Helium Leak Detection...A P A S S I O N F O R P E R F E C T I O N 2 VACUUM Helium leak detector...

A P A S S I O N F O R P E R F E C T I O N

Helium Leak Detection

Presented by:

Terry Rogelstad


■ 2

VACUUM

Helium leak detector basic principle

Helium

Q

Q = Helium flowrate in atm.cc/sec. ormbar. l/sec.

He

HELIUMSPRAYPROBETEST

PIECE

© Pfeiffer Vacuum 2016 • Leak Detection • Ron Ligthart • 2016


■ 3

Outboard or sniffing test

HeorH2 200PSI

He



■ 4

Few examples of Helium Leak Detection today

AutomotiveAirbags

Gas tanksCompressors

Fuel railsShock absorbers

ABS valvesRadiators

Wheel rimsManifoldsSensors

Oil coolersFuel cells

55 gallon drums

AerospaceEngines

Antennas Watches

AircraftHydraulic components

gyroswings

missiles

Food packaging

MedicalPacemakerCatheters

Blood FiltersSealed Packaging

SemiconductorMass flow controllers

Integrated circuits Vacuum systems

Gas linesGas cabinets

Quartz displays

ElectricalLampsTubes

TransformersPower plants

Circuit breakers



■ 5

Why helium?

Scarce (5 PPM)

Inert

Safe

Inexpensive

Light?

He

N

O

FNe

Cl

Ar

Kr

Xe

H



■ 6

Helium versus Hydrogen leak test gas

HeliumAmbient concentration: 5ppm

Inert: non-toxic, non-flammable High energy: Fast diffusion

Cannot be created via processNon-replenishable resource

Cost: $$$$Increasing cost rate: $$$

BLM surplus runs out ~2018Supply: intermitting issues

Hydrogen 95/5 (N2/H2)Ambient concentration: .5ppm

Non-toxic, non-flammable @95/5 Higher energy: Faster diffusion

Can be created via processReplenishable resource

Cost: $Increasing cost rate: $

Supply: No issuesMixing 95/5 onsite: ~1 cent cubic/ft.

© Pfeiffer Vacuum • Product Manager • Ron Ligthart • 2016


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Why helium/hydrogen leak detector?

High Sensitivity

Wide Range

Reliable

Quantitative

Dynamic Testing

Nondestructive

No “ghost” readings



■ 8

Various leak rate relationships

waterrunning1 mm

10mbar l/s

waterdripping100 µm

10E-1mbar l/s

watertight

30 µm

10E-3mbar l/s

Bacteriatight

10 µm

10E-5mbar l/s

Virustight3 µm

10E-7mbar l/s

“technically“tight

0.1 µm

10E-12mbar l/s

“gas“tight

0.8 µm

10E-9mbar l/s

µm = microns mm = millimeters mbar l/s = leak rate Note: Test conditions such as pressure will impact correlations



■ 9

Leak rate equivalents

refrigerantleakage in

ounces/year.

Time requiredfor one 3mm bubble to

form (dunk test).

Equivalenthelium leak. in

std. atm. cc/sec.or mbar l/s.

10.00 (284 grams)3.00 (85 grams)1.00 (28 grams)

.50 (14 grams).10 (2.8 grams).01 (.3 grams).00006 (.002 grams)

13.3 sec.40.0 sec.

145.0 sec.290.0 sec.24.0 min.

240.0 min.667.0 hrs.

1.8 x 10-3

5.4 x 10-4

1.8 x 10-4

9.0 x 10-5

1.8 x 10-5

1.8 x 10-6

1.0 x 10-8

NOTE: Above conditions are approximate with same test conditions.

ICE



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Helium leak detector wide range capability

1 x 10-1 atm cc/sec. 1X1 x 10-2 atm cc/sec. 10X1 x 10-3 atm cc/sec. 100X1 x 10-4 atm cc/sec. 1,000X1 x 10-5 atm cc/sec. 10,000X1 x 10-6 atm cc/sec. 100,000X1 x 10-7 atm cc/sec. 1,000,000X1 x 10-8 atm cc/sec. 10,000,000X1 x 10-9 atm cc/sec. 100,000,000X1 x 10-10 atm cc/sec. 1,000,000,000X1 x 10-11 atm cc/sec. 10,000,000,000X

CONCLUSION:

1 x 10-1 = 6 cc/minute He.1 x 10-11 = .03 cc/century He.

.01 cc/century Air. © Pfeiffer Vacuum 2016 • Leak Detection • Ron Ligthart • 2016

Mantissa Exponent

5.4 x 10-12 mbar L/S He.(.0000000000054 mbar L/S He.)


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Approximate range of leak rate methods

TEST METHOD

Bubble Testing

Pressure Decay

Helium Mass spectrometer

102 101 100 10-1 10-2 10-4 10-5 10-6 10-7 10-8

Leak Flow Rates in std. atm cc/sec. or mbar l/sec.

10-910-3


Foaming agent

Pressure rise

Flow controller

Infrared

Quartz sensor


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Leak rate units of measurement & conditions Leak rate units of measurement and conditions can severely

alter the real leak rate value.

=

Atm. cc/sec. He (drive force of leak at exhaust side x volume per unit time of helium)

1X10-8?Air?

Helium?Leak Rate?Pressure?

What?

Cc/second?Cc/minute?Cc/hour?Cc/day? ?Atm?

Torr?Mbar?

Pascal?InchHg?

PSI?

?5 PSIA?100 PSIG?

5 BAR?200 mbar?1000 Torr?

ASK:What gas?

What pressure?What unit of time?

What unit of pressure?

Additional details:What temperature?

What gas concentration?



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What is a leak?

Q = P x SWhere: Q = Flow Rate

P = PressureS = Pumping Speed

Q = P x VT

Where: Q = Flow RateP = Pressure ChangeV = VolumeT = Time Change

EXAMPLE: Assume a pressure change of 2 millibar within 80 secondsin a 10 liter volume.Q = 2mbar x 10Liters = .25 millibar liters/second.

80 seconds

LOOK! LOOK!A BUBBLE!

“Q”



■ 14

What is a leak?

Literal definition“…to enter or escape through an opening usually by a fault or

mistake.”

Everything leaks!It is a matter of degree or “How much?”

Modern leak detectors are extremely effectiveDetects leakage of molecules of tracer gas (< 0.0029 cc/year)

- Set test standard to leaks of practical concern -


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Determining Leak Rate Specifications

Typical Considerations What type of leakage will damage the product

Inflow of contaminant

Outflow of contents Vacuum requirement of process

How much material flow can be permitted

How long must the product last – “shelf life”

Competitors specification

Customer requirements


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Adixen turbomolecular high vacuum pump

ROTOR VANES

EXHAUST

STATOR VANES

CERAMIC BEARINGS

MOTOR

INLET



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Adixen Molecular Drag Pump (MDP)

IMPELLER

MOTOR

UPPER CERAMICBEARING

LOWER CERAMICBEARING

ROTOR

EXHAUST INTERSTAGE PORT

Robust “Holweck” design.

Low 27,000 RPM.

Mounted in any orientation.

100% field serviceable.

MTBF of 50,000 hrs.

Interstage pumping.(two HLD test ports)

Rugged single monoblock machining.

Superior compression ratio1,000,000,000 for N220,000 for He.

Full pump protection:air inrushes,thermal,movement.



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Adixen hybrid turbomolecular drag pump

TURBO STAGES

DRAG STAGES

EXHAUST

CERAMIC BEARINGS

MOTOR

ROTOR



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Counter-Flow Testing

Tracer gas (helium) flows past turbo exhaust

Helium molecules “migrate” back through turbo

Tolerable pressure at turbo exhaust is very high

Spec tube protected from heavier molecules

SpecTube

Gas Flow

Helium MoleculeFlow

Helium Gas

15 Torr

10-5 Torr

Exhaust

RP


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ACP28 mechanical dry pump

ROOTS TECHNOLOGY

FRICTIONLESSVACUUM

DISPLACEMENT SINGLE PHASELOW VOLTAGE

5 STAGECOMPRESSION

AIRCOOLED

16 CFM displacementFirst PM @ 22,000 hours



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Adixen HLD analyzer cell

+200VVAR.

FILAMENT

GROUND

TRIODE PLATEHEAVY IONS

HELIUM IONS

LIGHT IONS

REPELLER

HIGHPOWERSUPPLY

LOGAMPS

- +TARGET

RESISTANCE

HIGHVACUUMGAUGE

GROUND

2X



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Adixen helium calibrated leaks

HELIUM CALIBRATED Type FE 14 N

values Atm cm3/s +- 10%HeAtm cm3/s +- 10%Air

Date Temperature

8.4x10-8

3.1x10-8

2763

20 CTemperature Coeff. 3%/degree C. 2% loss/year

A L C A T E L

Nov. 23- 1996

150 cc volume / 8.4x10-8 atm. cc/sec. He = 1,785,714,286 seconds57 years

8.4x10-8 atm cc/sec. He - 2% loss = 8.2x10-8 atm cc/sec. He

HELIUM TO AIR CONVERSION IN MOLECULAR FLOWQhe = Qair Mair = Qair 29 = 2.7 8.4x10-8 atm. cc/sec. He = 3.1x10-8 atm. cc/sec. air

Mhe 4 2.7



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Pyrex Helium Calibrated Leak

Glass pyrex tube

Helium under pressure

Constant Helium permeation takes place until helium supply is depleted

Valve should always be open when in storage (permeation lives on).

Should glass break within. Your in trouble ($$$) and hide the evidence.

Helium pressure is directly proportional to helium permeation through glass pyrex.

Heliumrecharge

portHe

HeHe

He



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Integrated large vacuum chamber leak testing

Helium Helium


Leak detector in test cycle


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Leak rates versus permeation signature

Real Leak

Permeation

IntroduceHelium

RemoveHelium

Time

Leak

Rat

e Si

gnal

Note: Imperative that leak rate does not mimic permeation signature



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Vacuum purity levelsPressure Gas Density Mean Free Path760 torr 3 x 10+19 molecules/cc. 2.5 x 10-6 inches .76 torr 3 x 10+16 molecules/cc. 2.5 x 10-3 inches 7.6 x 10-3 torr 3 x 10+14 molecules/cc. .025 inches 7.6 x 10-6 torr 3 x 10+11 molecules/cc. 21 feet 7.6 x 10-8 torr 3 x 10+9 molecules/cc. 2100 feet7.6 x 10-10 torr 3 x 10+7 molecules/cc. 40 miles

“give me a pennyfor every moleculein one atm. cc. asmy annual salary?” boss

“DEAL!”

3 X 10+19/100 = $30,000,000,000,000,000

Mr.AVS



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Standard atmospheric pressureTotal pressure is the sum

of all partial pressures1013 millibar

760 mm of mercury760 torr (760,000 millitorr or microns)

14.7 PSIA

Particle density: 2.7E+19 per cubic centimeter(27,000,000,000,000,000,000 particles)

Partial pressure of Helium: 5E-3 mbarAtmospheric helium concentration: 5ppm

Partial pressures593 N2 torr159 O2 torr7.1 Ar torr.25 CO2 torr

1.4E-2 Ne torr4.0E-3 He torr8.7E-4 Kr torr4.0E-4 H2 torr6.6E-5 Xe torrH2O variable

760 torrTotal pressure



■ 28

Vacuum measurements reality

PRESSURE % VACUUM

14.7 PSI = 760 Torr13.5 PSI = 700 Torr11.6 PSI = 600 Torr

9.7 PSI = 500 Torr7.7 PSI = 400 Torr 5.8 PSI = 300 Torr3.9 PSI = 200 Torr1.9 PSI = 100 Torr

.2 PSI = 10 Torr.02 PSI = 1 Torr = 1000 Millitorr

.002 PSI = .1 Torr = 100 Millitorr .0002 PSI = .01 Torr = 10 Millitorr

.00002 PSI = .001 Torr = 1 Millitorr

0% 8%

21% 34%47%59%74%87%99%

99.9%99.99%

99.999%99.9999%



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Vacuum flow characteristics

PUMP PUMP

H L

VISCOUSFLOW

MOLECULARFLOW

Viscous flowPressure (millibar) X Diameter (centimeters) = >.66

Molecular flowPressure (millibar) X Diameter (centimeters) = <.02

1x10-4 mbar1 mbar



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Gas Flow

ViscousL/d < 0.01Rough Vacuum760 T to 1 TLR > 10-5 sccs

Transitional0.01 < L/d < 0.5 Medium Vacuum1 T to 10-3 T

MolecularL/d > 0.5High/Ultrahigh Vacuum< 10-3 TLR < 10-5 sccs

L

d


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In molecular flowConductance is the ability of an opening to allow a volume of gas

to pass through in a given amount of time. Most helium leak test applicationsunder vacuum are in molecular flow where conductance plays a

vital role for performance testing.

Simplified formula for air at 20 degrees C.

Cair = 12.1 x D3 / L

Where: C = liters/secondD & L = centimeters (Diameter & Length) Example: Assume a 3’ (91.4 cm.) tube length

with a ID of 1/4” (.64 cm)

C = 12.1 x .643 / 91.4 > C = whopping .034 l/s air

For Helium : C = 2.7 x .034 > C = .092 l/s. He.

For other gases than air Correction Factor

Cgas = CF x Cair

N21.0

O2.95

H23.8

Kr.59

AR.85

He2.7

CF Correction Factors for other gases



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In Series & More RealityConductance in series is where the total conductance is less

than the smallest of the conductances in series.

Series Conductance

1/Ctotal = 1/C1 + 1/C2

50 litervacuum chamber

“VS”Example 1: Assume C1 = 100 l/s.

And C2 = 50 l/s.Ctotal = 33.33 l/s.

More Conductance Reality

VS = C x SC + S

Example 2: C = Conductance of a 3’ bellow with 1/4” ID for Helium in molecular flow = .092 l/s.S = HLD He. inlet pumping speed @ 4.4 l/s.VS = He. pumping @ chamber= .09 l/s.

HLD Response time = 9.3 minutes.Example 3: If conductance of bellow “C” was 28 l/s. He.

(1 meter NW40)VS = He. pumping @ chamber = 3.8 l/s.HLD Response time = 13 seconds.

C

S

VS = pumping speed @ chamberC = conductance of bellowS = pumping speed of leak detector

C1 C2 = C total

This image cannot currently be displayed.



■ 33

adixen

adixen

ASM 142

2.0

Inboard or vacuum test

He

SPRAYPROBE

Spray heliumstarting at the

top of testitem.



■ 34

Helium leak detection involves the introduction of a tracer gas(helium) to one side of a containment wall and sampling for thattracer gas on the other side. Typically, there is a pressuredifferential across the chamber wall.

He To Leak Detector

P

P' < P

P'


■ 35

So What?

A helium leak detector does not detect leaks…

It detects helium

Helium signal, response time, & test results dependant on…

Test conditions, method & technique


■ 36

Outboard or sniffing test

He200PSI

He

Sniff heliumstarting at the

bottom oftest item.

LIMITATIONSNatural helium background of 5ppm.

Non global test (not cumulative)Operator dependent

adixen

adixen

ASM 142

2.0



■ 37

Sniffing basics & reality

TEST ITEMUNDER POSITIVE

HELIUM PRESSURE

Not avacuumcleaner

Noliquids

Filter stone

Conductancelimiter

Flow rate @ 1 cc/sec.

Adixen sniffing response and clean-up time isa function of tube length (.2 seconds/meter).

CRITICAL: Minimize helium concentration within working environment.

Angle probe tip along surfaces to assure flowrate and avoid clogging of any kind.

For maximum sensitivity probe speed shouldnot exceed 1 inch per second.

Helium5 ppm

Start at bottom of testitem and move yourway up.



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ASM 340 sniffing Helium/Hydrogen sensitivity.

Helium sensitivity:5E-9 mbar L/S. @100% He.

<.00006 oz. refrigerant loss per year*.

Hydrogen sensitivity:1E-8 mbar L/S. @100% H2.

.00006 oz. refrigerantloss per year*.

Non-flammable95/5 Nitrogen/Hydrogen

mix correction factor:.0012 oz. refrigerant

loss per year*.

*approximate based on refrigerant selected & test conditions

Response time for 16 ft. (5 meter). Sniffer hose ~ 1 second.(.2 seconds per meter of sniffer hose)



■ 39

Sniffing helium leak test example – step one

Helium

Test itemevacuation pump

Test item vent

Evacuationvalve

Heliumchargevalve

Vacuum

PressureCoil is evacuated andback-filled with helium

Note: coil is pressurized with nitrogenfor gross pressure decay test first. Helium vent must go outside

or reclaim system!

01



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Helium/Hydrogen outboard sniffer leak test Coil under test gas pressure

operator

ASM340Outboard sniffing mode

Helium or Hydrogentest gas

Standardor

Smart Probe



■ 41

Average molecular velocityAverage molecular velocity at room temperature (20 C).

HeliumAir

HydrogenHeliumAirNitrogenOxygenArgon

1.75 x 103 meters/sec. = 6,299 KPH = 3,915 MPH1.24 x 103 meters/sec. = 4,882 KPH = 2,997 MPH4.64 x 102 meters/sec. = 1,670 KPH = 1,038 MPH4.71 x 102 meters/sec. = 1,696 KPH = 1,054 MPH4.40 x 102 meters/sec. = 1,583 KPH = 984 MPH3.94 x 102 meters/sec. = 1,418 KPH = 881 MPH

Va = 8RT/ M = 1.455 x 102 T/M

WHERE: Va = Average velocity in meters/sec.R = Gas constant.M = Molecular weight in grams.T = Absolute temperature in Kelvin.



■ 42

Helium concentration impact

Helium leak test concentration is directly proportional to leak rate test results that can aid economic cost by means of helium dilution.

Example: Leak rate criteria = 1 x 10-6 atm. cc/sec. He.using 100% helium.

1 x 10-6 x 100% = 1 x 10-6 atm. cc/sec. He.

Example: Leak rate criteria = 1 x 10-6 atm. cc/sec. He.using 10% helium mix.

1 x 10-6 x 10% = 1 x 10-7 atm. cc/sec. He.

Example: Leak rate criteria = 1 x 10-6 atm. cc/sec. He.using 1% helium mix.

1 x 10-6 x 1% = 1 x 10-8 atm. cc/sec. He. Increasing test pressurecan bring back lost helium

sensitivity with helium dilution

1

2

3!



■ 43

Optimal spectrometer location

A CB

ROOTSBLOWER

ROTARY VANEMECHANICAL PUMP

OR DRY PUMP

HELIUMLEAK DETECTOR


Is it A or B or C?


■ 44

Subfab Leak Testing

*Not to scale

HeliumSpray gun

Exhaust at ~atmosphere

“Sniffing mode”


Helium spray


■ 45

Optimal spectrometer location?

A CB

ROTARY VANE WET ORDRY MECHANICAL PUMP

HELIUMLEAK DETECTOR

OPEN VALVE CLOSED VALVE

TURBO OR HYBRIDTURBODRAG PUMP



■ 46

Gas line “N2 BULLET”

PURGEGASN2

He

Maintainviscous flow



■ 47

“N2 Bullet” advantage

Conventional Vacuum BulletHelium background 1 X 10-10 atm cc/sec. <1 X 10-9 atm cc/sec.Helium spray time 5 seconds 5 seconds Appearance time 510 seconds (8.5 minutes) 25 seconds Peak value 1 X 10-6 atm cc/sec. 1 X 10-4 atm cc/sec. Time to obtain peak 1800 seconds (30 minutes) 35 seconds Clean up time More than 1 hour <2 minutes N2 flow rate Zero 150 sccm

•Conditions:•200 feet, 1/4” stainless steel gas line•Adixen ASM310•1 x 10-4 atm. cc/sec He leak rate at end of gas line•Research grade nitrogen for bullet principle•150 SCCM gas bullet flow rate

PURGEGASN2



■ 48

Reality of vacuum grease

Properly maintained vacuum seals don’t need grease—that is, really good technicians don’t need it.

Vacuum grease is a lubricant and retains helium.

Use only hydrocarbon or fomblin/krytox grease if needed.

If used, use enough for “glaze” but NO GOOPS!

Quick fix and long term consequences


???


■ 49

1E-2 mbar

Virtual leaks are small trapped volumes of gas at atmospheric pressurewithin the vacuum envelope which communicates with the vacuum

system by means of minute crevices, cracks, grooves, etc.

Outgassing is the desorption of atmospheric gases and contamination from the surface area within the vacuum system.

Virtual leaks & outgassing



■ 50

vacuum

solution

vacuum

“o” ring

Virtual leaks

vacuum

Top view with solution

Virtual leaks are small trapped volumes of gaswithin the vacuum envelope that can also “burp”

Virtual leaks within “O” rings



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Outgassing

Properties of Outgassing The rate of outgassing generally diminishes over time

The rate of outgassing increases with temperature

Sources include water vapor, synthetics, residual solvents, and contamination (finger prints)

The release of gases and vapor from the chamber walls and other materials inside a vacuum system


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What be happening here?



■ 53

Helium Background

Helium background is defined as a natural concentration ofhelium at a specific pressure.

Qb = P x S x (5 x 10-6)

Where: Qb = Helium background.P = Pressure in millibar.S = Helium pumping speed in liters/second.

5 x 10-6 = Standard ambient helium concentration.

Example:Assume: P = 1 x 10-3 mbar

S = 4.4 liters/second

Qb = (1 x 10-3) x 4.4 x (5 x 10-6) = 2.2 x 10-8 atm cc/sec.

CAUTIONHELIUMAHEAD



■ 54

Response time

Response time is defined as the time to reach 63% of the signal.

T = VS

Where: T = Time in seconds.V = Volume in liters.S = Helium pumping speed in liters/second.

Example 1: V = 50 liters. T = 50 = 250 seconds S = .2 liters/sec. .2 (4.2 minutes)

Example 2: V = 50 liters. T = 50 = 11.4 secondsS = 4.4 liters/sec. 4.4

12

3

6

921

4578

1011



■ 55

Disappearance timeDisappearance time is the time required for the leak detector to recover to its desired sensitivity after being exposed to a specificleak rate.

Td = V x 2.3 x log QS Qm

Where: Td = Appearance time in seconds.V = Volume in liters.S = Helium pumping speed in l/s. Q = Leak rate in atm. cc/sec.Qm = Smallest detectable leak.

Example 1: Example 2:Assume: V = 50 liters Assume: V = 50 liters

S = 4.4 liters/second S = .2 liters/secondQ = 1 x 10-4 atm. cc/sec. Q = 1 x 10-4 atm. cc/sec.Qm = 1 x 10-8 atm. cc/sec. Qm = 1 x 10-8 atm. cc/sec.

Td = 50 x 2.3 x log 1 x 10-4 = 105 sec. Td = 50 x 2.3 x log 1 x 10-4 = 2300 sec.=38 min.4.4 1 x 10-8 .2 1 x 10-8



■ 56

Parallel operation (split flow)

15

Sb

Sa

VacuumChamber

Qt

adixen

adixen

ASM 142

2.0

Qb = Sb x Qt (Sb + Sa)

Where: Qb = Helium detector reading in atm. cc/sec.Sb = Detector helium pumping speed in liters/sec.Sa = Helium pumping speed in liters/sec. of auxilary pump.Qt = Helium leak rate to be detected.

Example:Assume: Sb = .6 liters/sec.

Sa = 2.5 liters/sec.Qt = 1 x 10-8 atm. cc/sec. He.Qb = .6 x (1 x 10-8)

.6 + 2.5

Qb = 2 x 10-9 atm cc/sec. He.



■ 57

adixen

adixen

ASM 142

2.0

Pump down calculationT = 2.3 x V x LOG P1

S P2WHERE: T = TIME IN SECONDS

V = VOLUME IN LITERSS = INLET PUMPING SPEED

IN LITERS/SECONDP1= START PRESSURE

IN MILLIBARSP2= TEST PRESSURE

IN MILLIBARSEXAMPLE

CONDITIONS: V = 40 LITERSS = 3.5 L/SEC.

P1= 1000 MBAR.P2= 6 MBAR.

T = 2.3 x 40 x LOG1000 = 59 seconds.3.5 2

NOTE: ABOVE CONDITIONS ARE CLEAN, DRY,EMPTY AND CONSTANT PUMPING SPEED.

40 LITERS1.3 L/SEC.



■ 58

Viscous flow leak

Viscous flow leak is when the gas molecules are entrained by frictionto produce laminar flow as is encountered particularly in gross leaks.A cylindrical type leak in this regime can be expressed by the “Poiseuille’s” equation.

“Poiseuille’s”

QDnL

P1P2

= Leak rate in atm. cc/sec.= Leak Diameter in cm. = Gas viscosity in bar sec.= Length of leak in cm.= Test pressure on one side of leak (bar).= Test pressure on other side of leak (bar).

EXAMPLE: Assume a leak diameter of .005 millimeters with a length of 2 millimeters.Internal test pressure of 100PSI (6.8 bar) helium with surroundingambient pressure using the sniffing principle.

Q = .00054 (7.82 - 12) = 1.2 x 10-3 atm. cc/sec. helium. 256 (1.93 x 10-10) .2

“Q”

Q = D4 (P12 - P22)256nL



■ 59

Molecular flow leaksMolecular flow leak is when independent gas molecules no longerinteract with one another within the diameter of the leak due tothe mean free path. A cylindrical type leak in this regime can beexpressed with the “Knudsen” equation.

“Knudsen”

Q = 1 2 RT D3 (P1 - P2)6 M L

QRTM

= Leak rate in atm. cc/sec.= Ideal gas constant.= Absolute temperature.= Molecular weight of gas.

DLP1P2

= Diameter of leak in cm.= Length of leak in cm.= Test pressure on one side of leak (bar). = Test pressure on other side of leak (bar).

EXAMPLE: Assume a leak of .001 millimeters with a length of 2 millimetersTest item is under vacuum with helium being sprayed surrounding test item at atmosphere in a 20 C ambient environment.

Q = 1 2 (8.32 x 107) (293) (.0001)3 (1 - 0) = 1.63 x 10-7 atm. cc/sec. helium. 6 4 .2



■ 60

Fine leak rate realityTheoretical hole diameter calculation in molecular flow regime

D = 6 M L 1 Q .33

2 RT P1- P2CONDITIONS

1E-11 atm cc/sec. He leak rate.2 cm. wall thickness20 C ambient temperatureInboard He spray test

D = 6 4 (.2)(1)(1E-11) .33

2 (8.32E7)(293)

D = .0000045 cm. = .045 microns

Size (microns) .0001 .001 .01 .1 1 10 100 1,000 10,000Pollens

Skin cellsLints

Tobacco smokeHousehold dust

Industrial dustAcid fumesGas molecules

Finger printsAlcohol residue

1 meter=

1,000,000 microns

1E-11 1E-9 1E-7Theoretical leak rate values:



■ 61

How it works…1. Prior to Test: Measure and record raw signal (background)

2. Actual Test: Measure actual leak + background

3. Display: Actual test measurement – raw signal = Actual Leak

Helium Background Suppression (Zeroing)

CautionDo not “zero” signal while applying helium to test object

Signal(sccs)

Time (s)

Raw Signal

Activate ZERO Measured Value

Actual Leak + Raw Signal

Displayed Value(Measured Value – Raw Signal)

ZEROReference

RawSignal

Actual Leak


■ 62

Calibration Routine

Tune - Adjustment of ion source voltages

Calibrate - Adjustment of calibration factor (gain)

When do we calibrate?

Beginning of shift or test

After shock to leak detector

When in doubt

Leak Detector Calibration

Warm up leak detector for at least 30 minutes before calibrating


■ 63

Avoid Misidentifying Leak Location

Begin testing area with highest probability of leakage first

Seals, fixtures, recently maintained components, etc.

In still air, test from the top of the system down

‘Dead-Stick’ testing for accurate leak location

In high Air flow environments…

Use a wind barrier while fine testing

Isolate (“bag”) components being tested

Large Chamber Testing


■ 64

Helium leak detection Contact

Terry Rogelstad - Leak Detection InstructorTEL: 585-248-3208

Main contact numbers


Date post:	25-May-2020
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