Page:NIOSH DM DFM respirator evaluation draft.pdf/82

where the superscript (⁰) is used to indicate Case A with zero filter leakage. The second Case B is where the same respirator is worn with a DM or DFM filter against a contaminant size producing appreciable filter leakage. In this case, equation (10) yields

Linhaled = Lfilter + (1 - Lfilter) (@face wal/Qinhaled). Then, after assuming face seal is the same in both cases, combining equations (11) and (12) then yields (12) Linhaled = Lfilter + (1 - Lfilter) (L'inhaled). Rearranging the right side of this equation yields Linhaled = L'inhaled + (filler) (1 - L'inhaled). Since Linhaled VPF =O and for Case A, L'inhaled = Lface seal = 1/PF; face seal 1/PF = VPF "face seal + (Lfilter)(1 - V/PF face seal), thus (13) (14) (15) PF LOST = [(PF face seal)¹ + (Lfilter) - (Lfilter)/(PF face seal)]-¹. (16) As with Figure II for the simple-additive model, Figure III similarly illustrates for the improved model the predicted effects on PF er resulting from PF face seal values between 5 and 1,000 and Lar values between 0.10 and 0.50. There is a strong simi- larity between the two functions for PF in equations (2) and (16) respectively giv- en by the simple-additive and improved models. In spite of additional complexity in assumptions and derivation, the improved model has only one extra term, the nega- tive ratio Later/PF face seal. A comparison of user-PF values on Figures II and III for similar filter isoleakage curves indicates that the two models yield essentially iden- tical user-PF values except in the face-seal PF range 5 to 10. Even in that region the user-PF differences between the two models is about 10% at most. Note that the improved model yields slightly lower user PFs than those of the simple-additive mod- el. The small differences between the two models result from the minimal effect of the negative (Lfilter/PF face seal) term. For face-seal PF values > 10, this term rapidly approaches zero, thus it has minimal effect on computed user-PF values.