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PIV Leak Assessment of N95 Respirators

A TSI Fluid Mechanics Team Investigation

*Diese Seite ist leider nur in Englisch verfügbar.*

By Dan Troolin, Ph.D., Senior Applications Engineer, Fluid Mechanics
Ruben Hortensius, Ph.D., Regional Sales Manager, Fluid Mechaniics
Wing T. Lai, Ph.D., Marketing, Fluid Mechanics

Testing N95 masks for leakage with fluid mechanicsTSI fluid mechanics systems help researchers answer questions and solve problems related to air or liquid flow. In this investigation, our fluid mechanics team applied their skills and equipment to look at the problem of leaking N95 masks. 

What is an N95 Respirator?

  • Personal Protective Equipment (PPE)
  • Protects the wearer from airborne particles
  • Efficient filtration of airborne particles
  • “N95” ~ blocks 95% of very small (0.3 micron) test particles
  • No bypasses allowed; seal must be maintained
  • Respirators must be properly fitted

About Respirator Fit Testing

TSI's  PortaCount® Respirator Fit Tester was developed in 1987. This device tests the fit of a facemask to its wearer, using:

  • Particle counting technique
  • Comparative measurement
    • Particles concentration inside the mask
    • Particle concentration outside the mask

A PortaCount® Respirator Fit Test tells us whether a leak exists, but not the type.

  • Nose bridge, chin cuff, jaw line, facial hair, etc.

TSI's Fluid Mechanics team proposed an experiment to see whether flow visualization diagnostics using Particle Image Velocimetry (PIV), could reveal the location of an N95 mask leak.

What is Particle Image Velocimetry?

PIV measures the motion of illuminated seed particles within a flow to extract velocity vector maps. It also measures the average displacement of particles within interrogation window using a cross-correlation algorithm.

PIV formula

PIV Chart

  • ƒ  - first image
  • g – second image
  • x‘ – spatial variable
  • m – shift in correlation plane
  • Repeats for all interrogation windows
  • Divide by ∆ to calculate velocity
  • Move to next image pair; repeat

How PIV works

Investigation Setup

The Fluid Mechanics team set up a flow chamber, a particle generator, and a particle image velocimetry (PIV) system, consisting of a laser, high-speed camera, and software. They also acquired an "inhaling" mannequin head to wear the N95 mask during testing.

The experiment setup for testing mask leakage with PIV equipment


Results: No Mask

Clear suction flow at mannequin’s mouth is visible due to application of “inhalation” suction.

Results: Masked

Visible in the masked experiments:

  • Ambient fluid motion of air
  • Application of suction (inhalation), reveals stream of particles entering mask near the bridge of the nose



  • N95 respirator masks must be properly fitted to be effective
  • TSI PortaCount fit tester objectively assesses goodness of fit, but gives no indication as to the source/location of a poor seal.
  • TSI’s Particle Image Velocimetry (PIV) flow visualization system used a complementary tool to visualize the flow near a poorly sealed N95 respirator mask
  • PIV measurements were performed on an inhaling mannequin, both with and without a mask
  • No mask: suction flow at the mouth due to “inhalation”
  • With mask: Flow bypassing mask at leak near bridge of the nose
  • Similar experiments are being conducted by others, including Professor Eric Savory at Western University (Ontario, CAN)