ISO 14644 – What it is. | Mike Williamson Validation

ISO is the International Organization for Standardization, based in Geneva Switzerland¹. You may have heard of ISO 9000/9001 certification, which covers general quality management systems, but there are many more ISO standards – more than 10,000 total standards available at the time of this writing (September 2013), covering everything from IT systems configuration to milk production to statistical methods.²

A bummer, to me at least, is that the standards are not free, and they are not cheap. Copies of standards in electronic (PDF) or paper form can be purchased from the ISO website (store.iso.org) in Swiss Francs, or from ANSI³ or IEST⁴ in American Dollars. The 18 page ISO 14644-1 will cost you around $100. Note that the copy is for individual use only, not to be shared. Needless to say, to get yourself a complete set of ISO standards would cost you a pretty penny.

This post deals with ISO standard 14644-1 “Classification of air cleanliness.” There is currently (September 2013) two editions available: the official current standard 1999-05-01 and a draft revision, which “…is a draft circulated for comment and approval. It is therefore subject to change and may not be referred to as an international standard until published as such,” available since December 02, 2010. Note that the 1999 version is 18 pages, and the revision is 37 pages. I’ll discuss the differences at the end of this post.

Overview

ISO 14644-1 is part 1 of 10 parts of the “Cleanrooms and associated controlled environments” standard, which includes (current revision after colon)⁵:

14644-1:1999 Classification of air cleanliness
14644-2:2000 Specifications for testing and monitoring to prove continued compliance with ISO 14644-1
14644-3:2005 Test methods
14644-4:2001 Design, construction, and start-up
14644-5:2004: Operations
14644-6:2007 Vocabulary
14644-7:2004 Separative devices (clean air hoods, gloveboxes, isolators, and mini-environments)
14644-8:2013 Classification of air cleanliness by chemical concentration
14644-9:2012 Classification of surface cleanliness by particle concentration
14644-10:2013 Classification of surface cleanliness by chemical concentration

Note the standard’s titles changed somewhat in the revision, and 8-10 don’t exist in the 1999 version.

14644-1:1999 includes a foreword, an introduction, sections on Scope, Definitions, Classification, and Demonstration of compliance, and a bunch of Annexes. Let’s deal with each in turn.

Foreword

The foreword describes a bit about what ISO is all about and lists the parts of 14644, of which there were only 7 in 1999, and the titles were a bit different. The foreword also notes that Annexes B and C are integral to the standard, but annexes A, D, E, and F are for information only.

Introduction

Quick description of cleanroom purpose (to control airborne contamination) and scope of 14644-1 (assigns ISO classification levels, prescribes testing method and procedure for determining the concentration of particles). The Introduction also notes classifications are for a particular range of particle sizes only, but indicates there is a protocol for determining cleanliness levels for smaller or larger particles, if that is required by a particular process.

Scope

Mostly a repetition of the Introduction, but does state the range of threshold (lower limit) particle sizes is 0.1 µm to 5.0 µm. The standard calls particles smaller than 0.1 µm “ultrafine” particles and larger than 5.0 µm “macroparticles” and states populations of these may be quantified in terms of U descriptions and M descriptors, respectively, which we’ll see again below.

Definitions

Various terms are defined, notably:

Particle: solid or liquid object which is sized 0.1 µm to 5.0 µm
Particle size: diameter of the particle
Particle size distribution: “cumulative distribution of particle concentration as a function of particle size”
U descriptor: “measured or specified concentration of particles per cubic metre of air, including the ultrafine particles”
M descriptor: “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”

Classification

There are 9 ISO classes defined in the standard, as seen here⁶:

Class	maximum particles/m³
Class	≥0.1 µm	≥0.2 µm	≥0.3 µm	≥0.5 µm	≥1 µm	≥5 µm
ISO 1	10	2
ISO 2	100	24	10	4
ISO 3	1,000	237	102	35	8
ISO 4	10,000	2,370	1,020	352	83
ISO 5	100,000	23,700	10,200	3,520	832	29
ISO 6	1.0×10⁶	237,000	102,000	35,200	8,320	293
ISO 7				352,000	83,200	2,930
ISO 8				3,520,000	832,000	29,300
ISO 9				35,200,000	8,320,000	293,000

The maximum particles per cubic meter (m³) is calculated for each class number N as:

C_N = 10^N x (0.1/D)^2.08

Where

D is the consider particle size, in µm

0.1 is a constant with the units of µm

If you’re bored and want to recreate this in Excel, you can do it like this:

N	0.1	0.2	0.3	0.5	1	5
1	10	2	1	0	0	0
2	100	24	10	4	1	0
3	1000	237	102	35	8	0
4	10000	2365	1018	352	83	3
5	100000	23651	10176	3517	832	29
6	1000000	236514	101763	35168	8318	293
7	10000000	2365144	1017625	351676	83176	2925
8	100000000	23651441	10176251	3516757	831764	29251
9	1000000000	236514412	101762505	35167572	8317638	292511

Copy the formula in B2 to all cells: =(10^$A2)*(0.1/B$1)^2.08

Designation: to fully designate the classification of a given room, use 1) the classification number expressed as “ISO Class N”, 2) the occupancy state to which the classification applies (either at-rest or operational), and 3) the considered particles size(s) and related threshold.

E.g.: ISO Class 8; operational state; 1 µm (832,000 particles/m³), 5 µm (29,300 particles/m³)

This section ends by noting that if you’re going to measure more than one size in testing, but sure to measure a size at least 1.5 times the size of the smaller particle. E.g. if you measure 0.2 µm particles, don’t measure 0.3 µm particles, measure 0.5 µm particles.

Demonstration of compliance

This section includes a reference to Annex B, which covers the test method (details below), and the statement that the average particle concentrations and the 95% upper confidence limits shall be calculated. The average particle concentration shall not exceed the C_N value from the table and if you have at least two samples but not more than nine, you need to verify the 95% upper confidence limit is also not greater than C_N.

The test report should include:

a) the name and address of the testing organization, and the date the test was performed

b) the number and year of the standard used (e.g. ISO 14644-1:1999)

c) identification of the cleanroom tested and details on sampling locations

d) the specified design criteria, including ISO classification, occupancy states, and consider particle sizes

e) details of the test method used, including identification of the instruments employed with calibration certification

f) Test results, including measurement data for all sampling locations

Annex A Graphical illustration of the classes of Table 1 (Informative)

Includes a different way of looking at the classifications, but this was removed from the later revision.

Annex B Determination of particulate cleanliness classification using a discrete-particle-counting, light-scattering instrument

This is the annex that was reference in Section 4 as covering the standard test method. It turns out the standard test method is using a “discrete-particle-counting (DPC), light-scattering” instrument to determine the concentration of airport particles, equal to or larger than the specified sizes. The instrument should of course be calibrated prior to test.

Pretest checks may include airflow volume or velocity tests, air pressure difference tests, containment leakage tests, and filter leakage tests – i.e. standard cleanroom and HVAC IOQ tests should all be complete, naturally.

Minimum number of sample locations should be derived from the following equation (rounded up to the nearest whole number):

N_L=√A

where A is the area of the cleanroom in square meters

Sample locations should then be evenly distributed throughout the cleanroom and positioned at the height of work activity. If only one location is required, a minimum of three samples at that location should be taken.

The sample volume (in liters) per sample location is calculated as:

V_s=(20/C_n,m)x1,000

Where C_n,m is the class limit (number of particles/m³) for the largest particle size specified in the class (e.g. for ISO Class 5, 0.5 µm particles, C_5,0.5=3,520; for ISO Class 4, 0.2 µm particles, C_4,0.2=2,370) and 20 is the defined number of particles that could be counted if the particle concentration were at the class limit.

BUT the minimum sample volume should be at least 2 liters, and the minimum sampling time should be at least 1 min.

When the number of sample locations is less than ten, compute the overall mean of the averages, the standard deviation, and the 95% upper confidence limit. For one location or more than nine, the 95% upper confidence limit is not applicable.

The cleanroom is deemed to have met the specified air cleanliness classification if the averages of the particle concentrations measured at each of the locations and, when applicable, the 95% upper confidence limit calculated do not exceed the concentration limit.

Annex C Statistical treatment of particle concentration data

Includes equations for:

Average particle concentration at location x_i: = (x_i,1+x_i,2+…+x_i,n)/n where x_i,n is the nth measurement at location i and n is the total number of samples taken at location i.

Overall mean of the averages: = (x_i,1+x_i,2+…+x_i,m)/m where x_i,n is the average at location m and m is the total number of locations

Standard deviation s=√[(x_i,1-)²+(x_i,2-)²+ … +(x_i,m-)²]/(m-1)

95% upper confidence limit (UCL) = +t_0.95(s/√m) where t_0.95 comes from the following table:

m	2	3	4	5	6	7-9
t	6.3	2.9	2.4	2.1	2.0	1.9

Annex D Worked examples of classification calculations

Includes worked examples for calculating all of the above – not going to repeat it here.

Annex E Considerations for the counting and sizing of particles outside the range applicable for classification

This annex deals with cases where your process requirements have criteria for particles smaller than 0.1 µm or larger than 5 µm. It is not terribly interesting, just defining the U descriptor and M descriptor formats and referring to IEST guidance for test methods.

The U descriptor is used for < 0.1 µm (“ultrafine”) particles: U(x; y), where x is the maximum permitted concentration per m³ and y is the size in µm at which the particle counter counts such particles with 50% counting efficiency. IEST-G-CC1002 for test methods.

The M descriptor is used for > 5 µm (“macro”) particles: M(a; b); c, where a is the maximum permitted concentration per m³, b is the equivalent diameter associated with the method for measuring in µm, and c is the specified measurement method. IEST-G-CC1003 for test methods.

Annex F Sequential sampling procedure

Annex F covers sequential sampling, a procedure that evaluates sample results in real-time, to reduce the number of samples required for a passing or failing result.

Sequential sampling is based on upper and lower limit calculations as follows:

Upper limit: C = 3.96 + 1.03E

Lower limit: C = -3.96 + 103E

Where C is the observed count, and E is the expected count.

Graphically it looks like this:

By plotting sampling results in real time, you can theoretically get to a Pass or Fail more quickly.

2010 Revision

Foreword

Two additional parts to 14644 are noted:

Part 8 Classification of airborne molecular contamination
Part 9 Classification of surface cleanliness by particle concentration

ISO 14698, Cleanrooms and associated controlled environments – Biocontamination control is also noted:

Part 1 General principles and methods
Part 2 Evaluation and interpretation of biocontamination data

Introduction

Shortened from 5 to 2 paragraphs, but essentially the same.

Scope

Essentially the same, explicitly calls out the use of discrete-particle airborne counting and sizing instruments now.

Normative reference

New section – references ISO 21201-4:2007, Determination of particle size distribution – Single particle light interaction methods – Part 4: Light scattering airborne particle counter for clean spaces

Definitions

Added definition for unidirectional airflow (“controlled airflow through the entire cross-section of a clean zone with a steady velocity and approximately parallel airstream”)

Classification

No major change

Demonstration of compliance

Removes criteria for 95% UCL

Annex A Graphical illustration of the classes of Table 1 has been moved to Annex E in the new version.

Annex A Reference method for the determination of air cleanliness classification by particle concentration

Analogous to Annex B from the 1999 revision, but with some important changes:

Establishment of sampling locations is no longer N_L=√A, but based on the following table:

Which, if you compare to the previous, is quite a few more sample locations in some cases:

Area	Sqrt(Area)	ISO	Diff
1	1	1	0
2	1	1	0
4	2	2	0
6	2	3	1
8	3	4	1
10	3	5	2
24	5	6	1
28	5	7	2
32	6	8	2
36	6	9	3
52	7	10	3
56	7	11	4
64	8	12	4
68	8	13	5
72	8	14	6
76	9	15	6
104	10	16	6
108	10	17	7
116	11	18	7
148	12	19	7
156	12	20	8
192	14	21	7
232	15	22	7
276	17	23	6
352	19	24	5
436	21	25	4
500	22	26	4

The number of sample locations in Table A.1 are based on area units of 1, 2, and 4 m² to achieve 95% confidence that at least 90% of the total area does not exceed the class limit.