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BRITISH STANDARD BS EN1518:1998

The European Standard EN 1518:1998 has the status of a British Standard

ICS 19.100

NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW

Non-destructive testing ÐLeak testing ÐCharacterization of massspectrometer leak detectors

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This British Standard, havingbeen prepared under thedirection of the EngineeringSector Board, was publishedunder the authority of theStandards Board and comes intoeffect on 15 August 1998

© BSI 1998

ISBN 0 580 30042 0

BS EN 1518:1998

Amendments issued since publication

Amd. No. Date Text affected

National foreword

This British Standard is the English language version of EN 1518:1998. It supersedesBS 5914:1980 which is withdrawn.

The UK participation in its preparation was entrusted to Technical Committee

WEE/46, Non-destructive testing, which has the responsibility to:

Ð aid enquirers to understand the text;

Ð present to the responsible European committee any enquiries on theinterpretation, or proposals for change, and keep the UK interests informed;

Ð monitor related international and European developments and promulgatethem in the UK.

A list of organizations represented on this committee can be obtained on request toits secretary.

Cross-references

The British Standards which implement international or European publicationsreferred to in this document may be found in the BSI Standards Catalogue under thesection entitled ªInternational Standards Correspondence Indexº, or by using theªFindº facility of the BSI Standards Electronic Catalogue.

A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunity from legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, the EN title page, pages 2 to 12, an inside back cover and a back cover.

http://dx.doi.org/10.3403/00062874

http://dx.doi.org/10.3403/00062874

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CENEuropean Committee for Standardization

ComiteÂ EuropeÂen de Normalisation

Europa È isches Komitee fu È r Normung

Central Secretariat: rue de Stassart 36, B-1050 Brussels

© 1998 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN nationalMembers.

Ref. No. EN 1518:1998 E

EUROPEAN STANDARD EN 1518

NORME EUROPEÂ ENNE

EUROPA È ISCHE NORM April 1998

ICS 19.100

Descriptors: Non-destructive tests, gas permeability tests, leak detection, leak detectors, mass spectrometry, characteristics,instrument sensitivity

English version

Non-destructive testing Ð Leak testing Ð Characterization of massspectrometer leak detectors

Essais non destructifs Ð Contro Ã les d'eÂ tancheÂ iteÂ Ð

CaracteÂrisation des deÂtecteurs de fuite aÁ

spectromeÁtrie de masse

Zersto È rungsfreie Pru È fung Ð Dichtheitspru È fung Ð

Charakterisierung von massenspektrometrischen

Leckdetektoren

This European Standard was approved by CEN on 25 March 1998.

CEN members are bound to comply with the CEN/CENELEC Internal Regulationswhich stipulate the conditions for giving this European Standard the status of a

national standard without any alteration. Up-to-date lists and bibliographicalreferences concerning such national standards may be obtained on application tothe Central Secretariat or to any CEN member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the Central Secretariat has thesame status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, CzechRepublic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy,Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland andUnited Kingdom.

http://dx.doi.org/10.3403/01427921U

http://dx.doi.org/10.3403/01427921U

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EN 1518:1998

© BSI 1998

Foreword

This European Standard has been prepared byTechnical Committee CEN/TC 138, Non-destructivetesting, the secretariat of which is held by AFNOR.

This European Standard shall be given the status of a national standard, either by publication of an identicaltext or by endorsement, at the latest by October 1998,and conflicting national standards shall be withdrawnat the latest by October 1998.

According to the CEN/CENELEC Internal Regulations,the national standards organizations of the followingcountries are bound to implement this EuropeanStandard: Austria, Belgium, Czech Republic, Denmark,

Finland, France, Germany, Greece, Iceland, Ireland,Italy, Luxembourg, Netherlands, Norway, Portugal,Spain, Sweden, Switzerland and the United Kingdom.

Contents

Page

Foreword 2

1 Scope 3

2 Normative references 3

3 Definitions 3

4 Description of an MSLD 4

5 Apparatus for methods of characterization 5

6 Reference conditions for characterization 8

7 Test procedures 8

8 Results 11

9 Test report 12

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© BSI 1998

1 Scope

This European Standard specifies terms and

procedures for the characterization of massspectrometer leak detectors (MSLD). It is not intendedto give a complete set of specifications for anacceptance test but a description of procedures thatcan be used without particular calibration equipment.

An MSLD has an integral high vacuum system for maintaining the sensing element (mass spectrometer)at low operating pressure and for establishing a partial

pressure related to the incoming gas flow. This pressure is measured quantitatively by the massspectrometer. Such instruments are able to selectivelymeasure the flow of a tracer gas. In most cases thetracer gas will be helium, flowing in/out of an object

through a leak.The methods described in this standard are applicablewithout restrictions to helium as the tracer gas. For other gases, additional precautions may be necessary.

These methods are applicable to commonly-availableMSLD, based on the present level of technology, whichmay be able to measure leak rates down to10212 Pa´m3 /s.

2 Normative referencesThis European Standard incorporates by dated or undated reference, provisions from other publications.

These normative references are cited at theappropriate places in the text and the publications arelisted hereafter. For dated references, subsequentamendments to or revisions of any of these

publications apply to this European Standard onlywhen incorporated in it by amendment or revision. For undated references the latest edition of the publicationreferred to applies.

prEN 1330-8, Non-destructive testing Ð Terminology Part 8: Terms used in testing for leak tightness.

3 DefinitionsFor the purposes of this standard, the definitions given

in prEN 1330-8 apply together with the following.3.1 Terms

3.1.1

compression ratio of the counterflow stages of the high vacuum pump for tracer gas

with zero flow, the ratio of partial pressure of tracer gas at the exhaust of the counterflow stages of thehigh vacuum pump to the partial pressure at the inletof the mass spectrometer (MS)

3.1.2

display

a device which indicates visually the leakage ratemeasured. The units in which the leakage rate isexpressed may be selectableNOTE The display may be analog or quasi-analog (continuousscale with definite divisions) or digital (numbers with a definitenumber of digits) or a combination of both.

3.1.3

inlet pumping speed

the volume rate of flow at the test port of an MSLDwhen the instrument is operating with the MS below itsmaximum working pressure

NOTE The inlet pumping speed may be different for differentgases and different modes of operation. The inlet pumping speedfor the tracer gas determines the response time for the volumeunder test.

3.1.4

internal leak port

a flange directly behind the inlet valve, used to connecta small leak for zero drift determination

3.1.5

intrinsic pumping speed

in a direct flow leak detector, the volume rate of flowof tracer gas at the inlet of the MS

3.1.6

ion collector

the part of the MS where ions are collected andneutralized producing a current in the collector whichis a measure of the number of neutralized ions

3.1.7

sensitivity control

an electrical hardware or software control which may

be used to adjust the sensitivity of the instrument sothat a calibrated leak is indicated with its true leakagerate

3.1.8

sensitivity of the MS

the ratio of the ion current at the output of the MS tothe corresponding partial pressure of tracer gas insidethe MS

3.1.9

zero control

an electrical hardware or software control which may

be used to shift the output indication of the leakdetector to a determined point of the scale range inuse, usually zero

3.2 Terms related to the operation of the MSLD

3.2.1

peak (noun)

the trace showing a maximum on the chart recorder when the leak detector is scanned with respect tomass with gas present, usually the tracer gas, to whichthe detector is tuned

3.2.2

peak (verb)to set the scanning control (see 3.2.3) of a leakdetector so that the output due to a given tracer gasinput is maximized. It is a form of tuning (see 3.2.4)

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3.2.3

scan (verb)

to adjust the accelerating voltage (or other equivalentoperating parameter) of a leak detector, particularlyacross that range of voltage which includes the voltagenecessary to produce a tracer gas peak

3.2.4

tune (verb)

in leak detection, to adjust one or more of the controlsof a leak detector so that its response to a tracer gas ismaximized. Tuning by means of the scanning controlonly is called ªpeakingº (see 3.2.2)

3.3 Terms related to the specification of theMSLD

3.3.1 Operating conditions3.3.1.1 General

For the definition of operating conditions see prEN 1330-8.

NOTE The operating conditions are normally given by themanufacturer.

3.3.1.2

optimum working pressure

the pressure in the MS, at which the minimumdetectable concentration can be measured

3.3.1.3

maximum working pressure

the pressure in the MS above which normal operationis no longer possible

3.3.1.4

maximum inlet pressure

the maximum pressure at the test port at which theMSLD is able to detect leaks in a given mode of operation

NOTE For an MSLD to be connected directly to a system under test, the total pressure in the system has to be less than themaximum inlet pressure of the MSLD.

3.3.1.5

maximum gas load

the maximum pV-throughput of all gases emergingfrom the test specimen that the MSLD can pumpduring leak detection in a given mode of operation

NOTE For component testing, the MSLD is normally ready for measurement when the desorption of water vapour from the inner surfaces is less than the maximum gas load.

3.3.2 Tracer gas background signal

3.3.2.1

background signal drift

the relatively slow change in background signal, givenby the maximum change in a given period of time

3.3.2.2

background signal noisethe relatively rapid change in background signal givenby an average measure of scatter in a specified periodof time

3.3.3 Detection limit

3.3.3.1

minimum detectable concentration ratiothe smallest concentration of a given tracer gas in anair mixture that can be detected unambiguously whenthe mixture is fed into the MSLD at such a rate as thatis at its optimum working pressure

3.3.4

resolving power

the ratio of a given mass number to the peak widthmeasured at a specified (for example 10 %) height of the peak (in units of mass numbers)

3.3.5 Display resolution

3.3.5.1general

a quantitative expression of the ability of the leakagerate-display device to distinguish meaningfully betweenclosely adjacent values of the leakage rate indicated

3.3.5.2

linear display resolution

the constant difference between adjacent scaleintervals expressed in % full scale indication

3.3.5.3

logarithmic display resolution

the constant ratio between two adjacent scale intervalsexpressed as a percentage of the indicated value

4 Description of an MSLD

4.1 Main parts of an MSLD

An MSLD (see definition given prEN 1330-8) consistsbasically of a mass spectrometer and a high vacuum

pumping system for:

Ð maintaining the MS under appropriate vacuumconditions;

Ð producing a definite partial pressure of tracer gaswhen a specific throughput of tracer gas enters theleak detector.

An MSLD includes also a number of valves and pressure gauges to ensure the appropriate vacuumconditions within the system.

The leakrate output can be displayed in a number of ways, for example an electrical meter, or digitaldisplays of different types. In addition, a chart recorder output is usually available, which has to be used for the test procedures in this standard.

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The general structure of an MSLD is described by thefollowing list.

a) Mass spectrometer:Ð ion source;

Ð separation system;

Ð ion collector.

b) Pumping system:

Ð inlet system;

Ð inlet line;

Ð inlet valve;

Ð pump valve;

Ð test port;

Ð vent valve;

Ð internal leak port.

c) Sensitivity zero control.d) Display.

4.2 Operating principles of MSLD

4.2.1 Direct flow leak detectors

The leakage rate qx is given by the equation:

qx = px 3 S x

where

qx is the pV-throughput of tracer gas;

S x is the intrinsic pumping speed;

px is the partial pressure of tracer gas in the massspectrometer (MS).

4.2.2 Counterflow leak detectors

The leakrate qx is given by the equation:

qx = px 3 K x 3 S b,x

where

qx is the pV-throughput of tracer gas;

K x is the compression ratio of the counterflowstages of the high vacuum pump for tracer gas(see 3.1.1);

S b,x is the speed of the pumping system, for tracer gas, backing the counterflow stages of the high

vacuum pump;

px is the partial pressure of tracer gas in the MS.

4.3 MLSD specificationThe MLSD specification is described by the followinglist:

a) Operating conditions:

Ð optimum working pressure;

Ð maximum working pressure;

Ð maximum inlet pressure;

Ð maximum gas load.

b) Tracer gas background signal:

Ð background signal drift;

Ð background signal noise.

c) Non linearity.

d) Detection limit:Ð minimum detectable leakage rate;

Ð minimum detectable concentration ratio.

e) Resolving power.

f) Display discrimination.

5 Apparatus for methods of

characterization

5.1 Leaks

For the following tests, two known leaks are required:

Ð a small tracer gas leak for the test of minimumdetectable leakage rate;

Ð a large air leak for the determination of minimumdetectable concentration.

Both leaks have to have a specified temperaturecorrection.

For MSLD using the working principles indicatedby 4.2.1 and 4.2.2, depending on measurement range,more than two leaks may be necessary for

determination of detection limits and sensitivityaccording to the working principle.

5.1.1 Small tracer gas leaks

The small known leaks shall be of a size such that thetotal pressure in the MS is negligibly increased.

For the determination of sensitivity drift, the leakshould produce a deflection which is not less than50 times the smallest detectable signal. For driftmeasurements, as these leaks are usually of the

permeation type and have an appreciable temperaturecoefficient, ambient temperature variation should alsobe taken into account.

Should it be necessary to have a small known leak for the determination of background signal drift(see 7.2.1), this leak has to be small enough to give a signal in the lowest decade of signal output.

5.1.2 Large air leak

The large known leak shall be of such a size that,when ambient air is at its entrance and partial flowconditions do not apply, the pressure in the MSLD risesto the optimum working pressure. Such a large leakcan be obtained using a fine leak valve of highspecification.

5.2 Leak isolation valve (see prEN 1330-8)

(Refer to prEN 1330-8.)5.3 Test vessel

A test vessel is used for the determination of the inlet pumping speed of the MSLD. It is a vacuum chamber of known volume, preferably cylindrical or spherical,typically 10 l.

For connection to the MSLD, the test vessel shall havea flange of suitable diameter (not less than thediameter of the test port of the MSLD) and a secondflange for the connection of a calibrated leak. Typicalarrangements are shown in Figure 1.

The test vessel shall be made entirely of metal and itsinner surfaces shall conform to the requirements of high vacuum technology. Its leak tightness shall bebetter than 10210 Pa´m3 /s.

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Figure 1 Ð Vessel for determination of inlet pumping speed (see 5.3)

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© BSI 1998

Figure 2a Ð Direct flow leak detector test arrangement

Figure 2b Ð Counterflow leak detector test arrangement

5.4 Arrangement for test

5.4.1 Auxiliary pumping system

The MSLD is connected to an auxiliary system asshown in Figures 2a and 2b. (Frequently, the auxiliarysystem is an integral part of the MSLD.)

5.4.2 Materials

The use of rubber or plastics should be kept to a minimum in the system and preferably be restrictedonly to 0-rings. Accordingly, the ªleak isolation valveºshown in Figures 2a and 2b should be of all-metalconstruction. At least, it shall not act as a significantsource of adsorbed tracer gas.

5.5 Preparation for test

5.5.1 Power supply

The MSLD shall be connected to a power supplyconforming to the manufacturer's specification(voltage, frequency, etc.).

5.5.2 Warming up

The MSLD shall have been ªwarmed upº, according tothe manufacturer's specifications, prior to all test

procedures.

5.5.3 Adjustment

The MSLD under test shall have been adjusted for optimum detection of tracer gas in the manner specified by the manufacturer.

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5.5.4 Constant pumping speed

If the vacuum system of the MSLD is such thatadjustment of the volume rate of flow (pumping speed)

is possible, the selected rate shall not be varied duringthe test.

5.5.5 Constant compression ratio of the highvacuum pump

If the vacuum system of the MSLD is such thatadjustment of the compression ratio of the high

vacuum pump is possible, the selected compressionratio shall not be varied during the test.

6 Reference conditions for

characterization

6.1 Reference temperatureThe recommended ambient temperature is 23 8C. Other temperatures between 16 8C and 30 8C are possible andshall be stated.

6.2 Reference pressure

The recommended ambient pressure is 100 kPa. Other pressures are possible and shall be stated.

6.3 Reference humidity

The recommended ambient relative humidity is 50 %.Other humidities are possible and shall be stated.

7 Test procedures7.1 General

Test procedures are given only for those specificationsthat can directly affect measurement results withoutthe use of primary or secondary calibration equipment.

To determine the minimum detectable leakage rateand/or the minimum detectable concentration ratio, itis necessary to define the operation of the MSLD near the limit of its detection ability. The initial stage istherefore concerned with the determination of theminimum detectable signal in terms of backgrounddrift and noise. This is followed by the determinationof the overall sensitivity of the instrument. Sensitivity

measurements will only be meaningful with systemsthat are linear up to the point where the sensitivity ismeasured.

7.2 Minimum detectable leakage rate

7.2.1 Background drift and noise

The background drift and noise are determinedaccording to the following procedure (for the followingmeasurement, the inlet valve of the MSLD shall beclosed and the emission on).

a) Adjust the recorder so that zero on the recorder corresponds to zero of the MSLD output and fullscale of the recorder corresponds to full scale of the

lowest decade/range of the MSLD output.b) For instruments with manual zero control, adjustthe MSLD zero (or backing-off) controls, so that therecorder reading is approximately 50 % of full scale.

c) For instruments with automatic zero control,connect the small leak to the MSLD inlet, the MSLDbeing ready for measurement (the small leak should

give an output signal approximately in the middle of the lowest decade of signal output).

d) Record the output signal for 20 min with a chartspeed of not less than 1 cm/min. Mark the chartrecorder y-axis in units of leakage rate.

7.2.2 Auxiliary pumping system background

The background signal due to tracer gas memory effectin the auxiliary pumping system is determinedaccording to the following procedure.

a) Connect a blanking flange to the auxiliary pumping system as shown in Figures 2a and 2b.

b) Zero the output, with the emission on.

c) Open the leak isolation valve.

d) Initiate pump down and wait until the instrumentis ready for use. If there is no automatic pumpdown, open the pump valve and, when the inlet

pressure is low enough, open the inlet valve andswitch on the emission, if necessary.

e) Allow the pressure in the MS to reach the steady value, showing no observable change within 1 min.

f) Note the output reading when the output hasreached a steady value or no more than 3 min after the opening of the inlet valve.

g) Close the leak isolation valve as rapidly as

possible and, after a period correspondingapproximately to the estimated response time (90 %)of the system, note the output reading.

7.2.3 Sensitivity

For the determination of sensitivity, the small leak witha leakage rate at least 50 times that of the smallestdetectable leakage rate shall be used. The leak rateshall, however, be small enough to remain in the mostsensitive amplifier range in order that the timeconstant can be determined later. The leak has to belocated exactly in the position of the blanking flangethat was used for the auxiliary system backgrounddetermination. The sensitivity is determined according

to the following procedure.

a) Zero the output, with the emission on.

b) Open the leak isolation valve.

c) Initiate pump down and wait until the instrumentis ready for use. If there is no automatic pumpdown, open the pump valve and, when the inlet

pressure is low enough, open the inlet valve, closethe pump valve and switch on the emission, if necessary.

d) Allow the pressure in the MSLD to reach a steady value, showing no observable change within 1 min.

e) Note the output reading when the output signal

has reached a steady value, showing a change notexceeding the drift in 1 min. The units of thisreading should be the same as for the zero drift andfor the noise determination.

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7.2.4 Time constant

Immediately after the determination of the leakage rate

in the preceding step, the time constant of the systemand the MSLD shall be determined, using a recorder,according to the following procedure.

a) Close the leak isolation valve as rapidly as possible.

b) Observe the output continuously and determinethe time when it has reached 37 % of the originalreading.

7.3 Minimum detectable concentration

For the determination of minimum detectableconcentration the MSLD shall be capable of scanningthe range necessary to produce the appropriate tracer

gas peak.NOTE The following explanation is valid for helium used as thetracer gases. For other tracer gases similar considerations apply.

In a plot of MSLD output against scanning voltage, a helium peak on the tail of a lower mass peak, (H3

peak for magnetic sector MS and ªzero blastº for quadrupole MS) with the general features of Figure 3a,is produced. In practice, there is always some heliumbackground signal (Figure 3b); it is only the differencebetween the total signal and the background which isused for evaluation. In the absence of a lower masssignal, the minimum detectable concentration isdetermined only by the minimum detectable leakage

rate. In this standard the minimum detectableconcentration is defined as being equivalent to a helium peak showing no valley, but merely a shoulder on the tail of the lower mass peak. This is equivalentto the evaluation of 8.2.2. Figure 3c shows thissituation without any background signal for helium.

7.3.1 Background drift and noise

The determination of background drift and noise iscarried out according to the following procedure.

a) Adjust the recorder so that the zero on therecorder corresponds to the zero of the leakdetector output and the full scale of the recorder corresponds to the full scale of the selected decade

of the MSLD output.b) Connect the large known leak to the test port of the MSLD.

c) Admit atmospheric air (if helium is the tracer gas), or a mixture containing p.p.m levels of tracer gas, under reference conditions, to the leak. (Whenatmospheric air is used, the feed line should notitself act as a source of tracer gas and preferablyshould be of all-metal construction.)

d) Start a pump cycle or open and close therespective valves manually.

e) Adjust, after the MSLD is ready for measurement,

the gas flow through the leak so as to achieve theoptimum upper working pressure in the instrument.

f) Adjust the sensitivity control, if necessary, to thehighest setting that will give an on-scale reading on

the recorder.g) Record the output for 20 min. This record is usedfor the determination of drift.

h) Set, for determination of noise, the sensitivitycontrol to the maximum setting and bring theindication to mid-scale by means of the zero control.

i) Record the output again for 20 min. (If theenhanced sensitivity setting is not possible, the driftcurve is taken for determination of noise.)

7.3.2 Background signal

The determination of the background signal is carriedout according to the following procedure.

a) With the setup as before, feed a pure gas (notcontaining tracer gas), preferably evaporated liquidnitrogen, to the leak detector. If this is not available,close the leak isolation valve.

b) Adjust the leak detector to the greatest sensitivitythat will give on-scale readings.

It is preferable to use an xy-recorder for thefollowing measurement. In this case, the x-axis isconnected to the scanning voltage or another equivalent operating parameter and the y-axis isconnected to the recorder output of the MSLD. Thex-axis sensitivity of the recorder has to be set sothat the specified scanning voltage for the tracer gas

peak is approximately mid-range.

c) When the output signal has reached a steady value, showing no observable change in 1 min, scanthe tracer gas peak as specified for the instrument, if a yt-recorder is used, make sure that the scan speedis approximately constant.

d) Repeat the scanning at 15 min intervals until thereis no observable change over a 30 min period. Thefinal measurement is used for determination of thebackground signal.

7.3.3 Sensitivity

The sensitivity of the system is measured according to

the following procedure:a) Close the inlet valve of the MSLD.

b) Open the leak isolation valve.

c) Admit atmospheric air (if helium is the tracer gas), or a mixture containing p.p.m levels of tracer gas, under reference conditions, to the leak.

d) Start a pump cycle or open and close therespective valves manually.

e) Scan the tracer gas peak as specified for theinstrument when the leak detector is ready for useand the output signal has reached a steady value,showing no observable change in 1 min. (If a

yt-recorder is used, make sure that the scan speed isapproximately constant.) The curve will be used todetermine the sensitivity for the detection of smallconcentrations.

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Figure 3a Ð Identification of symbols and variables

Figure 3b Ð Total and background signals for He on the tail of lower mass peak (produced, forexample, in a reaction such as H2 + H+→ H3

+)

Figure 3c Ð Signal for the minimum detectable He concentration, appearing as a shoulder onthe tail of a lower mass peak

Figure 3 Ð Interpretation of curves showing the leak detector output versus scanning voltage

(for Helium as tracer gas)

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7.4 Inlet pumping speed for tracer gas

For the determination of the pumping speed for tracer

gas, the test vessel with known volume as describedin 5.3 is necessary.

The volume of the test vessel shall be selected so thattime constants can be evaluated from the traces on thechart recorder. For most leak detectors, this means a

volume between 10 l and 50 l. The following procedureis used.

a) Connect the test vessel directly to the test port of the leak detector (if direct connection is not

possible, a pipe with known conductance shall beused).

b) Fit the small leak to the test vessel. [There shallbe a leak isolation valve (preferably of all-metalconstruction) between the leak and the vessel.]

c) Evacuate the vessel with the leak open and waituntil the leak detector is ready for measurement andgives an indication resulting from the leak.

d) Set the chart recorder to a speed of 50 cm/min or more.

e) Start the recorder with the high chart speed,when the output signal has reached a steady valueshowing no observable change in 1 min.

f) Close, at a definite instant, the leak isolation valveon the vessel as rapidly as possible.

g) Record the decrease in the resulting output until

the signal is again stable at its low value(background indication).

NOTE If the initial leak indication is less than 10 times thebackground indication, the test has to be repeated with a larger leak.

h) Mark the time scale on the record.

8 Results

8.1 Minimum detectable leakage rate

8.1.1 Evaluation

8.1.1.1 Minimum detectable signal

The minimum detectable signal may be determinedaccording to the following procedure.

a) Take the record of the drift and noisemeasurement for smallest detectable leakage rate[7.1.1d)] and draw a series of line segmentsintersecting the recorder curve at right angles to thetime axis in 1 min intervals.

b) Draw straight line approximations for eachsegment of the curve between adjacent 1 min lines.

c) Determine that segment of the curve having thegreatest slope. (The signal change in this segment iscalled drift. If this is less than 2 % of the full scale of the recorder, determine the total change in outputover the 20 min period and divide this by 20.)

d) Determine the maximum deviation from thestraight line approximation for each 1 min segment

of the curve, in order to determine the noise. (Anylarge deviation occurring no more than once in thewhole length of the recorded curve shall beignored).

The noise is given by the average of the20 deviations multiplied by 2.

The minimum detectable signal is given by the sumof the drift in 1 min and of the noise. If the sum isless than the display resolution, the smallestresolvable indication is the minimum detectablesignal. (The minimum detectable signal is normallygiven in scale divisions.)

8.1.1.2 Background signal correction (for auxiliary

pumping system)

This correction may be evaluated according to thefollowing procedure.

a) Subtract the background reading obtained withthe leak isolation valve closed [7.1.2g)] from thatobtained with the leak valve open [7.1.2f)] and theblanking flange in place of the leak. The difference istermed the background signal correction (for auxiliary pumping system).

b) If the difference is negative, it shall be taken tobe zero.

8.1.1.3 Sensitivity

The sensitivity is determined according to thefollowing procedure.

a) Take the reading of the small leak [7.1.3e)].

b) Subtract the reading with closed leak valve [7.1.2g)] and the background signalcorrection (8.1.1.2).

c) Calculate the sensitivity by dividing this result bythe actual standard leakage rate of the calibratedleak used:

sensitivity = net signal due to calibrated leak

standard leakage rate of calibrated leak

The unit of sensitivity is:

signal unit/leakage rate unit

The time constant for this sensitivity has to be stated.It is given by determining the time for a decrease to37 % of the equilibrium leakage rate indication either directly using a stop watch or, preferably, by evaluationfrom the recorder chart (see 7.1.4).

8.1.2 Expression of minimum detectable leakagerate

The minimum detectable leakage rate is calculated bydividing the minimum detectable signal by sensitivity.Together with the minimum detectable leakage rate,the time constant shall be stated.

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EN 1518:1998

© BSI 1998

8.2 Minimum detectable concentration

8.2.1 Evaluation

For the following evaluations, it is necessary to smooththe measured scans [7.2.2d) and 7.2.3e)] as indicatedin Figure 3 by the dashed lines.

Points A, B, C, D shall then be marked and thecorresponding distances from the x-axis measured withthe following meanings:

A, a 9 point directly below scan maximum on thesmoothed curve and corresponding distance;

B, b9 point of scan maximum and correspondingdistance;

C, c9 point directly below scan minimum on thesmoothed curve and corresponding distance;

D scan minimum.

8.2.1.1 Minimum detectable signal

The minimum detectable signal is determined as beforefor the evaluation of the minimum detectable leakagerate.

8.2.1.2 Background signal correction

In the scan of tracer gas peak, with the admission of pure gas (not tracer gas) or with the leak isolation valve closed [7.2.2c)], the distance AB is determinedand, if not zero, is taken as the background signalcorrection value, s9.

8.2.2 Expression of minimum detectable concentration

The minimum detectable concentration is calculated bythe following formula:

C d = C mc9 2 a9

(b9 2 a9) 2 s9

where

C d is the minimum detectable concentration;

C m is the concentration of tracer gas mixture(5 ppm, if atmospheric air was used and

helium is the tracer gas);s9 is the background signal correction value;

a9, b9, c9 as defined above for the measurementresult of 7.2.3.

If (c9 2 a9) is less than the minimum detectable signal,the formula is:

C d = C m I d

(b9 2 a9) 2 s9

where

I d is the minimum detectable signal.

8.3 Inlet pumping speed

8.3.1 Evaluation

The recorder trace of the cleanup measurement (7.1.4)on the vessel with small leak is used for the initialdetermination of the time constant with this volume.This is accomplished by making a log plot of theoutput signal divided by the equilibrium signal versustime t. This means plotting the function:

ln = 2 (1/ t ) 3 t I (t)

I 0

where

I (t) is the time dependent output signal;

I 0 is the initial equilibrium signal;

t

is the time constant;t is the time.

The slope of the linear portion of the resulting curve isequal to 21/ t , so the required time constant is givenby:

t = 21/slope

NOTE Some leak detectors already provide a logarithmic output.

8.3.2 Expression of inlet pumping speed

The time constant is related to the pumping speed bythe expression:

t = V / S v,x

where

V is the test vessel volume;

S v,x is the pumping speed at the vessel port for tracer gas.

The pumping speed is calculated using the formula:

S v,x = V / t

NOTE If the vessel is connected to the leak detector through a given conductance, the pumping speed at the leak detector inlet port is given by:

S LD,x = C x 3

S v,xC x 2 S v,x

where

S LD,x is the pumping speed for tracer gas at LD inlet port;

C x is the pipe conductance for tracer gas.

9 Test report

a) The test results shall be given together with thereference conditions.

b) The tracer gas used shall be stated explicitly.

c) A statement of precision shall be given with every

result.

d) The mode of operation shall be stated for everytest result.

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