Welcome back to What In The Tech — the LabCertTech series where we pull back the curtain on the instruments field certifiers carry into your facility. If you missed Episode 1 on the TSI VelociCalc 9565, start there. This week we are looking at the instrument that sets the stage for one of the most critical tests in your certification cycle.

Episode 2: The ATI 6D Aerosol Generator

If a field certifier wheels a compact stainless steel box into your facility before a HEPA filter leak test, you are looking at an aerosol generator. This particular model — the ATI TDA-6D — is a Laskin nozzle-based aerosol generator, and it is the instrument responsible for creating the upstream challenge that makes a HEPA filter leak test possible in the first place.

Without this instrument, there is nothing to detect. The aerosol generator is what gives the photometer — which we will cover in Episode 3 — something to measure.

What Is It Actually Doing?

The ATI 6D works by forcing compressed air through submerged Laskin nozzles inside a liquid reservoir filled with PAO (poly-alpha-olefin). The pressurized air creates turbulence in the liquid that atomizes it into a fine aerosol mist. That mist is then carried by the airstream up through the generator’s output port and injected into the supply duct or plenum upstream of the HEPA filter being tested.

Looking at the front panel of the instrument, the controls are straightforward but each one matters:

• Jet Control — selects how many Laskin nozzles are active. The ATI 6D can run 2 jets (½ nozzle) or up to 6 jets (1½ nozzles). More active jets means higher aerosol output concentration. The certifier adjusts this based on the upstream challenge concentration required for the specific filter system being tested.
• Jet Pressure gauge — the Ashcroft pressure gauge on the front face shows the operating pressure being applied to the nozzles, typically in the range of 20–25 PSI. Consistent pressure means consistent aerosol output.
• Jet Pressure control knob — the right-side knob adjusts the pressure delivered to the nozzles, allowing the certifier to fine-tune output concentration.

Why PAO and Not DOP?

Older certification records and some legacy documentation still reference DOP — dioctyl phthalate — as the challenge aerosol for HEPA leak testing. At LabCertTech we use PAO exclusively, and there is a clear reason for that.

DOP was flagged by both the FDA and the U.S. Surgeon General for potential health concerns, particularly with repeated occupational exposure. In pharmaceutical and healthcare environments where personnel safety and product integrity are paramount, the industry moved to PAO as the standard substitute. ISO 14644-3:2019 §C.5.3 lists PAO among the accepted aerosol source substances for installed filter system leakage testing. In sterile compounding environments specifically, PAO is the expected material — not DOP.

PAO is a clear, synthetic oil with low toxicity and low outgassing characteristics, which makes it appropriate for use in classified pharmaceutical spaces. The ATI 6D is designed to work with PAO, and the aerosol particle size distribution it produces — a mass median diameter typically in the 0.3 to 0.7 micron range with a geometric standard deviation up to 1.7 — is consistent with what ISO 14644-3:2019 Annex B §B.7.2.5 describes for photometer-based leak testing.

What the Upstream Challenge Concentration Requirements Are

The ATI 6D does not just generate aerosol indiscriminately — the certifier is targeting a specific upstream concentration range. ISO 14644-3:2019 Annex B §B.7.2.6 provides guidance that the upstream challenge concentration should be between 1 mg/m³ and 100 mg/m³. That is the window within which the downstream photometer can reliably detect a leak as a percentage of the upstream reference.

Before scanning begins, the upstream concentration must be verified and confirmed to be uniform across the filter face — guidance in ISO 14644-3:2019 Annex B §B.7.2.6 states that upstream concentration measurements should not vary more than ±15% from the average. If the aerosol is not mixing uniformly in the plenum before it reaches the filter, the downstream readings are unreliable. The certifier takes upstream samples at defined injection and sampling points to confirm that uniformity before the scan starts.

It is worth noting again that these concentration and mixing requirements come from an informative annex in ISO 14644-3 — they are best practice guidance, not normative mandates from that standard alone. However they reflect sound technical principles that are widely adopted in the field and underpin the reliability of the entire leak test.

Where This Instrument Is Used in Your Facility

Ceiling HEPA Filters in Buffer Rooms and Anterooms

For cleanroom suite certification under USP <797> §5, ceiling HEPA filters must be leak tested as part of every recertification cycle. The ATI 6D is connected to the supply plenum upstream of the filters — typically through an injection port or by temporarily introducing the aerosol output into the return air path upstream of the air handling unit — to challenge the entire filter bank simultaneously. The certifier then scans downstream with a photometer while the 6D maintains a steady upstream concentration.

LAFWs, BSCs, and Isolators

For Primary Engineering Controls, the aerosol generator introduces the PAO challenge aerosol into the upstream airstream of the device — ahead of the HEPA filter — and the certifier scans the downstream filter face and frame with a photometer probe. For BSC certification under NSF/ANSI 49 Annex N5, HEPA filter integrity testing using a PAO challenge and photometer scan is a required test element. The ATI 6D is sized appropriately for PEC applications — it can generate sufficient challenge concentration for both individual device testing and larger filter banks by adjusting the number of active jets and operating pressure.

The Instrument That Comes Next

The ATI 6D generates the challenge. What measures it downstream is the aerosol photometer — and that is exactly what we are covering in Episode 3. The photometer probe is what the certifier moves across the filter face in overlapping strokes to detect any localized spike in downstream concentration that indicates a leak. The two instruments work together as a system, and understanding one helps make sense of the other.

Both the aerosol generator and the photometer must be listed on your certification report per CETA CAG-003 §8.5 with make, model, serial number, and calibration date. The ATI 6D should have a current calibration certificate on file. An aerosol generator that has not been serviced or calibrated can produce inconsistent output concentration — which means your upstream reference is unreliable and your leak test results are not trustworthy, even if the numbers look fine on paper.

That is Episode 2 of What In The Tech. Next time we cover the aerosol photometer — the instrument that actually detects the leak. Have a piece of equipment you have always wondered about? Let us know.