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Build a monitoring plan around the facility’s contamination risks, then use an appropriately specified optical particle counter (OPC) to track particle counts by size. Choose locations, sampling conditions, and a cadence that fit the site; define alert and action levels; and investigate changes alongside filter condition and facility activity. A particle count is evidence about airborne particles—not a measure of particle mass, particle chemistry, or corrosive gases.
Start with a monitoring plan, not a one-off reading
A single measurement is a snapshot. Monitoring becomes useful when repeated measurements can reveal changes and support decisions. ISO 14644-2:2015 describes a monitoring plan as a way to obtain evidence of air cleanliness by particle concentration. Its concepts can inform data-center monitoring, but their use does not by itself mean a data center is a classified cleanroom.
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Base the plan on the facility’s risks and the decisions the measurements should support. For example, monitoring may help establish a baseline, identify a change after work in a room, or evaluate whether a suspected ingress path warrants investigation. ISO allows monitoring to be continuous, sequential, or periodic; it does not set one universal sampling interval for every data center. For periodic sampling, document the chosen frequency and why it suits the site.
Set user-defined alert and action levels before relying on the data. An alert is an early warning for increased attention or corrective action; an action level calls for immediate intervention, investigation, and correction. Record the method, locations, sampling conditions, cadence, thresholds, and response steps. Review the plan when installation or process conditions change.
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Choose a method that answers the right question
Optical particle counters: counts by size
An OPC illuminates individual particles and interprets scattered light to estimate particle sizes and count particles in size ranges. This gives size-selective data in near real time, which is useful for observing patterns and changes. Some modern designs can detect particles down to 0.1 μm, but that is not a universal capability: check the specific instrument’s stated size range.
ASHRAE’s description of the OPC method for Standard 52.2 media-filter testing covers 12 particle-size ranges from 0.3 to 10 μm. That method-specific range is not a specification for every counter or a required range for every data-center application.
Gravimetric sampling: integrated mass concentration
Gravimetric sampling draws air through a pre-weighed filter and determines the collected mass per volume of air. It produces an integrated mass result rather than the size-bin counts generally reported by an OPC, and can require more effort when concentrations approach indoor-air levels. Decide whether the question is about particle counts by size or particle mass; the results are not interchangeable.
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ASHRAE Handbook, Chapter 11: Air Contaminants (2017)
Select and use an OPC with its limits in mind
When choosing an optical particle counter, match its specification and setup to the measurement task rather than assuming counters are interchangeable. Check:
- Size range: Confirm the particle sizes and bins the instrument can report, including whether its lower detection limit covers the intended question.
- Concentration range: Choose a counter suitable for expected conditions. Some cleanroom counters can overload in other applications without clearly indicating that they are out of range.
- Calibration and sizing: OPC size estimates depend on a calibration curve developed using particles with particular optical properties. Ambient particles may have different properties and produce significant sizing errors.
- Sampling arrangement: Decide whether portable spot checks or fixed, continuous monitoring better fit the plan, and ensure the instrument can support the intended approach.
- Data handling: Check that the instrument can log results in a form that supports consistent comparisons and trend review.
For fast-moving duct air, ASHRAE advises isokinetic sampling: the inlet velocity of the sampled air should match the surrounding airflow. If the inlet is faster or slower than the duct flow, the sample can contain too many or too few particles. Record sampling location and conditions so readings taken at different times can be compared fairly.
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ASHRAE Handbook, Chapter 11: Air Contaminants (2017)
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An OPC counts particles in size ranges; it does not identify their chemical composition or directly measure corrosive gases. PM2.5 and PM10 are mass-concentration measures, so they should not be treated as equivalent to OPC particle counts. If gaseous contamination is a concern, use a separate method.
ASHRAE describes copper and silver coupons as a way to quantify the effects of gaseous contaminants: coupons are typically exposed for around one month and then analyzed in a laboratory. The exposure period and laboratory analysis make this a distinct monitoring approach, not a function of a particle counter.
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ASHRAE Handbook, Chapter 20: Data Centers and Telecommunication Facilities (2023)
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Investigate changes before assigning a cause
A changed particle reading is a reason to investigate, not proof of a particular source. Compare it with recent facility activity, air-handling conditions, filter observations, and plausible routes for contamination. ASHRAE identifies possible sources including outdoor contaminant ingress, boxes or dirty equipment stored in the room, equipment unpacking, and nearby printing or paper dust.
ASHRAE notes that contamination can clog air-conditioner and IT-equipment filters, reduce cooling effectiveness, shorten equipment life, and increase energy use. The particle reading alone does not establish that any of these effects occurred or identify their cause.
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Detailed visual inspection of air-handling filters can help detect airborne dust, but it is a complementary check—not a quantitative substitute for particle-count monitoring. If readings rise, use the result to guide inspection and follow-up measurements while checking for changes in sources and airflow.
ASHRAE Handbook, Chapter 20: Data Centers and Telecommunication Facilities (2023)
Use filtration as a control, not as proof of cleanliness
Monitoring helps identify conditions; filtration and source control address them. ASHRAE’s 2009 data-center contamination guideline states that data centers must meet ISO 14644-1 Class 8 cleanliness and describes different filtration approaches for facilities without air-side economizers:
| Facility context in the 2009 guideline | Filtration approach described |
|---|---|
| Without air-side economizers | Continuous room-air filtration using MERV 8 filters |
| Without air-side economizers | Filtering incoming air with MERV 11 or MERV 13 filters |
| With air-side economizers | Filter selection depends on site conditions |
These are recommendations in a 2009 guideline, not proof that a particular filter rating makes a specific facility meet a cleanliness classification. In particular, the guideline says filter selection for sites with air-side economizers depends on conditions at that site. Use particle monitoring, inspection, and the facility’s contamination risks to inform control decisions rather than treating a filter rating as a cleanliness result.
ASHRAE, Particulate and Gaseous Contamination Guidelines for Data Centers (2009)
Quick Recap
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