ISO/TR 14644-21:2023

TECHNICAL ISO/TR
REPORT 14644-21
First edition
2023-08
Cleanrooms and associated controlled
environments —
Part 21:
Airborne particle sampling techniques
Salles propres et environnements maîtrisés apparentés —
Partie 21: Techniques de prélèvement des particules en suspension
dans l’air
Reference number
© ISO 2023
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative documents . 1
3 Terms and definitions . 1
4 Determination of airborne particle concentration . 2
4.1 General . 2
4.2 Classification . 3
4.3 Monitoring . 4
4.4 Other LSAPC applications . 4
5 Sampling airborne particles – things to consider . 5
5.1 General . 5
5.2 Instrument selection . 5
5.2.1 General . 5
5.2.2 Considered particle size selection . 6
5.2.3 Required sample volume and sample flow rate . 7
5.3 State of occupancy . 7
5.3.1 General . 7
5.4 Sample locations – points to consider. 7
5.4.1 General . 7
5.4.2 Sampling locations for classification . 7
5.4.3 Sample locations for monitoring . 9
5.5 Instrument measurement issues. 10
5.5.1 General . 10
5.5.2 Sampling errors . 10
5.5.3 Sample measurement errors . 13
5.5.4 Sample tubing issues . 14
5.6 Decision tree . 16
5.7 Examples of use of the decision tree . 19
5.7.1 General . 19
5.7.2 Example 1 . 19
5.7.3 Example 2 . 21
5.7.4 Example 3 . 23
6 Verifying a system .25
Bibliography .26
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use
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www.iso.org/iso/foreword.html.
This document was prepared by ISO/TC 209, Cleanrooms and associated controlled environments.
A list of all parts in the ISO 14644 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
This document provides clarification on the application of sound airborne particle sampling techniques
in support of ISO 14644-1:2015 for classification of cleanrooms and clean zones, and ISO 14644-2:2015
for airborne particle monitoring, to provide evidence of cleanroom performance related to air
cleanliness by particle concentration. It provides information on how to gather appropriate, accurate and
repeatable data, and how to interpret this information for the purpose of improving process protection.
This also includes information on the choice of measurement methods and apparatus configuration,
calibration, repeatability/reproducibility and the uncertainty associated with measurement. In short,
what can be reasonably attained with the current technology.
This document addresses potential misinterpretation of the use of ISO 14644-1:2015, C.4.1.2 in
informative Annex C, which suggests the use of limited tubing length for sampling macroparticles. The
phrase in question has been applied beyond the context intended in ISO 14644-1, to other applications.
This document also provides extra clarity on the use of the M Descriptor in ISO 14644-1:2015, Annex C,
specifically in relation to consideration of >5,0 µm alongside ISO Class 5 (EU-PIC/S GMP Grade A and B
at rest).
It provides information on the uncertainty associated with sampling particles ≥5,0 µm and macro-
particles, and measures that can be taken to reduce that uncertainty.
It addresses the importance of understanding that:
— for classification, the quality of the sample is the most important factor;
— for monitoring, the quality of the data is the most important factor;
— direct sampling without tubing is preferred. However, sample tubing is sometimes necessary to get
a representative sample at a significant or critical location;
— to reduce sampling loss in tubing, this tubing is as short and straight as possible;
— a sampling system is evaluated to assess the impact of any compromises in its set up.
An evaluation of existing sampling systems can deem them suitable for continued use even if the system
is assessed as less than optimal.
The scientific basis for airborne particle counting, and the performance characteristics of airborne
particle counters, particularly LSAPC, is amply documented in established technical publications (see
Bibliography).
v
TECHNICAL REPORT ISO/TR 14644-21:2023(E)
Cleanrooms and associated controlled environments —
Part 21:
Airborne particle sampling techniques
1 Scope
This document discusses the physical limitations of probe and particle counter placement, and any
tubing that connects the two, particularly in providing representative samples where particles 5
micrometres and greater are of interest.
The document further identifies the key factors of sampling performance when classifying and
monitoring, and good practice to determine and maintain an acceptable compromise between
attainable accuracy in counting and feasibility of counting in real-life situations.
This document includes a decision tree, used to identify key considerations when sampling airborne
particles, and whether the system requires further assessment. There are also examples provided to
illustrate typical application challenges and show how the decision tree can be used.
It is assumed that this document is read in conjunction with ISO 14644-1 and ISO 14644-2. This
document is not a manual, but an explanatory document. It does not describe measurement methods,
which is handled in ISO 14644-1 and ISO 14644-2, but provides information to help make effective
choices of sampling configuration, when evaluating a new or existing system.
2 Normative documents
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org
3.1
classification
method of assessing level of cleanliness against a specification for a cleanroom or clean zone
Note 1 to entry: Levels should be expressed in terms of an ISO Class, which represents maximum allowable
concentrations of particles in a unit volume of air.
[SOURCE: ISO 14644-1:2015, 3.1.4]
3.2
monitoring
observations made by measurement in accordance with a defined method and plan to provide evidence
of the performance of an installation
Note 1 to entry: Monitoring may be continuous, sequential or periodic; and if periodic, the frequency shall be
specified.
Note 2 to entry: This information may be used to detect trends in operational state and to provide process
support.
[SOURCE: ISO 14644-2:2015, 3.2]
3.3
particle size
diameter of a sphere that produces a response, by a given particle-sizing instrument, that is equivalent
to the response produced by the particle being measured
Note 1 to entry: For discrete-particle light-scattering instruments, the equivalent optical diameter is used.
[SOURCE: ISO 14644-1:2015, 3.2.2]
3.4
macroparticle
particle with an equivalent diameter greater than 5 μm
[SOURCE: ISO 14644-1:2015, 3.2.5]
3.5
M descriptor
designation for measured or specified concentration of macroparticles per cubic metre of air, expressed
in terms of the equivalent diameter that is characteristic of the measurement method used
Note 1 to entry: The M descriptor can be regarded as an upper limit for the averages at sampling locations. M
descriptors cannot be used to define air cleanliness classes by particle concentration, but they may be quoted
independently or in conjunction with air cleanliness classes by particle concentration.
[SOURCE: ISO 14644-1:2015, 3.2.6]
4 Determination of airborne particle concentration
4.1 General
Airborne particle concentration is the primary attribute and essential parameter that determines and
denotes the cleanliness level of a cleanroom or clean zone.
In classification, ISO 14644-1 states that “The use of light scattering (discrete) airborne particle
counters (LSAPC) is the basis for determination of the concentration of airborne particles, equal to and
greater than the specified sizes, at designated sampling locations”.
ISO 14644-1 does not provide for classification of particle populations that are outside the specified
lower threshold particle-size range, 0,1 µm to 5 µm. According to ISO 14644-1, an M descriptor (see
ISO 14644-1, Annex C) may be used to quantify populations of macroparticles (particles larger than
5 µm).
In monitoring, an LSAPC is also used for airborne particle counting, often supported by other methods,
as indicated in ISO 14644-2.
For both classification and monitoring, the choice of the counter takes account of the effective particle
size range and sampling flowrate with regard to sample size.
For monitoring, the effectiveness of the sampling system is determined by the appropriate choice of
sampling location and the ability of the system to capture particles and deliver them to the counting
mechanism.
A key driver in quality management is continuous improvement. When we apply the dynamic
cycle tool Plan-Do-Check-Act (PDCA) to cleanroom contamination control, key information is used
from classification and monitoring activities to establish and demonstrate that control. Changes
and improvements will lead to re-evaluation and a repeat of this cycle. Figure 1 below illustrates
the importance of classification and monitoring and how these influence the process of continual
improvement.
Figure 1 — Strategy for contamination control
All particle counting systems have the potential to delay or prevent a particle from reaching the
counting mechanism of the LSAPC. The likelihood of this increases with the particle's size and the
length and complexity of its pathway through the system. Good practice is applied to minimise particle
loss and potential gain through shedding and false counts. The following sections explain the relevant
aspects of the determination of airborne particle concentration that is performed in classification and
monitoring. Guidance will be given in Clause 5 on the application of particle counting technologies and
associated sampling methods.
4.2 Classification
In classification, the cleanliness of the cleanroom is specified for a state of occupancy and considered
particle size(s). The sizes available for classification are defined in ISO 14644-1:2015, Table 1, and all
sizes considered are measured simultaneously, with the same LSAPC instrument. Sampling locations
and minimum sample volumes are selected using the procedure described in ISO 14644-1. Satisfactory
airborne particle concentration results obtained by sampling at these locations provide confidence that
the volume measured will comply with the specified class.
The reference method for classification supposes a degree of reproducibility to establish control for a
specified state of occupation, both initially and when classification is reconfirmed. As such, the quality
of the sample taken and precision of the values recorded is paramount.
The measurement of airborne nanoscale particles, <0,1 µm, uses another type of instrument, not
included in this document. Values obtained at the nanoscale are not appropriate for classification. This
application is considered in ISO 14644-12.
ISO 14644-1:2015, Table 1 provides the range of particle sizes and particle concentrations deemed
statistically appropriate for classification.
NOTE ISO 14644-1, Table E.1 shows application of the same threshold particle sizes for decimal classes, and
ISO 14644-1, Formula E.1 enables calculation of the maximum particle concentration for intermediate particle
sizes within the normative range.
Note f) to ISO 14644-1:2015, Table 1, indicates a special case for recording larger particles when
classifying at ISO 5. It provides an example where particles ≥5 µm are sampled with a specified limit
of 29 particles/m as a complement to classification at ISO 5 measured on sizes that are included in
ISO 14644-1:2015, Table 1 as appropriate for that class. This complement is expressed via the M
Descriptor, in this case as “ISO M (29; ≥5 µm); LSAPC” for ISO 5 and refers on to ISO 14644-1:2015,
C.7. For some applications, it can be necessary to also sample other sizes outside those for which
ISO 14644-1:2015, Table 1 defines concentrations. ISO 14644-1:2015, C.7 in informative Annex C
provides an example of adaptation of the M descriptor to accommodate consideration of ≥ 5 µm particle
size for ISO Class 5 at this particle size threshold, where ISO 14644-1:2015, Table 1 does not specify a
concentration limit.
ISO 14644-1:2015, Annex C is concerned with the broader techniques of measurement of macroparticles
(particle size thresholds not in ISO 14644-1:2015, Table 1), by a variety of appropriate methods and
instrumentation, including the LSAPC, and expression of the result. This subject is not considered
within the scope of this document. When measuring and recording particles > 5,0 µm (macroparticles)
alongside those measured within the ISO 14644-1:2015, Table 1 range, their value is expressed as an M
descriptor, in support of classification.
4.3 Monitoring
Monitoring is explained and illustrated in ISO 14644-2:2015. The activity involves recording and
analysing data on a variety of parameters to support demonstration of control.
The trending of data obtained from monitoring with LSAPC is used to understand the variation in the
number of particles over time and its relationship with the specified and maintained cleanliness of
the cleanroom or clean zone. Data obtained and analysed is used to evaluate whether air quality is
maintained at a satisfactory level at locations considered significant or critical for the process, at the
appropriate occasions. This data can be analysed along with other monitoring parameters to provide a
holistic approach to demonstrating control.
Monitoring is applied to demonstrate that the air cleanliness at a defined location complies with the
required level at a defined point in time, during operation, and often during periods at rest. Particle
sizes used for monitoring can be different from those required for classification. This is clearly shown
in EU & PIC/S GMP Annex 1 (2022) where monitoring is subject to limits for particle sizes (>5,0 µm)
not identified as reliable for the purpose of classification at ISO5. Results are often expressed per unit
of time, rather than accumulated to a unit volume. Other indirect indicators may also inform on the
satisfactory operation of the installation or equipment.
In ISO 14644-2, guidance is provided on where, when and how to monitor the airborne particle
concentration, and how to complement this with other indicators of cleanroom or process zone
performance.
In monitoring, the reproducibility of sampling efficiency, and the ability to detect variations over time,
are more important than absolute precision in the data obtained at a particular point in time.
Analysis of variations over time can help to understand the process, and provide trending data to assist
the management of critical parameters in order to maintain control as required.
Monitoring performed using a mobile or fixed LSAPC can be periodic or continuous. In monitoring,
minor particle loss in the sampling system is typically accepted since this will not affect the trending of
the local air cleanliness unless the alert or action level is very low.
NOTE It is important that the system is verified as able to record the considered particle sizes in a
satisfactory manner.
4.4 Other LSAPC applications
LSAPCs are also used for measuring high airborne particle concentration at defined sample
locations during the Recovery Test (ISO 14644-3:2019, B.4), the installed filter system leakage test
(ISO 14644-3:2019 B.7) the containment leak test (ISO 14644-3:2019, B.8) and the Segregation Test
(ISO 14644-3:2019, B.11). This document does not examine the detail of these and other applications.
In addition, the LSAPC can be used for investigation of particle concentrations at specific locations for
the purposes of characterisation, without any attempt to predict compliance of a whole surface or room
to a specified class. This activity is not expressly considered in ISO 14644-1 and ISO 14644-2, except in
ISO 14644-1:2015, Annex C but can be a valuable part of establishing and demonstrating control.
5 Sampling airborne particles – things to consider
5.1 General
The sampling of airborne particle concentration at a specific location, to determine air cleanliness,
cannot always be performed by placing the LSAPC directly at the location, due to limitations on access,
instrument size and the need to avoid disturbance of a critical volume by the counter exhaust and heat
gain. Consequently, a sample will often need to be drawn from the test location to the instrument for
measurement.
The approach taken ensures that:
— the sample is representative;
— a suitable volume is taken, relative to the particle size;
— collecting the sample does not affect the operation of the process;
— the particles sampled do reach the device;
— the location of the LSAPC is not influenced by other factors.
Consideration of influencing factors is discussed further in this clause.
5.2 Instrument selection
5.2.1 General
There are several types of LSAPC that can be used to determine classification, and in routine monitoring.
The choice of instrument type, for classification or monitoring, is typically a balance between:
— Sample volume required for statistical significance:
— a minimum number of particles is required to determine a classification state;
— low concentrations of particles require large sample volumes, and/or sequential sampling.
— Total test time:
— particle counter sample at a fixed volumetric flow rate;
— minimum sample volumes determine the time required to sample at each location;
— higher flowrates will reduce test duration;
— sample tubing requirements.
Manufacturers will provide recommended dimensions for tubing.
Table 1 describes different types of instrument and their typical applications.
Table 1 — Comparison of particle counter types
Function Portable particle Handheld particle Remote particle sensor
counter counter
Number of size channels Typically 4-6 Typically 2-6 Typically 2-6
Smallest channel Application dependant, Application dependant, Application dependant,
(sensitivity) typically 0,1 µm, 0,3 µm or typically 0,2 µm, 0,3 µm or typically 0,1 µm, 0,2 µm,
0,5 µm 0,5 µm 0,3 µm or 0,5 µm
Sample flow rate 28,3 l/min (1 cfm) or Low flowrate of 2,83 l/min Flowrates of 28,3 l/min or
greater (50 l/min, 75, l/ (0,1 cfm) 2,83 l/min
min, 100 l/min)
Power Mains or battery Mains or Battery Mains, or power over
Ethernet
Mobility Carried by hand, placed Lightweight carried in Fixed location mounted to
on mobile cart, probe on hand or placed on tripod/ equipment or infrastruc-
tripod/stand or fixed to stand ture
the top of the LSAPC
Display Local interactive display Local display with fewer None – connected to
with many operator fea- operator features central system for data
tures display
Printer Onboard External accessory None – connected to cen-
tral system
Typical applications Cleanroom classification, Cleanroom classification, Continuous monitoring of
a
portable and in-situ mon- and portable monitoring, individual locations
itoring. HEPA filter leak recovery tests
testing, recovery tests,
a
Fixed monitoring devices in cleanrooms and clean zones are sometimes used to support classification, on condition
that the sample volume considered is consistent with the requirements of ISO 14644-1, and the timing and duration of
the sample are designated for classification activity. Appropriate sensitivity for capture of the specified particle sizes,
positioning of the sampling head and absence of disturbance of the critical location are determined for each case, to assess
the suitability of this use of the remote sensor.
5.2.2 Considered particle size selection
The range of particle sizes considered in the ISO 14644-1, Table 1 classification is ≥0,1 µm, ≥0,2 µm,
≥0,3 µm, ≥0,5 µm, ≥1,0 µm and ≥5,0 µm. Selected particle sizes will include all particles equal to or
greater than the selected size.
For classification and for monitoring, the chosen size(s) can be driven by the significance, for product
or process cleanliness, of specific particle sizes and concentrations, but are also often determined by
applicable regulation or industry guidance. It is also possible to select specific sizes not featured in
ISO 14644-1:2015, Table 1. Concentrations for intermediate sizes within the table can be calculated.
The measurement of airborne nanoscale particles, <0,1 µm, uses another type of instrument, not
included in this document. This application is considered in ISO 14644-12.
In some scenarios, alternative levels of air cleanliness are selected, at specific particle sizes larger
than 5 µm which are not within the size range applicable to classification. These are defined as
macroparticles and ISO 14644-1:2015, Annex C provides guidance for sampling only these larger
particles. Their measured concentration can be described by use of the M Descriptor, with mention of
the measurement method used.
This specific mention does not mean that the whole of ISO 14644-1:2015, Annex C applies to
classification. The rest of the guidance expressed in this informative Annex is solely concerned with the
measurement of macroparticles per se.
In particular, the guidance given in ISO 14644-1:2015, Clauses C.3, C.4, C.5 is not applicable to
classification of particles in the size ranges detailed in ISO 14644-1:2015, Table 1.
For monitoring applications, additional particle sizes to those used for classification can be beneficial
for trending if considered relevant.
5.2.3 Required sample volume and sample flow rate
The need to perform either classification or monitoring will typically determine the type of instrument
and the flow rate chosen, as high sample flowrate instruments allow for a specific sample size to be
taken in a shorter amount of time. Shorter sample duration time is beneficial for routine classification
and facility monitoring where individual samples are taken throughout an area and the instrument is
moved between samples. Sample time is balanced against the statistical accuracy of the measurement
and the potential for monitoring a sample location’s change of state over time.
5.3 State of occupancy
5.3.1 General
In ISO 14644-1, three occupancy states are defined:
— as-built;
— at-rest;
— operational.
For each occupancy state an ISO class and particle size or sizes will be designated for a specific
cleanroom or clean zone.
At-rest and operational states are typically used for routine classification and monitoring, classification
as-built can be useful for new projects, particularly if the cleanroom is to be commissioned before
equipment is installed.
Particle size distribution will vary between the at-rest and operational states. Larger particles will
settle out as part of the room recovers to the at-rest state, whereas in operation there is variation
through the particle size distribution due to a more diverse set of particle sources. Depending on the
application considered, this can influence alarm and action limits for monitoring.
5.4 Sample locations – points to consider
5.4.1 General
The selection of an appropriate sampling location is important, to ensure a representative sample
is obtained when classifying, and to reflect activity at a critical or representative location when
monitoring.
5.4.2 Sampling locations for classification
For classification, ISO 14644-1:2015, Annex A, A.4.2 describes the selection of locations. It can be
necessary to position the sample probe at a number of different locations, potentially at different
heights.
Some particle loss always exists in airborne sampling. Therefore, a prudent approach to minimise
particle loss can be to use no tubing at all, so that the sample is drawn directly from the sample probe
into the LSAPC. However, in some cases it is not possible to place the counter directly at the location. In
these cases, sample tubing will link the probe to the counter positioned at some distance.
ISO 14644-1:2015, A.5.1 states “Set up the particle counter (see A.2) in accordance with the
manufacturer’s instructions.” The following clause A.5.2 concerns orientation of the sampling probe.
There is no explicit restriction on tube length. Such restriction can be included in the manufacturer’s
recommendations. The LSAPC can be placed at convenient location(s) with the sample probe mounted
on a tripod, so that height and location are easily adjustable. This is true for cleanrooms, unidirectional
flow workstations, isolators, restricted access barrier systems (RABS), safety cabinets etc.
5.4.2.1 Practical challenges associated with counting without use of sample tubing
ISO 14644-1 states that sample heights can differ between areas under test and even between sample
locations within a single area under test. Ensuring the LSAPC sample probe is at the correct height
when fixed directly to the counter, without tubing, may require some attention. An LSAPC with 28,3 l/
min or greater sample flow rate is not a lightweight instrument.
a) In cleanrooms, the LSAPC is placed either freestanding, or on a cart/trolley bench. Effective use of
a trolley bench requires that this be adjustable to meet various specified sample heights. This is not
a practical solution. For some locations, it is not always possible to place the LSAPC on a flat surface
to ensure correct probe orientation. This situation also excludes use of a trolley.
NOTE Only a handheld particle counter can be used to respond to this latter challenge, but sampling
efficiency for large particles is limited by the low flowrate (2,83 l/m) of such a counter. For unidirectional
horizontal flow workstations, it is impossible to sample correctly without recourse to a sample tube. The
probe cannot be positioned in the correct direction, nor at the required height.
b) For unidirectional vertical flow workstations, class II safety cabinets, downflow isolators etc. the
LSAPC can sit inside the critical environment. However, depending on the shape, size and flowrate
of the LSAPC such placement can disturb the airflow characteristics of the clean zone. Also, this
limits the sample probe position to one height.
5.4.2.2 Practical challenges associated with use of a short - ≤1 m - length of tubing
a) Some particle loss will inevitably occur with use of tubing. This can be minimised by maintaining a
straight run of the tubing but some loss is to be expected.
b) For cleanrooms, a tripod can be used. This adds some flexibility to location positions and heights
(but limited to a maximum of 1 m above the LSAPC). This short length of tubing restricts the
practical movement and placement of the LSAPC, which can sit virtually under or next to the
probe (which is mounted on a tripod). Process equipment and chosen sample heights can limit that
possibility.
c) Sampling at significant height above practical counter placement presents specific challenges, due
to the distance between the counter and the sampling location.
d) If the tripod in a cleanroom is placed too far from the counter, the tubing can restrict distance, and
pull over the tripod and sample probe.
e) In horizontal unidirectional airflow workstations, where the critical working area is on or above a
flat surface or bench, a small tripod with probe attached by tubing to the LSAPC can be difficult to
direct horizontally. This is because the counter must be out of the airflow, and placed on the floor
or trolley bench. The tube can be too short for this distance and smaller tripods are more prone to
toppling over, especially with the higher flowrate LSAPC and consequent wider tubing radius (and
tube rigidity). The alternative is to set up a large tripod at the exit plane and turn the sample head
toward the airflow. However, this is sampling at the exit and not necessarily in the working area.
It can be a workable solution if the LSAPC can be placed below the exit plane opening and near the
tripod.
f) For Isolators the issues can be more complex. The tubing comes from the probe, which is likely to be
on a small tripod, out of the isolator and onto the LSAPC, all within 1 m. Some manufacturers install
connection ports in the front fascia/screen or on the top of the critical working area (chamber)
to connect a tube from the tripod to the port and then another tube from the port to the LSAPC.
This creates difficulty, in some scenarios, in keeping the total length under 1 m, especially with
multiple sample locations. There are also potential issues again with tripod stability. A practical
compromise can be to use a cut and taped glove finger to feed the tubing into the isolator, and close
the required sampling point. This can work, unless the glove is in use (operational activity).
g) Entry/Exit transfer boxes also present challenges. Often there are no gloves or ports. Consequently,
the LSAPC is placed inside the small inside space. Accordingly, there is no tubing length issue, but
there are very restricted options on location, and a risk of compromising airflow. If the volume
sampled is a port, then again the issues in e) above are replicated.
h) Open-fronted downflow cabinets and class II safety cabinets have similar issues to those above:
small tripod, inflexible tubing, and length from tripod to counter. These tend to be less of a
restriction/issue in the field.
i) Sampling hot air volumes, as for instance in depyrogenation tunnels, presents particular challenges.
An LSAPC cannot take in high-temperature air. Cooling the sample often involves longer tubing.
This requires special attention.
5.4.2.3 “Operational” classification with tubing presents additional practical challenges
a) The routing of tubing, especially within confined environments such as isolators, can make short
tubing a real problem. The tubing run can obstruct the process.
b) The locating of the probe and/or tripod can interfere with the process, process equipment or
operations.
5.4.3 Sample locations for monitoring
Sample locations within the cleanroom or clean zone are determined by a risk-based approach and
often defined as ‘critical locations’ as part of a contamination control programme.
The critical location is a process point or small area, or perhaps represents a more general background
environment.
For a continuous monitoring system, the sample inlet is more likely to be fixed, or perhaps fixed and
connected with a probe and tripod, where movement within a defined localised area is required.
For a routine (periodic) monitoring programme, the sample probe can be connected to a portable
counter and enable a number of different locations to be sampled, along with potentially different
orientations, as for classification (i.e. pointed towards primary direction of airflow).
Some particle loss always exists in airborne sampling. Where particle loss in the sample is of concern,
a prudent approach to minimise such loss can be to use no tubing at all, so that the sample is drawn
directly from the sample inlet into the LSAPC.
This is possible in some scenarios, where the counter can be directly positioned at the critical location.
However, for most applications, continuous or routine, some form of tubing will be required to connect
the sample inlet and LSAPC.
For routine monitoring where a movable particle counter is used, the challenges and considerations
detailed in 5.4.2 are also applicable.
For monitoring using a fixed particle counter, the following considerations are important.
a) The choice between a counter fitted with an integral pump or linked to a central vacuum pump
is fundamental. Noise, size, and heat gains are likely to be factors that influence the location for a
fixed LSAPC. Integral pump units are larger than those connected to a central vacuum system, but
avoid the need for complex vacuum pipework and plantroom space for the pump.
b) In cleanrooms, the LSAPC is often mounted on a wall, a pillar, or a fixed structure. This can limit
the location options, and not enable satisfactory positioning to obtain a representative sample.
Depending on the room grille distribution, airflow patterns and working height, a location near
the wall could be misrepresentative of the cleanroom and critical location. If tubing is required, the
sample tubing length will often be quite short. However, if sample tubing is required to gain access
to a critical location at working height, this can hinder the process or movement of personnel.
c) For unidirectional flow workstations (both horizontal and vertical flow variants) the isokinetic
sampling probe needs to be close to the critical location without hindering the process or restricting
the process equipment. An isokinetic probe and sample tubing are required, connected from within
the critical work zone to an LSAPC location outside the work zones but as close as possible to the
critical location. The LSAPC is lower than the sample height to avoid sampling upward through
tubing.
d) In horizontal or vertical unidirectional flow workstations, isolators and RABS, it is likely the
tubing run will be longer than ideal, and is also likely to require a number of bends and possibly
connectors or valves. Large radius bends contribute to the overall length of a tube, and can force
a sampling location to be above the working height. These factors all influence the efficiency of
particle sampling. It is therefore likely that in many installations an assessment can help to
understand the potential losses in the sampling system. It can also be useful to make a choice
between the tubing materials and finishes available. Tubing runs can be examined and optimised
before implementation. Special installation fixtures and tailor-made solutions can be considered at
the early stage of separative device design for the shortest possible sampling system.
5.5 Instrument measurement issues
5.5.1 General
When reviewing the potential errors associated with particle counting, there are several associated
risks, which can be collated into three main topics for discussion:
a) sampling errors (5.5.2);
b) sample measurement errors (5.5.3);
c) sample transportation errors (5.5.4).
When a sample is taken without the need for transport (tubing) between the sample location and
measurement of the sample by the particle counter, the sampling error and measurement error are
primary areas of concern.
Sampling errors and sample measurement errors relate to the particle counter and its sample probe
and are covered below in 5.5.2 and 5.5.3. Sample transportation errors relat
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