A conventional P100 respirator upgraded for use as COVID-19 PPE by the addition of an intercom and a means of exhaled-air treatment
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("Supplementary Data" to you science types.)
Ensuring a good fit:
In occupational settings, the all-important fit of the mask to the face must be professionally tested before use and every year thereafter. Here is another link re fit testing (both last accessed 02-06-2021). I got my present mask at a hardware store outside of an occupational context but did a DIY fit test with powdered stevia sweetener puffed out of a rubber bulb just after giving the bulb a good shake. The stevia seems to lose its sweetness slowly after removal from the packet, so I grind it fresh for each test. Will try saccharin next. (02-20-2021: saccharin test passed, but self-administered.) NB: I have two years of experience in wearing a respirator occupationally. If you are new to this, you should practice a lot in a safe setting until you know what a good and bad face seal feel like and the moves you need to get a good one. My big discovery was that moving the mask up or down on my face often did the trick.
I learned how to wear a respirator in a paint shop where there are smelly fumes that you can smell if they leak into your mask. This facilitated the learning process and I am not surprised that OSHA provides a smell option for fit testing involving water solutions of isoamyl acetate; using smell is so much more convenient than taste (no need for a "nebulizer" that often blocks up) and makes a more stringent and foolproof test because to pass, the mask has to keep out individual molecules, which are vastly smaller than the tiniest particles of concern in infection control. You just swap organic vapor cartridges for filter cartridges for the duration of the test, then swap 'em back afterward.
The above smell-based procedure seems designed for testing only the mask-to-face seal. What if you are concerned about the seal between the filter cartridges and the mask, or whether there is a hole in a filter? Let's say you have visions of an enemy, who is a dead ringer for the Green Goblin, sneaking into your apartment while you are on the shitter and sticking a nail through the intake grille (or maybe you have kids). Then, I guess you need to use small, taste-able particles for testing, but having previously tested the face seal (e.g., by smell) should simplify the process of testing the filter cartridges, because the cartridges are smaller, more rigid, and geometrically simpler than the face seal. For testing the cartridges and their seals, perhaps all the bowing and talking to be performed during the fit test, as prescribed by OSHA, would be unnecessary.
In cold weather, a mask leak can fog up my glasses and thus be detected easily if it occurs alongside my nose, which is a problematic area anyway. Fogged glasses can be good for detecting leaks.
Making a flared exhalation tube:
I first prepared a 6-cm dia. plastic funnel by sanding the places where future hot glue would go. I then cleaned off the sandings. I cut off the hanger tab and then encircled the funnel with a bead of glue to help hold the filter. I then cut off the stem and drilled small holes near the bottom to pass the hold-down wire. I then assembled the flared tube on the mask and used hot glue to seal the seam between the funnel and the straight part of the tube (cut from a 1.5" dia. aspirin bottle along the top and bottom of the label). I then secured the ends of the hold-down wire with more glue. (See below). The straight part of the tube fits snugly into the exhalation valve without leaks, but getting it to go in required a combination of force, accurate alignment, and stuffing in the parts that boinked out. A lubricant might have helped; haven't tried it. Something else to try would be filing the end of the tube on a bevel.
04-16-2021: Repair hack: OK, that didn't work too good. So don't do everything I say. What am I doing wrong? <PS 04-21-2021: Not roughening the dental dam in addition to the mask before gluing. This doesn't produce wrinkle-free either, but way better.>
04-15-2021: 1) It's looking like: I can repair holes in the mask with a tire repair kit, but not the dry-repair kind: tire-repair rubber cement seems to be required (I used "Certified" brand). For intensely curved areas, a piece of rubber dental dam seems to work as a patch (see picture); tire-repair patches are way too stiff for these areas. Before use, I cleaned the piece of dam with glass cleaner and dried it, then just followed the tire repair kit instructions. Moreover, there are signs that dental dam is seriously weakened by 0.1% chlorine bleach and badly wrinkled by rubbing alcohol. Going forward, I plan to disinfect the mask by spraying with rubbing alcohol while holding a wet, soapy piece of paper toweling over the dental dam-- to disinfect the dam while keeping the alcohol off it.
04-15-2021: 2) After some use, the joint between the exhalation tube and the mask got loose, so I hot-glued it also, after sanding and cleaning the area (see picture).
03-25-2021: Intercom with automatic volume control (AVC): Boom! Ready to go into the box. The Flying Spaghetti Monster (i.e., the first build) is getting kicked out. This will become the canonical version of the project if it passes field testing. <PS 03-30-2021: It passed.> <PS 04-23-2021: The AVC upgrade yielded a vast improvement in intelligibility, but speaking clearly and slowly and getting the volume control setting right are still important. I am now a firm believer in AVCs.>
03-22-2021: The AVC-augmented intercom circuit is finished and working. Will post diagram soon. The blue button cuts in the AVC function so that I can easily tell if this function is working.
03-21-2021: NB: Today I found a nick in the sealing ring on the right filter cartridge and a smaller nick on the corresponding mask-integrated sealing ring on the opposite side of the same gasket. Have replaced the cartridge with a hopefully intact spare and am crossing my fingers about the lesser nick. Obviously, the integrity of the seals is something you need to check regularly. The giveaways were seeing a streak of dust on the gasket and hearing a difference in the sounds the two sides made when inhaled through individually.
03-12-2021: Coming soon: automatic volume control.
02-20-2021: I have successfully field tested a version of the intercom system that has the speaker box and intercom box taped together for convenience. Weight: 315 g. There is now only one cable that matters, which improves safety by reducing the opportunities for cables getting caught on things, and there is now no possibility of getting two cables mixed up when connecting the system. The photo gallery has been updated.
02-07-2021: build details of microphone board
The microphone capsule was reinforced around the base with epoxy. Here is what the conductor side of the board looks like, to save you some time if you decide to build the project:
The small holes are on 1/10" centers. The large holes are 9/10" apart. Sound reproduction quality is sensitive to the position of the mic capsule inside the mask. A previous build placed the mic slightly higher and had worse essing.
01-09-2021: Flared exhalation tube, January experience
The backpressure from the exhalation is still too much under winter conditions when the cloth mask can get wet from condensate after about 15 min of use outdoors. Using the next-larger size of funnel (having a diameter of 8 cm rather than the existing 6 cm) to create the flared section seems advisable. However, I have not tried this yet and it's not clear that a typical cloth mask would be big enough to go over it. (With a larger flared section, additional hold-down points will also be necessary, probably four...Read more »
The speaker is protected from disinfection solutions by plastic film taped inside the enclosure, and the speaker is clamped onto the film using mirror brackets bent to fit in a drill-press vise, to provide further mechanical stabilization. (I disinfect the speaker and the mask with 70% isopropanol spray.) The white strap on the speaker box is a creative use of straps sold for holding the corners of unfitted sheets on mattresses. The other straps come from a spare P100 mask. (At the start of the project, I had 3 identical P100s, all bought before the pandemic. They cost the equivalent of $47 US apiece in Ontario in fall of 2019.)
The microphone board is secured inside the mask with two stainless bolts that also serve as the signal conductors. There are 3 nuts on each bolt, all stainless. The first secures the external brass connection lug, the second seats on the white nylon mask insert with a stainless lock washer underneath, and the third is an acorn nut that seats on the internal signal lugs, which I place on top of the mic board. I also include a rubberized-fiber washer under the board to prevent short circuits. No messy protective coatings were applied except around and under the microphone capsule; I am putting all my faith in stainless steel. However, I am not quite satisfied with the amount of mechanical stability this arrangement of the nuts gives me. The alternative seems to be an additional nut, to place a nut just above and below the hard nylon insert to clamp it tight, but this would have required somewhat longer bolts, which I had difficulty finding. In use, the acorn nuts need re-tightening now and then.
When drilling the holes in the mask for the microphone bolts, there is a risk of the elastomer ("rubber") part wrapping itself around the drill bit and coming away in a big chunk. Not good, considering what we are about here. This happened to me when drilling from the inside out but not when drilling from the outside in. The trick may be to go slow and keep the rubber pressed firmly to the nylon insert while drilling, which is not an issue if drilling from the outside in. It's a female dog to fully separate the rubber and nylon parts so they can be drilled/punched separately. Remember to leave sufficient flat area around the hole to seat a future nut, especially inside, and don't leave any turnings between the rubber and nylon parts.
The mic is protected from humidity by a collet pried out of a PL-3020 push coupling, 3/8-inch OD (Watts or Sioux Chief), with the teeth clipped off. The top of the collet was covered in a layer of double-sticky carpet tape followed by a layer of plastic food wrap. I left a space of 1/16" between the film and the microphone capsule. Without this space, reproduction quality is unsatisfactory.
The battery is lithium-ion type, 9V. It connects to the intercom box with an adapter that plugs into a barrel jack on the box. The wires of this adapter tended to break at the point where they enter the 9-V clip, so I reinforced this spot with epoxy and a zip tie. A better solution is to use an adapter with a molded, hard-plastic 9-V clip such as Adafruit 80, not a vinyl-covered clip.
A problem with the P100 is that there is no filtration on the exhalation valve, so others are not protected if the wearer is infected by SARS-CoV-2 in spite of taking unusual precautions. You can mitigate this by wearing a second mask over the P100. The second mask is of fabric and hooks around your ears per convention. Make sure the cloth mask covers the exhalation valve. The project log has more to say on this.
To make the little brass adapter (pictured), I removed the business end from a quick disconnect and soldered it to an 1/8" tang, 5/16" long, protruding from a brass cutout. The brass stock was .03" thick and a template for it is provided in the project log.
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