How come the Overall Fit Factor calculated by the PORTACOUNT isn't the arithmetic average of the individual exercise fit factors?

Averaging fit factors does not produce a valid result. The correct way to compute an overall fit factor is to calculate the average penetration and then convert it to a fit factor. This is what the PORTACOUNT does internally and is the method described in OSHA's Respiratory Protection Standard 29CFR1910.134.

The number you get is exactly the same as you would have obtained with a chamber system. Refer to the PORTACOUNT Operation and Service Manual for more detail. More.

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