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© DOP Solutions Limited July 2008 Page 1 of 17



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DOP Solutions Technical Document:

Guide to in situ leak testing of HEPA filter configurations
that cannot be conventionally scan tested

Issue 1:
Prepared by Neil Stephenson and Tim Triggs and edited by John Neiger
4 July 2008








Contents:

Page

0 Introduction 2
1 Scope 2
2 Principal references 3
3 Testing principles 3
4 Method statements 5
5 Reporting requirements 5
Annex A Examples of test methods for filter configurations that
cannot be conventionally tested
6
Annex B In situ leak test report sheet 9
Annex C Calculations of the effect of volumetric testing compared
with a scan test
13
Annex D Theoretical calculation of the size of hole that can be
detected by means of the aerosol leak test
14
Bibliography 16












© DOP Solutions Limited July 2008

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0 Introduction

Where HEPA filters are fitted, it is important to ensure that the filters with their
housing and sealing devices do not permit the passage of particles from the
upstream side to the downstream side. If possible, this is checked by
challenging each filter with an aerosol of particles dispersed upstream of the filter
and scanning over the downstream face to ensure that there are no leaks that
exceed a specified level of penetration. PD 6609: 2007 provides information
supplementary to the provisions of BS EN ISO 14644-3:2005 for such a test.
However the scope of PD 6609:2007 is limited to the leak testing of HEPA air
filters that can be conventionally challenged with aerosol and face scanned.

There are many configurations for HEPA filters, notably in separative devices as
covered by BS EN ISO 14644-7:2004 and microbiological safety cabinets as
covered by BS EN 12469:2000, where it is difficult to apply a uniform upstream
challenge, or to carry out a full downstream scan-test.

The guidance contained in this Technical Document is for the leak testing of
HEPA filters that are installed in configurations that cannot be leak tested in
accordance with BS EN ISO 14644-3:2005 and PD 6609: 2007, either because
the filters are not readily accessible for face scanning, or because the challenge
cannot be applied in the specified manner.

This Technical Document is for use by test engineers, to assist them with their
testing, and by design engineers, to help them ensure that there is a suitable test

method for all HEPA filters incorporated into the equipment that they design.

The object of the tests described in this Technical Document is to determine if
filters have been damaged between manufacture and installation, and during
subsequent use.

In addition, where the in situ test method to be used after installation is included
in the purchase specification of a HEPA filter (which is strongly recommended),
this method should be used after installation to check that the HEPA filter has
been correctly supplied.

All test methods should be properly documented and validated, with clear
pass/fail criteria.


1 Scope

This Technical Document provides information supplementary to the provisions
of BS EN ISO 14644-3:2005 and PD 6609:2007. In particular, it gives
recommendations and explanatory guidance for in situ leak testing, using an oil
aerosol challenge and photometer, of HEPA filters that cannot be conventionally

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scanned or challenged, either because the filters are not readily accessible for
face scanning, and/or because the challenge cannot be applied in the specified
manner.


2 Principal references

BS EN ISO 14644-3:2005, Cleanrooms and associated controlled environments
– Part 3: Test methods

PD 6609:2007, Environmental cleanliness in enclosed spaces – Guide to in situ
HEPA filter leak testing


3 Testing principles

3.1 Preparation and safety

Unlike HEPA filters that have the sole purpose of supplying clean air into
cleanrooms or controlled environments, HEPA filters covered by this Technical
Document might be operating in environments that are subject to chemical,
microbiological or radioactive contamination. Due consideration must therefore

be given to safety issues and a risk assessment carried out prior to testing or
modification (for testing) in relation to each particular HEPA filter installation to
be tested.

3.2 Filter challenge methods

3.2.1 Principles of the challenge aerosol

The challenge aerosol presented to the upstream side of the filter should be
stable, homogeneous and have a concentration of between 20 µg/l and 50 µg/l in
accordance with BS EN ISO 14644-3:2005 and PD 6609:2007. In order to
ensure that this is achieved this there should be provided: -
a. a defined injection point
b. provision for homogeneous mixing
c. an upstream sampling point

3.2.2 Provision for homogeneous mixing

For ducted systems, it is generally accepted that that in order to ensure
homogeneous mixing of the challenge aerosol, the aerosol should be injected
into an upstream duct at a distance that is at least 15 duct diameters from the
upstream face of the filter.

There are other means of ensuring a homogeneous challenge: -

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a. Bends, dampers, baffle plates and sound attenuators in a duct all aid
mixing and reduce the distance required between the injection point and
the upstream face of the filter.
b. Static fan blades have been fitted inside ducts to promote mixing.
c. Where it is necessary to inject the challenge very close to the filter face,
this can be done using sparge pipes.

Some of these methods may require the use of an aerosol injection pump.

3.3 Access for downstream measurement

Access can be: -

a. Direct, where the filter face is accessible for scanning with a standard
probe
b. Through a glove port to allow scanning with a standard probe inside an
enclosure
c. Through a hatch

d. Through a single scanning port (see 3.3 Scanning probes)
e. Through a series of scanning ports
f. By means of a permanently installed grid of sampling points
g. By means of a single sampling point (for volumetric measurements)

3.4 Scanning probes

Scanning probes can be in the following form: -

a. Standard scanning probe
b. L-probe
c. T-probe
d. Permanently installed grid arrangement

3.5 Detection of individual leaks

Quantitative methods for the detection and measurement of individual leaks are
strongly preferred but these are not always possible. Non-quantitative methods
should only be used if there is absolutely no alternative and then only for non-
critical applications.

3.6 Volumetric methods

It should be noted that volumetric leak test methods, where the downstream
samples give the overall penetration rather than the local penetration, are greatly
inferior to scan methods. (See Annex C). Again, quantitative methods, with
defined pass/fail criteria, are greatly preferred to non-quantitative methods. With

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non-quantitative methods, any detectable penetration at all indicates a leak.


4 Method statements

Every test should have a full method statement consisting of: -

• Outline description of method

• Detailed description of method stating in particular: -

a. The method whereby a homogeneous upstream challenge is achieved
and maintained
b. The method whereby the concentration of the upstream challenge is
measured
c. The method whereby the downstream challenge is scanned and

measured
d. Sketch or drawing of all relevant parts of the facility showing the
location of the upstream aerosol injection point(s), the location of the
upstream aerosol sampling point(s) and the location and type of the
downstream measuring points
e. List of test equipment to be used including any special test equipment
f. Operating condition of the facility during testing
g. Reference to the standard or guideline applicable
h. Pass/fail criteria
i. Risk assessment leading to the clear specification of:
i. conditions for the granting of a ‘permit to work’
ii. decontamination measures to be taken prior to the test
iii. personal protective equipment to be used
iv. handling and safe disposal of contaminated waste
including the filter
v. any other relevant safety measures
vi. Requirement to give the name or job title of the person
responsible for ensuring the facility is safe to test
j. Requirement to give the name or job title of the person who will carry
out the test


5 Reporting requirements

A proforma test report form is shown in Annex B and follows the guidance on
information to be recorded in BS EN 14644-3:2005 and PD 6609:2007. .

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Annex A: Examples of test methods for filter configurations that cannot be
conventionally tested

A.1 Introduction

The aerosol photometer is capable of being used in many different ways. It
should therefore be possible to design a suitable alternative HEPA filter leak test
for most filter configurations. Examples are given in Table A.1 below.

Note: Whatever test method is provided, it must be fully documented. This
applies to the possible tests suggested in this Annex and to any other test that
might be devised. The test engineer has a basic obligation, which is to carry out
a repeatable test that is appropriate for the filter to be tested.

Table A1: Examples of HEPA filter configurations with alternative test
methods

Configuration Issues Possible alternative tests
1 Ducted exhaust filters Whilst it might be possible
to apply and calibrate the
upstream aerosol
challenge, there may be
no easy access to the
downstream face of the
filter (with the ductwork
connected) in order to
carry out a downstream
scan-test.
A sufficient number of
downstream access points
should be provided to allow the
whole area of the filter to be
scanned. The probe tube may
be an L-probe to facilitate
access to every part of the filter
face.
Alternatively, a T-probe may be
permanently installed
downstream of the filter face.
For testing, this is connected to
the aerosol photometer through
a sealed access port and drawn
slowly across the face of the
filter at a rate defined by the
formula in PD 6609:2007 (3.3).
The location of the leak will only
be defined by one coordinate.

2 Dual in line filters,
where there is
insufficient access to
scan the downstream
side of the first filter or
to apply an even
aerosol challenge on
the upstream side of
the second filter
The choice of test
depends on whether
verification of the
combined overall leakage
is sufficient or whether it
is necessary to check for
leaks in individual filters.
Where verification of the
combined overall leakage is
sufficient, consideration should
be given to setting tighter
pass/fail criteria for the
combined filter than would be
appropriate for either of the
individual filters.
Where it is necessary to check

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for leaks in individual filters, it
might be necessary to remove
the secondary filter in order to
test the primary filter and then to
reinstall the secondary filter and
use a sparge pipe to ensure
even distribution of the upstream
aerosol challenge for the
secondary filter.
3 Filters where there is
no provision for
measuring the
upstream challenge
This may be a problem
that faces test engineers
on older installations and
equipment that are not or
cannot be equipped with

suitable test ports.
It is possible to apply a constant
upstream challenge and then to
‘calibrate’ the photometer
against the ‘worst case’ media
penetration on the downstream
side. The filter is then scan-
tested in the normal way. A
penetration of one or two times
the media penetration would be
considered to be a leak.
4 Filters where there is
no provision for
providing an upstream
challenge
This may apply in older or
poorly designed
installations.
An upstream challenge point
should be installed by the test
engineer.
5 Filters where it not
possible to apply the
aerosol challenge with
the system running
In these situations, it is
not possible to inject the
aerosol challenge into the
airflow. Therefore, the
upstream challenge

cannot be measured.
One solution would to use the air
pressure generated by the
aerosol generator system,
assisted if necessary by an oil
aerosol injection pump, to ‘flood’
the upstream space with the
aerosol challenge and then scan
in the normal way. This test is
not quantitative and therefore
any leak that is identified is
taken as a fail. Care must be
taken not to over-challenge and
wet the filter with the challenge
aerosol.
6 Filters, such as
cartridge filters where it
is not possible to carry
out a downstream scan
in accordance with BS
EN ISO 14644-3 - B
The construction of
cartridge filters is such
that a scan-test is simply
not possible.
Depending on the feasibility of a
suitable challenge concentration
(which might be difficult with
smaller sizes of cartridge filter),
an overall (volumetric) test can

be carried out in accordance with
BS EN ISO 14644-3 – B.6.4.
Note: This test cannot detect
individual local leaks.











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7 Filters, such as v-form
filters where the actual
downstream faces of
the filter elements are
inaccessible for
conventional scanning.
The challenge
concentration penetrating
a leak in a filter element is

diluted by the time it
reaches the plane in
which it can be scanned.
The face of the filter casing is
scanned. This will detect leaks,
but at a sensitivity that is less
than if the leak were detected at
the face of the element itself.
8 Filters where there is
easy access for the
aerosol challenge on
the upstream side but
no access for scan-
testing on the
downstream side
Certain designs of
separative device have a
return air or exhaust filter
where the upstream side
is readily accessible from
the work space but where
the downstream side
leads to inaccessible
plenums and airways.
One possible solution would be
to introduce the aerosol
challenge using a scanning
pattern that covers the whole
face of the upstream side of the
filter. The photometer probe is

then set at maximum sensitivity
and placed at a suitable point as
far from the filter as possible
(without the addition of dilution
air) in the downstream airway.
Note: This test gives a non-
quantitative indication of leaks
9 Filters in safe change
filter boxes
Systems that are
designed to allow bagging
of contaminated filters for
safe disposal are often
prone to leaks at filter
seals.
The filter and seal should be
tested in the normal way with
special emphasis on the seal.
Where access is difficult,
manufacturer’s instructions
should be followed.
10 Open-faced filters High efficiency filters are
used as room extract or
return air filters. In this
case the upstream side of
the filter is usually open-
faced to the room so
special measures are
required to provide a
homogeneous upstream

aerosol challenge. More
often than not the
downstream face is in an
inaccessible duct so
downstream scanning is
also difficult.
A uniform challenge may be
achieved by attaching a
temporary box and duct to the
open upstream face of the filter.
The box includes a sampling
port for measuring the upstream
challenge concentration. The
length of the duct is at least 15 x
the duct diameter. The
challenge is introduced in the
normal way at the opening of the
duct so that by the time it
reaches the filter it is fully mixed.
It is usually more practical to
scan a cross section of the
downstream duct than the
downstream face of the filter
itself. This can be done through
a series of scanning ports in the
duct or through a permanently
installed grid of sampling ports.

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Annex B: In situ leak test report sheet

B.1 Introduction

Table B.1 sets out a proforma test report form which has been prepared using
the guidance provided in BS EN ISO 14644-3:2005 (sections 5 and B.6.7) and
PD 6609:2007 (Annex B).

Table B.1

Name and address of the
site where the facility to be
tested is located:

Name of customer
contact:

Job title:

Name and address of
testing organisation:




Name of test engineer:


Testing qualification:
Date of test::
Clear identification of
facility to be tested, i.e.
name of department or
location on site:







ISO classification in
operational occupancy
state:

Description of facility to be
tested and specific
designations/locations of all
filters to be tested, all
aerosol injection points and
all upstream and
downstream sampling
points:



State if by reference to
sketch/plan:

State if sketch/plan is
attached:
Details of the test method
that has been agreed
between the customer and
the supplier:







Method Statement
reference number:

State if Method Statement
is attached:
Specific
Standards/Guidelines on
which the Method
Statement is based (in
order of relevance):




Special conditions: Departures from the
Method Statement:











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Details of DOP
Photometer:



Serial Number:
Calibration status (date of
calibration certificate):
Calibration certificate
attached (tick):
Details of probe:


Serial Number:

Details of aerosol
generator:


Serial Number:
Service and test status
(date of certificate):
Certificate attached (tick):
Details of sparge pipe or
any other equipment
provided to facilitate
aerosol mixing:

Details of aerosol injection
pump:

Serial Number:
Service and test status
(date of certificate):
Certificate attached (tick):
Details of any other test
equipment:



Service and test status if
applicable (date of
certificate):
Certificate attached if
applicable (tick):

Filter 1:

Filter 2:

Filter 3:

Rated air volume flow
through filter m
3
/s:

Rated air volume flow

through filter m
3
/s:

Rated air volume flow
through filter m
3
/s:

Actual air volume flow
through filter m
3
/s:

Actual air volume flow
through filter m
3
/s:

Actual air volume flow
through filter m
3
/s:

Upstream challenge
concentration at start of
test mg/m
3
:

Upstream challenge
concentration at start of
test mg/m
3
:

Upstream challenge
concentration at start of
test mg/m
3
:

Acceptance filter
penetration %:

Acceptance filter
penetration %:
Acceptance filter
penetration %:

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Maximum measured filter
penetration %:

Maximum measured filter
penetration %:
Maximum measured filter
penetration %:
Acceptance frame and seal
penetration %:

Acceptance frame and seal
penetration %:
Acceptance frame and seal
penetration %:
Maximum measured frame
and seal penetration %:

Maximum measured frame
and seal penetration %:
Maximum measured frame
and seal penetration %:
Upstream challenge
concentration at end of test

mg/m
3
:

Upstream challenge
concentration at end of test

mg/m
3
:
Upstream challenge
concentration at end of test

mg/m
3
:
Filter pass/fail:

Filter pass/fail:

Filter pass/fail:

Frame and seal pass/fail:

Frame and seal pass/fail:

Frame and seal pass/fail:

Corrective action (if fail):

Corrective action (if fail):



Corrective action (if fail):




Filter 4:

Filter 5:

Filter 6:

Rated air volume flow
through filter m
3
/s:

Rated air volume flow
through filter m
3
/s:

Rated air volume flow
through filter m
3

/s:

Actual air volume flow
through filter m
3
/s:

Actual air volume flow
through filter m
3
/s:

Actual air volume flow
through filter m
3
/s:

Upstream challenge
concentration at start of
test mg/m
3
:

Upstream challenge
concentration at start of
test mg/m
3
:

Upstream challenge

concentration at start of
test mg/m
3
:

Acceptance filter
penetration %:

Acceptance filter
penetration %:
Acceptance filter
penetration %:
Maximum measured filter
penetration %:

Maximum measured filter
penetration %:
Maximum measured filter
penetration %:
Acceptance frame and seal
penetration %:

Acceptance frame and seal
penetration %:
Acceptance frame and seal
penetration %:
Maximum measured frame
and seal penetration %:

Maximum measured frame

and seal penetration %:
Maximum measured frame
and seal penetration %:











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Upstream challenge
concentration at end of test

mg/m
3
:
Upstream challenge
concentration at end of test

mg/m

3
:
Upstream challenge
concentration at end of test

mg/m
3
:
Filter pass/fail:

Filter pass/fail:

Filter pass/fail:

Frame and seal pass/fail:

Frame and seal pass/fail:

Frame and seal pass/fail:

Corrective action (if fail):



Corrective action (if fail):



Corrective action (if fail):

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Annex C: Calculations of the effect of volumetric testing compared with a
scan test

This annex will be added at a later date.

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Annex D: Theoretical calculation of the size of hole that can be detected by
means of the aerosol leak test

It is possible to estimate, very approximately, the size of a leakage hole that the
aerosol leak test can detect by using the formula for the velocity of a fluid
through an orifice in a thin plate. This is particularly useful for the purpose of
comparing the sensitivity of the aerosol test with other test methods such as the
pressure decay test which is a standard test method for the pressure integrity of
isolators (separative devices).

Whilst the methods described in this Technical Document (and in PD 6609:
2007) apply to HEPA filters together with their housings and sealing devices,
they can also be applied to separative devices. Where such separative devices
rely on both HEPA filters and physical barriers to contain particulate
contamination or to prevent the entry of particulate contamination, it is entirely
relevant to be able to relate the extent of leakage through the filters with the
extent of leakage through seals etc. in the physical barrier.

The formula is:

U = √(2∆P/ρ)

Where:

U is the velocity of the fluid through the orifice in m s
-1

∆P is the differential pressure across the orifice in Pa
ρ is the density of the fluid in kg m
-3
(which for dry air is 1.205 kg m
-3
at
atmospheric pressure (101.3 kPa) and 20
o
C)

The principal assumption is that the leakage hole has the characteristics of an
orifice in a thin plate. In the case of a HEPA filter, this assumption is not
unreasonable as a hole that is detected by the aerosol test is likely to have one
particular place where the cross-section of the leakage hole is at its most
constricted. The formula would apply at this point.

For the purpose of the calculations that follow, it is assumed that ∆P is 100 Pa.
Therefore, using the above formula, U = 12.88 m s
-1
. The calculations can be
repeated for pressure differentials greater or smaller than this value.

In the aerosol leak test, an upstream challenge is applied to the filter to be
tested. A probe connected to an aerosol photometer is used measure the
aerosol concentration of the upstream challenge and then to scan the
downstream face of the filter for leaks. When a leak is detected, what is
measured is the upstream challenge concentration at the volume flow rate of the
leak diluted by the much larger sampling volume flow rate.












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If: V
o
is the volume flow rate through the leakage hole in m
3
s

-1
V
s
is the sampling volume flow rate in m
3
s
-1

which is typically 1 cfm (cubic foot
per minute) = 4.72 x 10
-4
m
3
s
-1

C
u
is the upstream aerosol challenge concentration
C
d
is the downstream sample concentration measured by the photometer

Then: V
o
x C
u
= V
s
x C

d

Therefore V
o
= V
s
x C
d
/C
u

When the DOP photometer shows a leak penetration of 0.01%, C
d
/C
u
= 1 x 10
-4

When the DOP photometer shows a leak penetration of 0.001%, C
d
/C
u
= 1 x 10
-5


If: A is the effective cross-sectional area of the leakage hole in m
2

d is the effective diameter of the leakage hole in µm

Then V
o
= U x A m
3
s
-1

So A
a
= V
o
/U = V
s
x (C
d
/C
u
)/U m
2
=

V
s
x (C
d
/C
u
)/U x 10
12

µm
2

And d = √(4A/π) µm

Using these formulae and values, it is possible to calculate the hole area and
hole diameter at various penetrations. These are shown in Table D.1.

Table D.1

Penetration % C
d
/C
u
A
a
= effective area of hole d = effective diameter of hole
0.01 1 x 10
-4
3664.6 µm
2
68.3 µm
0.001 1 x 10
-5
366.46 µm
2
21.6 µm

By comparison, the ‘single hole equivalent’ has been calculated for various
classes of separative device (isolator). These are shown in Table D.2.

Table D.2

Class of isolator
b
Hourly leak rate (h
-1)
Effective diameter of SHE (µm)
c

3 1.0 x 10
-2
464
2 2.5 x 10
-3
232
1 5.0 x 10
-4
103

a
It should be noted that the effective cross sectional area of the hole (as calculated by
the formula) is slightly smaller that the actual area.
b
As defined in BS EN ISO 14644-7: 2005.
c
The values calculated are for an isolator with a volume of 1m
3
.

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Bibliography

Standards publications

BS 5295-1:1989, Environmental cleanliness in enclosed spaces
Part 1: Specification for clean rooms and clean air devices
(withdrawn)

BS 5295-2:1989, Environmental cleanliness in enclosed spaces
Part 2: Method for specifying the design, construction and commissioning of
clean rooms and clean air devices
(withdrawn)

BS 5726-1:1992, Microbiological safety cabinets

Part 1: Specification for design, construction and performance prior to installation
(withdrawn)

BS EN 12469: 2000 Biotechnology. Performance criteria for microbiological
safety cabinets

BS EN ISO 14644-1:1999 Cleanrooms and associated controlled environments
Part 1: Classification of air cleanliness

BS EN ISO 14644-2:2000 Cleanrooms and associated controlled environments
Part 4: Specifications for testing and monitoring to prove continued compliance
with ISO 14644-1

BS EN ISO 14644-4:2001 Cleanrooms and associated controlled environments
Part 4: Design, construction and start-up

BS EN ISO 14644-5:2004 Cleanrooms and associated controlled environments
Part 5: Operations

BS EN ISO 14644-6:2007 Cleanrooms and associated controlled environments
Part 6: Vocabulary

BS EN ISO 14644-7:2004 Cleanrooms and associated controlled environments
Part 7: Separative devices (clean air hoods, glove boxes, isolators and
minienvironments)

BS EN ISO 14644-8:2007 Cleanrooms and associated controlled environments
Part 8: Classification of airborne molecular contamination

BS 3928:1969, Method for sodium flame test for air filters (other than for air

supply to
I.C. engines and compressors) (confirmed 2003)











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BS EN 1822-1:1998 High Efficiency air filters (HEPA and ULPA)
Part 1: Classification, performance, testing, marking

BS EN 1822-2:1998 High Efficiency air filters (HEPA and ULPA)
Part 2: Aerosol production, measuring equipment, particle counting statistics

BS EN 1822-3:1998 High Efficiency air filters (HEPA and ULPA)
Part 3: Testing flat sheet filter media

BS EN 1822-4:2000 High Efficiency air filters (HEPA and ULPA)
Part 4: Determining leakage of filter elements (Scan method)

BS EN 1822-5:2000 High Efficiency air filters (HEPA and ULPA)
Part 5: Determining the efficiency of the filter element

Recommended practices of the Contamination Control Division of the
Institute of Environmental Sciences and Technology, Illinois, USA.

IEST-RP-CC001.4. HEPA and ULPA Filters

IEST-RP-CC006.3. Testing cleanrooms

IEST-RP-CC007.1. Testing ULPA Filters

IEST-RP-CC034.2. HEPA and ULPA Filter Leak Tests