How the Portacount works

The most widely used Quantitative Fit Test method is the Portacount . ThePortacount provides as assessment of respirator fit by comparing the concentration of microscopic ambient dust particles outside the facepiece to the concentration of particles that have leaked into the facepiece. The ratio of these two concentrations is called the Fit Factor:

TSI Portacount Equation

Since the microscopic particles in the air are not able to pass through the HEPA filter used on the face mask, any particles which are detected inside the facepiece are assumed to have come from a faceseal leakage.

The Portacount is based on a miniature, continuous flow Condensation Nucleus Counter (CNC). This type of device takes particles that are too small to be easily detected and grows them, using condensation of alcohol vapour, to a size where they are large enough to be detected.

The Portacount uses a bespoke computer programme which runs an automated fit testing procedure. The software controls the test sample timing, test exercises and pass levels and allows the operator to input candidate / mask specific information. Data in the form of Fit Testing Records can be printed as individual reports showing all results achieved or selected on the basis of a number of parameters including test date or location.

Training Dates


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Thursday 22nd November


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