MEASUREMENT PROCEDURE
The filtration efficiency of 10 samples were tested, consisting of 5 samples each of 2 materials, using a method based on the method detailed in BS EN 14683:2019 (Annex B).
The materials were grouped as described on delivery, as “Material 1” and “Material 2”.
The test deviated from the methods prescribed in BS EN 14683:2019 (Annex B) in several aspects. Using an ultrasonic mesh nebuliser and an 8 % NaCl solution, a challenge aerosol of particles with a calculated Mass Median Stokes Diameter of nominally 3 μm was generated and used in place of a bacterial culture. The aerosol was dispensed at a rate of nominally 0.25 ml min-1 with an air flow of 28.3 l min-1 through a 7-stage Andersen cascade impactor. The cascade impactor plates were washed in nominally 250 ml of deionised water and the conductivity of the water measured and used to calculate the mass of salt collected on each plate.
Two positive control runs were performed without test specimens in place; one before and one after the specimens were tested, for each configuration. The vacuum pump was activated and the rate of air flow through the cascade impactor adjusted to 28.3 l min-1. The challenge aerosol was then delivered for nominally 4 minutes. The cascade impactor plates were then washed, in turn, in nominally 250 ml of deionised water. The conductivity of the water and total mass of the solution were recorded. These values were then used to calculate the mass of salt collected on each plate.
Each test specimen was secured by an O-ring inside an additional Stage F, situated between Stage 0 and Stage 1 of the cascade impactor. The vacuum pump was activated and the rate of air flow through the cascade impactor adjusted to 28.3 l min-1. The challenge aerosol was then delivered for nominally 8 minutes. The cascade impactor plates were then washed in turn, in nominally 250 ml of deionised water. The electrical conductivity of the water and total mass of the solution were recorded. These values were then used to calculate the mass of salt collected on each plate.
For each test specimen the filtration efficiency (FE) was calculated according to the following equation:
Where 𝑚 is the total mass of salt measured and 𝑚𝑐 is the average of the total masses of salt measured for the two control runs.
The test specimens were pre-conditioned at nominally 21.0 °C and 85 %RH in an environmental chamber for at least 4 hours prior to measurement.
RESULTS
The results of the filtration testing are represented in the table below.
The reported uncertainties include the following type A uncertainties: The short-term reproducibility of the electrical conductivity meter and the short-term reproducibility of the mass balance in the measurements performed, and the following type B uncertainties: the calibration of the electrical conductivity meter, the resolution of the electrical conductivity meter, and the resolution of the mass balance. The reported uncertainty does not include contributions from the mass flow meter, concentration of the NaCl solution, or the total amount of nebulised NaCl solution during the mask testing.
The results and uncertainties given in this report are valid for the specified samples tested and refer to the measurements made during testing only.
Reference: M4R/10073_3