The differential pressure (continuous inhalation) across 4 novel face masks (two designs and two filter materials) were measured on a headform 3D printed using Polylactic acid.
The headform was based on the ISO 18526-4:2020 1-M headform but was modified by creating a nominally 42 mm hole, originating at the mouth of the headform, through to the back of the headform; such that an insert could be fitted. The fitted insert was required to perform the measurements of continuous inhalation and pressure at the front of the mouth.
The headform was further modified to allow for a maxillofacial rubber to be used for the face of the headform. The modification of the headform, and manufacture of the silicone rubber face was carried out at King’s College London (see Figure 1). The skin of the face was made from Technovent M511 Addition Platinum Silicone Rubber, with a standard cure hardness of approximately Shore A25. The remainder of the face was packed with Tecnovent M510 Gel Catalyst.
This modification was carried out, in order to mimic human skin on the headform, such that the “fit” of the mask would be more comparable to that of a human wearing the mask than that of a rigid plastic.
The masks provided, consisted of 2 designs, one with a red face seal and the other with a green face seal, both with removable filter holders. Head straps were not provided with the masks, therefore straps were retrofitted on the mask from a YINGHUA RM101 FFP2 mask using the connecting loops on the filter holder.
The measurements were performed by first donning face mask on to the modified headform*. A vacuum pump connected to the back of the insert was then turned on and regulated to a continuous flow rate of 8.0 l min-1, until the flow rate equilibrated. After which the differential pressure, flow rate, and ambient conditions, were recorded. The flow rate was then increased to 20.0 l min-1, and allowed to equilibrate before recoding the differential pressure, flow rate, and ambient conditions. This process was then repeated at 30.0 l min-1, and 95.0 l min-1. The flow rate was then decreased, and the flow pump deactivated.
The mask was then doffed, and re-donned and the above measurements were repeated. This was performed for a total of 5 sets of measurements at each flow rate. The results were subsequently averaged for each mask.
The above measurements above were also performed on a commercially available CE marked FFP2 face mask, in order to facilitate comparison with the sample masks.
All measurements were made at nominal ambient conditions; a relative humidity of (48 ± 2) %, barometric pressure of (997 ± 1) hPa, and temperature of (19.7 ± 0.2) °C.
The filter material was pre-conditioned at nominally 21.0 °C and 85 %RH in an environmental chamber for at least 4 hours prior to measurement.
The results of the full mask breathability on a modified headform measurements are shown in the following table.
The reported expanded uncertainties are based upon standard uncertainties multiplied by the coverage factor k given in Table 1, providing a coverage probability of approximately 95 %.
These values include the following type A uncertainties: The reproducibility of the calibrated mass flow meter during the measurements, and the reproducibility of the differential manometer during the measurements, and the following type B uncertainties: the calibration of the mass flow meter, the resolution of the mass flow meter, the ambient conditions, the uncertainty of the differential manometer, and the resolution of the differential manometer.
The results and uncertainties given in this report are valid for the specified samples tested and refer to the measurements made during testing only.