bombing the objects are tested for helium emission in a vacuum

vessel, following the same procedure as in the hood test. This test

permits detection of the smallest leak rates and is especially used for

objects, which cannot be gas-filled by other means. The bombing test

can for example be used for examination of transistors, ampoules etc.

Fig 5: Helium Leak Test, Overpressure Method, Bombing test.

5) Overpressure method - Sniffer test

At this type of test the suspected areas of the test object are carefully explored by means

of a detector probe, a "sniffer", which is connected to the leak detector. The objects to be

tested are under helium search gas overpressure. The sensitivity of the method of the

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method and the accuracy of localizing any leaky points depend on the nature of the search

gas, the design of the sniffer and the time constant of the actual leak-testing device.

The sniffer test can for example be used for examination of vessels, heat exchangers,

seals, flange connections, weld seams etc.

Fig 6: Helium Leak Test, Overpressure Method, Sniffer test.

The leak detection system discussed in this ITG is a mass spectrometer leak detector

tuned to detect small quantities of helium. It is utilized by pacemaker electronic

component manufacturers to test electronic components (integrated circuits, transistors,

capacitors, etc.) for hermetic loss. The system is typically portable (on casters) or bench

mounted and operates from a 115 volt, 60-Hertz power source. The system contains one

or more vacuum pumps, a magnetic mass spectrometer and auxiliary components

necessary for proper operation.

Industrial users employ mass spectrometer leak detection on all sizes of objects from

miniature components to large systems and there are various detection methods used. The

most popular leak detection method used by pacemaker manufacturers to leak test

electronic components is the bell jar or hood method using helium as the tracer gas. In

this method, the test object is placed in a pressure chamber and the chamber is filled with

commercially pure helium at a specified pressure. The test object is held in the

pressurized chamber (soaked) for a specified time (bomb time). Pressure and bomb time

vary according to the test specification used. Typically, bomb pressure is four

atmospheres minimum and bomb time is one to four hours. If there is an opening in the

test object, the pressurized helium will be forced through the opening into the test object.

The chamber pressure is then released and the test object is transferred to the leak

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detector hood or bell jar. The bell jar interior is connected through a valve to the leak

detector mass spectrometer tube. Transfer time from chamber to bell jar should be kept at

a minimum to prevent loss of helium from the test object. Military and industrial

specifications typically specify a maximum time between removals from the pressure

chamber to detection of 30 minutes.

When the detector test cycle is initiated, the test station automatically evacuates the free

volume under the test dome to a vacuum level compatible with the interior of the mass

spectrometer. The valve then opens allowing any tracer gas leaking from the test object to

enter the detector spectrometer tube.

Operation of the mass spectrometer is similar to standard mass spectrometer operation. In

the described system, the gases entering the spectrometer tube, such as nitrogen, oxygen,

carbon dioxide and helium (if a leak occurs) are ionized by an electron beam. The

spectrometer magnetic field separates the resulting ions according to mass. The helium

leak detector magnetic field is arranged such that only helium has the right mass to reach

the detector. As the helium ions strike the detector, a minute current flow is generated.

The current flow is amplified and the amplified flow (which is proportional to the amount

of helium in the tube) appears as a visual leak rate indication on the leak indicator meter.

6 The leak rate indicated on the detector meter is the equivalent air leak rate.

The measured leak rate is the quantity of gas in cubic centimeters that flows through an

aperture or porous wall in one second as determined under specified conditions. 5 It is

assumed that the gas is air at room temperature, and is at one atmosphere pressure on the

high-pressure side of the leak and the low-pressure side (vacuum) has a negligible effect

on the flow rate. Leak rate is commonly given in units of atm-cc/sec. The air leak rate can

be converted roughly to a helium leak rate by multiplying, Air leak rate x 2.8 = helium

leak rate.

The helium leak detector discussed here is used to detect leak rates in the 10 -4 to 10-10 atm

cc/sec range (fine leak) although units are now being developed to detect gross leaks, 10 -4

atm cc/sec or greater. There is presently no technical basis for maximum allowable leak

rate specifications. The reason for this is that there are no data presently available which

can be used to relate leak to component life. Acceptable leak rates may vary as the

internal free volume of the test object varies.

The sensitivity of the described system is such that gross leaks, or leaks with flow rates

10-4 atm cc/sec or greater cannot be accurately detected. Also, the tracer gas would be

removed from test objects with gross leaks when the system evacuated the bell jar and

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little or no helium would reach the spectrometer tube. Some helium is removed from

objects with fine leaks during evacuation but the evacuation time is small (typically three

seconds) and detector manufacturers state that enough helium remains for detection

purposes. This is a questionable area. It is obvious that bomb time and pressure become

important. It is also apparent that test dome or bell jar volume should be kept small to

decrease evacuation time.

For leaks 10-4 atm cc/sec and larger (gross leaks) a bubble test is commonly utilized. The

test object is soaked in a pressurized chamber filled with helium or other gas (as done for

fine leak testing) and then the test object is immersed in a silicone or mineral oil, or

fluorocarbon liquid and observed for bubbles emanating from the object. Gross leak

testing should not be performed before the fine leak test, as there is a possibility that the

test liquid could temporarily plug a fine leak.

Calibration of the helium leak detector is presently accomplished using a calibrated

helium leak. The calibrated leak is typically in the form of a cylinder charged with helium

at atmosphere pressure. The cylinder contains a filter through which helium exits at a

fixed calibrated rate when the cylinder valve is opened. The temperature at which the

calibrated leak was calibrated is marked on the cylinder (typically 22-23 C) and the

calibrated helium leak cylinder should be at this temperature when calibrating the system

or a temperature compensation factor should be provided and used in calculating the test

object leak rate. The actual accuracy of calibrated leaks are questionable due to the lack

of standardization in calibration methods and the disagreement between different

calibration labs as to the accuracy of the calibrated leak rate. When using the calibrated

leak to set the sensitivity of the helium leak detector, the detector meter is set for direct

readout at the air leak rate figure marked on the calibrated leak cylinder.

Radioisotope and weighing are two other methods used for leak detection. Radioisotope

leak testing is generally felt to be a better leak testing method than helium leak detection.

In this method, the test object is soaked in a pressurized chamber of radioactive gas. The

object is then removed and the emissions of gamma rays penetrating the walls of the

object are counted, thereby measuring the amount of radioactive tracer gas trapped within

the leaking object. An Atomic Energy Commission license is necessary for possession

and use of radioisotope test equipment and manufacturers are reluctant to use this method.

Radioisotope leak testing will be covered in more detail in a future ITG.

A weight test method is also used in which the test object is weighed before and after

being pressurized in a test liquid, or before and after an extended time period.

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The detection of loss of component package integrity is important because entrance of

damaging contaminants will reduce the components effective life. Water vapors both

sealed-in and that, which leaks into the package, is a contaminant of major concern.

Moisture inside the package may result from package leaks or the moisture may be sealed

in during component manufacture. Sealed-in moisture may result from improper or

inadequate handling or processing of materials. For example, the walls of ceramic

packages are a sink for moisture, which may later serve as a moisture source after sealing.

Glass, epoxies, shellacs and polyamides are also sinks. A proper bake-out period and

subsequent sealing in a moisture free environment can minimize moisture in these areas.

Hermetically sealed components are typically evacuated or are sealed in a dry nitrogen

atmosphere. Some component manufacturers are now including helium as part of the

component internal atmosphere to facilitate leak testing. The tubing through which the

sealing gas passes may emit moisture and contaminate the sealing gas. Also sufficient

moisture may penetrate the tubing to contaminate the package. Dynamic flow conditions

should exist to minimize moisture. Sealed-in moisture may be sufficient to block fine

leaks so that they are not detected.

To minimize leaks in the package hermetic seals, care should be taken in controlling

sealing materials. For example, sealing material additives designed to adjust the thermal

expansion coefficient between metal and glass may be contaminated or not be in proper

balance. Inadequate control over the mechanical handling of packages can also result in

degradation of the package seal.

There are presently a number of test specifications in use, both military and commercial,

for performing helium fine leak detection and gross leak detection. In the absence of a

standardized widely accepted test method, it is important that device manufacturers

a) Know the scientific capabilities of the methods being used

b) Conform to their own stated procedures and specifications and

c) Properly calibrate and maintain the device

6) THE HELIUM MASS SPECTROMETER

LEAK DETECTOR (MSLD)

The Helium mass spectrometer leak detector (MSLD) is a complete system for locating

and / or measuring the size of leak into or out of a device or a container. In use, this

method of leak detection is initiated when a tracer gas. Helium is introduced to a test art

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that is connected to the MSLD system .The helium leaking from the test part diffuses

through the system, its partial pressure is measured and results are displayed on a meter.

The MSLD operating principle consists of ionization of gases in a vacuum and

accelerating of the various ions through a voltage drop and a magnetic field. The helium

ions are separated and collected, and the resulting ions current is amplified and indicated

on the meter.

An MSLD consists of a spectrometer tube, quantitively sensitive to the pressure of

helium; a vacuum system, to maintain adequate low operating pressure in the

spectrometer tube; mechanical pump(s), to evacuate the part to be tested; valves to

transfer the connection of the evacuated part from the mechanical roughing system to the

spectrometer vacuum system; amplifier and readout instrumentation, to monitor the

spectrometer tube output signal; electrical power supplies and control, for valve

sequencing, protective circuits etc., and fixing for attachment to the part be leak tested.

Major components:

a) Spectrometer Tube: -

The heart of the MSLD is the spectrometer tube, which is essentially a partial pressure

ionization gauge that measures only the helium pressure in the system rather than the total

pressure. In spectrometer tube operation electrons produced by a hot filament enters the

ion chamber and collide with gas molecules, creating within the chamber ions

quantitatively proportional to the pressure in the ion chamber. These ions are repelled

out of the ion chamber through the exit slits by the repeller field. The combined

electrostatic effect of the repeller, exit slit, focus plates and ground slit collimates the ion

beam so that it enters the magnetic field as a straight “ribbon” of ions.

At the entrance to the magnetic field, ions of all gas species are present in the ion

chamber and if an ion collector were placed at this point, its voltage would be

proportional to the total pressure in the ion chamber. However, as the ions pass through

this magnet magnetic field, they are deflected in direction proportional to their mass to -

change ration. The spectrometer tube is typically designed and adjusted so that hydrogen

ions are deflected 1350, helium ions 900, and all heavier species less than 900.

Consequently, only helium ions pass through the field exit slits and arrive at the collector.

The collector current is therefore proportional to the partial pressure of helium in the

spectrometer tube and, within the normal operating pressure range of the MSLD, is not

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affected by the pressure of the residual gases. The collector current is measured by an

electrometer amplifier and displayed on the meter.

A cold cathode ionization gauge is an integral part of the tube. It serves two functions

1) To monitor total pressure within the tube to initiate protection of the

filament in case of an excessive pressure rise.

2) Because of its location between the filament and the vacuum system, to

minimize the quantity of gaseous hydrocarbons reaching and damaging the

filament.

The ion sources is a replaceable assembly consisting of two tungsten filament that

provides a source of electrons; an ionization chamber into which the electrons are beamed

to ionize gas molecules; a repeller plate that guides the positive ions through an exit slit;

and two focus plates that direct the ion beam towards a slit in the ground potential plate.

The magnetic fields are provided by an alnico V alloy block. The fields are adjusted by

fixed and movable pole pieces. The magnet provides fields for the cold cathode gauge

and ion source as well as the field, which separates the helium ions from the other gases.

The preamplifier assembly contains an ion collector assembly to translate helium ions

into an electrical signal from external interference and stabilize the meter reading.

During use, the ion source keeps the spectrometer tube assembly at normal high

temperature. When the ion source is turned off, however the 15-watt cartridge heater is

used to prevent condensation of contaminates in the tube and to keep it normal operating

temperature.

2) Vacuum system.

In all conventional helium leak detectors, the spectrometer tube is mounted on the inlet

side of the diffusion pump, which is connected to a mechanical pump (fore pump). The

diffusion pump keeps the spectrometer tube at the proper vacuum level. In addition,

conventional leak detectors utilize a liquid nitrogen trap to provide additional pumping

for condensable gases, such as water vapors. The trap also keeps the spectrometer tube

clean by collecting condensable contamination from the test pieces. A second mechanical

pump (roughing pump) may be used to provide faster pump down to the roughing

vacuum in the test port. A valving manifold used to affect transfer to high vacuum pump

(diffusion pump) and thereby bring the test piece to the same pressure as the spectrometer

tube.

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The spectrometer tube pressure is maintained at less than 0.2 milliTorr during use. A

roughing pump is provided to evacuate the test port to a vacuum level that will not disturb

the diffusion pump’s operation (approximately < 100 milliTorr). This is typical of most

MSLD’s used in production today.

Supplementary aids to leak detector operation:

3) Helium Spray Probe: -

In order to locate leaks when the leak detector is used to evacuate the test piece, it is

necessary to have a controllable source of helium so that the helium can be directed at the

point of leakage in small quantities. This procedure uses a spray probes and is an

examples of the outside-in test technique. The spray probe assembly consists of a flexible

hose connected to a regulated helium supply, a spring close valve, and a fine capillary

nozzle to direct the helium to a small area.

4) Sniffer Probe: -

The Sniffer probe is used in the leak location technique known as the detector probe

mode. This device attached to the inlet of the leak detector with a long flexible tube and is

used to pinpoint leaks from test parts pressurized with helium. The probe at the end of the

tube is a small orifice, which allows vacuum to be maintained in the tube at 100 milliTorr

in a Contra-Flow system or much lower in a conventional leak detector. This probe may

be a fixed orifice matched to the pump size or a variable orifice (needle valve) that can be

adjusted over the operating range of the pumping system. Any leakage in excess of 10-5

std. cc/sec can be readily located.

5) Calibration leak: -

This device is an external reference standard that permits setting up the leak detector to

read the leak rate directly on the leak rate meter. It is a necessity if leak rated are to be

measured or if leak rates are to be measured or if the leak detector is to reject leaks in

excess of a predetermined value. The calibration leaks are normally supplied in the 10-7 or

10-8 std. cc/sec ranges. The leak can be verified by actuating a switch.

7) TEST- CYCLE DESCRIPTION

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Leak detector capable of detecting leaks from as large as 1 cc/sec to as small as 2x10 -10

cc/sec (about 1 cc in 30 years). Major assemblies, which comprise the leak detector and

include – a mass spectrometer tube; a vacuum diffusion pump; thermocouple vacuum

gauge; a series of valves sequenced by an operating handle; a test port coupling for

connecting the objects to be tested; and two bar-graph displays, one for use with the

thermocouple gauge to indicate leak rate in the test object.

Initially, both the diffusion pump and the mechanical pump are running and the test port

is open (unplugged). With the operating handle in the VENT position, the vent valve is

open and the rough valve is closed, so that air and other gases in the inlet manifolds are

vented to the atmosphere. At the same time, the test valve is open, so that the diffusion

pump evacuates the spectrometer tube and the mechanical pump evacuates the diffusion

pump through the fore line. When the diffusion pump is warned up, the spectrometer tube

is low enough (< 2 x 10-4 Torr) to permit the ion-source filament in the spectrometer tube

to be energized (there are actually two filaments, but only one is energized at a time.)

With the diffusion pump warms up and the ion-source filament energizes, the test object

is placed in (or connect to) the poet and secured, establishing a vacuum-tight seal. The

operating handle is then turned to START position, where upon the vent valve and the

test valve are closed, and the rough calve is opened. Under these conditions, the

mechanical pump is diverted from the primary function (as fore pump to the diffusion

pump) long enough to “rough” pump test object (Fig.) This “rough” pumping action may

take from several seconds to several minutes, depending on test object volume and

mechanical pump capacity. When the test port pressure is reduced from atmosphere down

to a safe level (approximately 100 milliTorr as indicated by the thermocouple gauge and

its associated horizontal bar graph display), the operating handle can be moved to the

TEST position. In this case, the test valve is opened and the mechanical pump is again

restored to its primary function as fore pump, while continuing to pump the test object.

Vacuum communication is now established between the test port and the spectrometer

tube. At this point, helium is applied sparingly to the test objects, such as from a spray

probe. If there is a leak in the test object, helium entering through the leak will spread to

all parts of the evacuated system. Some of this helium will be exhausted through the

mechanical pump to atmosphere; the rest of helium will be diffused through the diffusion

pump (against or “contra” the normal flow) and will reach the spectrometer tube is

indicated on the associated vertical bar-graph display and can be monitored on an internal

or optional external loudspeaker, as desired.

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As long as helium from the spray probe enters a leak in the test object, the leak rate will

be displayed on the front panel of the leak detector. When the spray probe is removed, the

leak rate will drop rapidly as helium is quickly evacuated from the leak detector by action

of the diffusion and mechanical pumps. The end result is a rise and fall of the leak rate

indication on the display, which is a directly proportional to the leak rate in the test

object.

7) CONCLUSION: -

The helium leak testing is capable of detecting leaks as small as 10 -11 std cc/sec and it is

also capable of finding leaks as 1 std cc/sec. It’s extreme sensitivity in combination with

it’s capability for reliable quantitatively measurement have made it particularly popular in

many industrial application requiring exacting leak rate specification. During the last

decade this method has been simplified and automated, and it can be easily adapted to

difficult production application.

By 1970 a number of new developments has given the helium MSLD far more sensitivity

that was needed for most requirements & hence in subsequent design sensitivity was

traded for speed by simplifying & automating the cycle by integrating it into computer &

microprocessor controlled production line.

Improvements to simplify manual operations by automation reduced maintenance and

increased productivity of the helium leak detector have made it possible to expand it’s

usage and acceptance to areas not previously considered. Pharmaceutical, medical and

diagnostic equipments industries are beginning to utilize helium leak detection techniques

as appliance, automotive, chemical and similar industries have been doing in the past

decade. Meanwhile the electronics, R & D aerospace and nuclear industries that were,

once the sole domain of the helium leak testing, continues to enjoy the benefits of new

and more sophisticated leak detection equipments.

To summarize, some of the capabilities of the Helium Leak Test are listed below.

a) Leak rates down to 10-10 mbar l / s can be detected

b) Both objects with vacuum or overpressure can be examined

c) The objects can be examined during operation

d) The examination can be performed fast and effective

e) Non poisonous, non explosive and inexpensive gasses are applied

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f) Leaks can be detected in due time to prevent safety or operational hazards

g) Leaks can be detected in due time to prevent unplanned and expensive shut downs

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