Development of Filtering facepiece respirators (FFR) filtering performance testing test rig

The present research focuses on developing a filter performance testing competency of filtering face-piece respirator’s (FFR) at the University of the Witwatersrand (Wits). This activity is in response to the expected shortage of medical personal protective equipment (PPE) related to the spread of COVID -19 in South Africa. A problem identified is the shortage of original equipment manufacturer (OEM) N95 FFR for medical staff. Consequently, the team at Wits has been designing and prototyping facemasks using rapid prototyping methods to help mitigate the shortage of OEM N95 facemasks.

Within the School of Mechanical, Industrial & Aeronautical (MIA) Engineering, a filter performance testing facility has been setup to quantify the efficacy of the alternative filter designs that have been developed by the Wits team, which are required to provide the same level or exceed the filtering protection that is offered by the N95 FFR. The filters are tested using a generated NaCl aerosol, where the particle concentration upstream and downstream of the filter are measured by the present filter testing facility to determine the filtering efficiency. In addition, the pressure drop across the filter is measured at varied air flow rates. This facility also enables alternative filter combinations that have been proposed in the social media domain to be tested and the filtering performance validated. Hence, such testing is also in the public’s interest as the level of filtering protection (or lack of) that are provided by these “improvised” face masks can be investigated and the findings reported to the public. In addition, the present filter performance test facility can accommodate 3D printed face mock-ups with a fitted face mask to test the aerosol dispersion and aerosol leakage around the mask during inhalation, exhalation, coughing and sneezing.

Specialized filter testing equipment used for the certification of facemasks (e.g., N95 standard) is prohibitively expensive and are only available a number of months (e.g., 8-10 months) after the order has been placed, which is not ideal as a rapid response to the Covid-19 pandemic is required. The present approach has utilized existing flow measurement equipment and adapted the system for the specific requirements of filter performance testing. While it is acknowledged that the present filter testing competency cannot substitute the requirement for filter certification by a recognized standards authority, the efforts at Wits does fulfil an important pre-certification filter testing role, which aids the research and development of more effective facemasks. The present filter testing method has also revealed certain advantages in improving the fundamental understanding of the function of facemask filters, which is not achieved with existing filter testing methods. For example, the particles that challenge and penetrate the filter can be the directly visualized and the trajectories analysed using cross correlation with the present filter testing method.