Battery life

Battery life limits the service life [100] of the product. To test the battery life, the battery was fully charged and the PAPR was fully assembled (connected to the AV3000 facepiece (mask) with a CPAP hose). The mask was sealed with cling wrap to simulate a person wearing the mask. This left all air to exit through the exhalation valve, as would be the case during normal use. The PAPR was turned on with the PWM controller set to 100%, as this is expected to be the case with maximum current-draw. The voltage of the battery at the input terminals on the PWM controller was measured every 15 minutes until the battery died (the blower turned off). This was tested

with no pre-filter and with a surgical mask pre-filter installed, offering the lowest and highest resistive loads for the blower.

When running the PAPR at 100% duty cycle, starting with a fully charged 6000 mAh battery, the PAPR ran for over 7 hours in both cases. The expected use time, according to Greg Ball from the Oakland Township Fire Department in Michigan, is around 70 minutes. This means that airflow will continue in some capacity for any expected use of this PAPR. However, the voltage of a rechargeable battery decreases as it is used. This battery’s voltage dropped from a starting

voltage of around 12.2 V to around 9 V by the time the battery died. This voltage decrease has an effect on the airflow as shown in Figure 35.

In order to assess how long the PAPR can provide sufficient airflow according to NIOSH

standards, the battery voltage over time was plotted, then overlaid with the expected use time and the approximate voltage at which airflow drops below 115 L/min with a HEPA pre-filter. This is shown in Figure 36, which indicates that airflow will drop below the NIOSH standard after around 280 minutes of use, or 4 hours and 40 minutes. This is 400% of the expected use time (calculated by dividing _{𝑙𝑙𝑒𝑒𝑒𝑒𝑙𝑙𝑎𝑎𝑎𝑎𝑙𝑙𝑒𝑒}𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑙𝑙𝑙𝑙𝑙𝑙_{𝑎𝑎𝑢𝑢𝑙𝑙}).

Figure 36. Battery voltage as a function of run time with no pre-filter and with a surgical mask prefilter.

This testing suggests that stepping down to a 3000 mAh battery could be a reasonable design change to save on cost and weight, although this has not been tested. An important detail that was discovered during testing is that the indicator lights on this particular battery do not give a good idea of the remaining life of the battery. As can be seen in Figure 37, three of five lights on could indicate a remaining battery life anywhere from 20 minutes to over 4 hours (250 minutes). As such, it is highly recommended that the battery simply be brought to a full charge after each use, while following instructions provided with the battery on charging and maintaining the battery.

Figure 37. Battery life indicator lights. 8. Safety

Neither the safety nor the effectiveness of this product is certified by NIOSH, 3M, or Michigan Tech. This product has been shown to provide sufficient airflow to allow the PAPR to meet NIOSH requirements, and it is built with components that were selected with NIOSH

certifiability as a high priority. It has been qualitatively tested to show that breathing is possible even without the blower running and the hose was selected and constrained in the design in order to prevent kinking or blockages in the hose. This means that suffocation due to malfunction of the device is highly unlikely, provided that it is used in the expected manner. This device is not intended to be used without the blower on, as extended use has the potential to cause CO2

rebreathing, which can be harmful to one’s health [76].

The HEPA filter used in this design does not contain fiberglass, according to Sears Parts Direct [101]. Regardless of this, Smart Air Filters, a manufacturer of filters, has noted that

experimentation has shown with some confidence that any fibers shed by a HEPA filter fall far below requirements set by NIOSH and WHO and should not be harmful to respiratory health [102].

In order to produce this device commercially, further development and testing is required in order to receive NIOSH approval for use as a CBRN PAPR [103], which is the intended use of this design. There are two classes of PAPR defined by the NIOSH standard: HE and PAPR100. Each of these has different but overlapping requirements listed in 42 CFR 84 subpart K.

PAPR100 devices have been recently introduced, removing some old requirements while adding new features. Both have requirements for airflow resistance, fitment, filter efficiency, noise level, and breath response. HE devices must sustain silica dust loading tests, while PAPR100 devices must undergo particulate loading tests and communication performance tests. PAPR100 devices must also have a low-flow warning device [72]. To meet CBRN requirements from the CDC, PAPRs (both classes) must also undergo tests from other sections of 42 CFR 84, which challenge the longevity of the PAPR against physical and chemical abuse [74].

This study is intended to outline and detail the design process for an open source device, to provide a framework on which to build such a device, and to provide a low-cost, accessible option for PPE in such an event as the current COVID-19 pandemic. Distributed manufacturing of open hardware has been shown to be an effective means to meet demand for a wide array of medical supplies during this immense supply shortages observed in the current pandemic [104- 114].

Safety procedures for the use (donning and doffing) and cleaning of this equipment should be determined by local health personnel. 3M and other sources provide a wide array of resources for identifying and developing safety and disinfecting procedures as previously discussed.

9. Discussion