A Guide to Particle Measurements and ISO 14644-1:2015  in a Data Centre

What is ISO 14644-1 and why is it used to classify data centre ‘cleanliness’?

Although applicable to all different types of technical spaces, the International Standards Organisation’s Clean Rooms and Associated Controlled Environments. Part 1 Classification of Air Cleanliness by Particle Concentration (ISO 14644-1:2015) has been adopted by most of the data centre community to characterise data centre environmental cleanliness.  Technical spaces are classified as ISO Class 1 through to ISO Class 9. Class 1 describes the cleanest environments such as those found in silicon chip clean rooms. ISO Classes 7 and 8 are considered the most appropriate standards for IT facilities.

Why is Class 6 (or better) not relevant for data centres?

Class 7 and Class 8 ISO standards do not specify any limits on particles smaller than 0.5 micron. However, to be eligible for a Class 1 to Class 6 rating, smaller particles (0.3 um and smaller) must be measured and limited.  In a data centre environment, air filters that block particles smaller than 0.5 micron would also dramatically impede air flow.  Therefore, although Class 6 would signify a ‘cleaner’ room, it may come at a higher cost to cooling than is practical.   

To be classified as a Class 7 environment, there can be no more than 352,000 particles of a size 0.5 micron or larger per cubic metre of air (pcm). Class 8 allows 3,520,000 such particles pcm and each class also has limits on the number of 1 and 5 micron particles allowed.  

In any environment, particles above 1 micron in size will eventually fall to ground and, unless disturbed or removed by cleaning, will remain there. A few such particles may reach filters when carried in directed air flows and will be trapped but the majority can be removed from the environment only by detailed cleaning.  

Particles smaller than 0.5 micron pass through the filters and are not efficiently removed from the room but instead remain circulating. The 0.5 micron size is affected by sedimentation in still air, and by filtration from air flows. That is why this particular size is used as a reasonably realistic marker for room preparation, cleanliness and potential cleanliness.

Image of boxes filled with balls, to represent Class 7 (few balls) and Class 8 (more balls) of ISO 14644-1

How do we measure the particles?

Particles are measured using a particle counter.  We follow the ISO standard method which specifies the frequency of sampling locations for any particular space.  To this we also take measurements at other known risk areas such as fresh air intake, AHU exhausts, around doors etc. We then interpret, fully explain and present the measurements in a report.  If we spot an anomalous reading, we will attempt to find the reason.  All our particle counters are calibrated annually. Although not necessary for data centres, we can, if requested, measure particles as small as 0.1 micron, which is a requirement for ISO Class 6 or better.

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Image describing some of the different particle sizes measured in ISO-14644-1.  Text describes the action of each particle size within a data centre.  0.3 micron size remains in air flow, not trapped by computer room air filters, 0/5 micron size remains in air flow but will settle out, is trapped by almost all data centre air filters, 1 micron size falls to ground in air flow

What is the significance of the 0.5 micron size in the data centre?

The suite of ISO 14644 Standards was revised late in 2015, the original versions dating from 1999. Many of the changes are directed toward industries (such as pharmaceutical, silicon chip production, food prep, health care) that are concerned with particles smaller than 0.5 microns. In addition, industries involved with nanoparticles will now be guided by a completely new standard ISO 14644-12.

For ISO Classes 7 and 8, the relevant classes for IT space, there were no changes to the specified limits for either the number or the size of particles. The revised standard has made a small increase in the minimum number of test points by better defining the requirement by room size. The frequency of testing required for continued compliance confirmation has been slightly relaxed.  The update also heralds a move toward standardising particle counter calibration. In this 2015 update, adhering to the new calibration process (ISO 21501-4) is encouraged but not mandated as it will generally require a firmware update to the particle counter and this may not always be possible to an otherwise perfectly acceptable counter.  Therefore any counter that can trace its calibration back to documented bead sizes remains acceptable.

In practice, the revised ISO 14644-1 changes very little for a well executed particulate survey of IT space. RITEL has always tested more than the minimum number of locations required because doing so increases our confidence in the results.  In addition, if we discover an anomaly in our results, we target further measurements to track down the cause.

Update regarding the revised ISO 14644:2015 Standard

Click here for the latest information on how the revised ISO 14644-1 affects measurements in a data centre.

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