Separate the two forms of evidence
Differential pressure indicates the driving force between spaces. Fog shows how the air moves through the available opening under the tested condition. Both can change when doors, exhaust, supply systems, equipment or people move.
Where qualitative visualization is useful
Door undercuts
Observe direction and distribution across the opening with the door closed.
Open doorways
Capture transient movement when the door opens, remains open and closes.
Pass-throughs
Observe airflow during closed, opening, transfer and closing states.
Transfer grilles
Visualize direction, mixing and obstructions while recording pressure.
Penetrations
Identify visible pathway tendencies around approved, accessible openings.
Curtains and barriers
Observe how flexible barriers or partial separations influence movement.
Define state combinations before recording
| Variable | Example states | Why it matters |
|---|---|---|
| Doors | Closed, opening, held open, closing | Transient behavior can differ from steady state. |
| Supply / exhaust | Normal, reduced, alarm or standby | Changes the driving pressure and available flow. |
| Adjacent rooms | Occupied, process running, equipment off/on | Neighboring loads can alter the boundary. |
| Pass-through | Empty, loaded, one door open, interlock sequence | Geometry and operation affect the visible path. |
| People / carts | Absent, stationary, moving | Wake and blockage can dominate a small opening. |
Do not turn a doorway video into a building-envelope claim
- No quantified leakage rate.
- No proof of pressure stability outside the recorded period.
- No guarantee of contaminant containment or product protection.
- No replacement for alarm, control or pressure-sensor verification.
- No permission to fog occupied, hazardous or product-exposed areas without approval.
Questions this page should settle
Can fog determine whether a room is positive or negative?
It can show the observed direction at an opening, but the pressure differential should be measured and recorded with an approved instrument.
Why test the door opening sequence?
Opening motion, people and pressure decay can create transient movement that is not visible when the door remains closed.
Turn the airflow question into an equipment specification.
Send the environment, scale, purity constraint, target visibility, delivery geometry, runtime and operating restrictions. Applied Physics can compare current ultrasonic and LN₂ platforms without treating one model as a universal answer.