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Use a risk-based plan that combines particle-counter measurements with consistent sampling, trend review, and investigation of likely sources. One reading cannot show whether particle conditions are stable, and no single monitoring interval or counter suits every data center.
What particle monitoring can—and cannot—tell you
Airborne-particle monitoring measures particle concentration in air, often as counts grouped into size ranges. A monitoring program can show whether readings change over time and provide evidence to investigate air cleanliness. ISO 14644-2:2015 describes monitoring as a way to obtain evidence of cleanroom or clean-zone performance related to particle concentration; its concepts can inform a data-center plan, but a data center is not automatically a classified cleanroom. ISO 14644-2:2015 preview
A particle counter does not identify particle chemistry or directly measure corrosive gases. Particle counts also are not interchangeable with PM2.5 or PM10 mass concentrations: counts describe how many particles fall within instrument size bins, while mass concentration describes particulate mass per volume. If corrosive gaseous contamination is a concern, assess it separately; ASHRAE describes copper and silver coupons exposed for around one month and then analyzed in a laboratory as one means of evaluating gaseous contaminant effects. ASHRAE Handbook, Chapter 20, 2023
Build a monitoring plan around facility risk
Start by stating what the monitoring is meant to detect or support: for example, changes associated with outdoor-air ingress, facility work, or a suspected contamination source. Then specify the method, sampling locations, measurement frequency or continuous approach, and response levels. ISO 14644-2 allows continuous, sequential, or periodic monitoring; periodic plans should set a measurement frequency. It does not establish one interval for all data centers.
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Choose locations and cadence based on the facility’s airflow, contamination risks, and purpose for collecting the data. Record the rationale so results can be interpreted consistently. Review the plan when installation or process conditions change, rather than assuming the original locations and schedule remain representative.
Set user-defined alert and action levels that fit the plan. Under ISO 14644-2, an alert level is an early warning calling for increased attention or corrective action; an action level requires immediate intervention, investigation, and correction. Establish these levels for the facility and measurement method—do not present them as universal data-center thresholds.
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Choose a measurement method and instrument
Optical particle counters for size-bin counts
An optical particle counter (OPC) illuminates individual particles and uses the scattered light signal to assign them to size ranges. OPCs provide real-time, size-selective particle-count data. Some modern designs can detect particles down to 0.1 micrometres, but the actual lower size limit and other specifications depend on the instrument. Check the manufacturer’s stated size and concentration ranges for the intended application. ASHRAE Handbook, Chapter 11, 2017
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Gravimetric sampling for mass concentration
Gravimetric sampling draws air through a pre-weighed filter and determines the mass collected per volume of air. It provides an integrated sample rather than generally real-time readings, and can take more effort as concentrations approach indoor-air levels. Choose this method when the question is about particle mass, not size-bin counts. The two outputs answer different questions and should not be treated as directly interchangeable. ASHRAE Handbook, Chapter 11, 2017
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Compare instruments against the measurement need
- Output: Confirm whether you need particle counts by size bin or integrated mass concentration.
- Size range: Ensure the instrument covers the particle sizes relevant to the investigation; the stated lower detection size varies by design.
- Concentration range: Select a counter suitable for expected conditions. Some counters designed for cleanroom use can overload in other applications without indicating that they are out of range.
- Calibration: OPC sizing uses a calibration curve developed with particles having particular optical properties. Real-world ambient particles can produce significant sizing errors, so interpret displayed sizes as instrument-based estimates rather than chemistry or identity.
- Deployment and records: Decide whether portable spot checks or fixed/continuous measurements best support the plan, and ensure readings can be logged and reviewed as trends.
No single model is established as best for every data center. The relevant choice is an instrument whose calibration, size range, concentration range, and recording capability match the conditions and question.
Make samples comparable
Define sampling locations and conditions clearly so repeated readings can be compared fairly. In fast-moving ducts, ASHRAE says sampling should be isokinetic: the velocity of air entering the sampling inlet should match the surrounding airstream. If inlet speed is higher or lower than duct airflow, the sample may contain too many or too few particles, respectively. ASHRAE Handbook, Chapter 11, 2017
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Keep the measurement method and sampling setup consistent when tracking a location. Document where and how each sample was taken, along with relevant conditions, so a change in readings is not mistaken for a change in contamination when the sampling approach itself changed.
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Investigate changes before assigning a cause
A rise or other change in particle readings is a signal to investigate, not proof of a specific source. Compare results with filter inspections, facility activities, air-handling conditions, and possible ingress paths. ASHRAE identifies outdoor contaminant ingress and sources brought into the facility—including boxes or dirty equipment stored in the room, equipment unpacking, and nearby printing or paper dust—as possible contributors. ASHRAE Handbook, Chapter 20, 2023
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ASHRAE notes that contamination can clog air-conditioner and IT-equipment filters, reduce cooling effectiveness, shorten equipment life, and increase energy use. Inspect air-handling filters visually as a complementary practice: detailed inspection can reveal airborne dust, but it is not a quantitative substitute for particle-count measurements. Gaseous contaminants may require separate seasonal or periodic coupon testing. ASHRAE Handbook, Chapter 20, 2023
Connect monitoring to filtration and source control
Monitoring helps identify conditions and trends; it does not remove particles. Use findings to guide investigation and control at the source, alongside appropriate filtration. Filter selection depends on site conditions, especially where air-side economizers are used.
ASHRAE’s 2009 data-center contamination guideline states that data centers must meet ISO 14644-1 Class 8 cleanliness. For facilities without air-side economizers, it describes continuous room-air filtration using MERV 8 filters, or filtering incoming air using MERV 11 or MERV 13 filters. The guideline says filter selection for facilities with air-side economizers depends on conditions at the site. These are recommendations in a 2009 guideline, not proof that a particular filter rating or installation achieves a cleanliness classification at a specific facility. ASHRAE, Particulate and Gaseous Contamination Guidelines for Data Centers, 2009
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