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Tuesday, March 17, 2020

PPE from the pVp

I have been working extensively with people over at a slack Channel called Helpful Engineering.  This is mostly closed conversation but you can sign up here if you want to help out.  One of the big problems related to COVID-19 is the developing shortage of PPE (Personal Protective Equipment), specifically N95 masks.  Some places are trying to figure out safe ways to reuse them.  Others are trying to figure out alternative methods of making them. 

The CDC has issued Recommended Guidance for Extended Use and Limited Reuse of N95 Filtering Facepiece Respirators in Healthcare Settings  The design I am sharing here meets these guidelines as a "powered air purifying respirator"

This is also something that people could make at home for their own use, or perhaps in situations where they need to provide care to another family member at high risk, for example an elderly person.  This device is to be worn by the person that needs protection from infection, where it would be highly effective, and not the person who is already infected where it could actually increase risk of transmission.  

I have not built a device nor intend to get heavily involved in this design.  I am busy on working on the ventilator program elsewhere. I just put this out as an informational idea.

I know something about the design of these things, as I had to be “fitted” for these masks many times in my career.  So basically an N95 mask filters out 95% of the particles at a virus size so you only breath in 5% (or less) of the particles.  The specific design should be fitted to your face by a trained fitter using specialized equipment so that it will  less than 1% or so leakage even when you move your head all around. Here are some examples of medical style masks intended for hospital use and an N95 dust mask intended for industrial use.

Duckbill style N95 Medical Mask

3M N95 Medical Mask

These masks have to be made of a fine filter material to do this job.  The material gets damp from the breathing and it that makes it become more difficult to breathe in and out over time.  This is not only uncomfortable but also increases danger because the leakage rate rises with this as well (blow-by).  Also C02 rates increase making the wearer feel very tired over time.  To prevent this from happening the health care worker will replace the mask many times during the day and throw out the wet one.

Here is a commercial solution to this problem that requires a special mask and a special blower.  They are sold out of course.

This mask will accept a blower.  See the  Video Link

Now a mask like this can be worn all day (battery lasts 4 hours) because the continuous air flow prevents the moisture in your breath from “wetting” the material

So todays design is “pandemic “ version of something like this that  meets the criterial of the project in that everything must be able to be made from components already available in a pandemic with no functional supply chain.  It is basically a computer fan, run by a battery, using a HEPA filter to provide a continuous supply of air to a mask

It uses a fan similar to what is used in a computer. The fan speed is controlled via pulse width modulation using an Arduino or similar.  There is some sort of control the user can adjust to control comfort and maintain sufficient air flow.  The air is filtered before it is blown to the mask. through HEPA filter material that would be cut from the large bags used in commercial HEPA vacuums.  Make and model does not matter.

A cheap dust mask can be used for the face mask portion.  Fit is not important because the flow of air is continuous and always blows out of the mask.  A blow-off  valve may have to be fitted to the mask to prevent strenuous exhaling. The feed tubing can be any flexible tubing.  Kinking should not be a major problem as there is always a positive pressure in the pipe.  The filter housing could be 3D printed

One of my readers suggested a (better) alternative to my dust mask.  Use a snorkel mask like this:
This would give eye protection as well as providing a very good seal.  This brings the system closer to the functionality of a professional PAPR, or Positive Air Pressure Respirator. for a fraction of the cost and made of non-medical components that you can probably get during a pandemic.  Here is a PAPR below:
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  1. Seems like you are going for a PAPR. A PAPR doesn't even need a face filter. Adequate airflow is passed into a (reusable) hood, and is designed to leak, while always maintaining a positive pressure inside the hood. These are needed for staff with facial hair as well as those who the N95 does not fit. They are actually somewhat more effective than an N95, but in a normal situation they are much more expensive and less convenient.

    1. Thank you for the input. The connection to the part can go through the filter material then to improve flow. Whatever is used on the face has to cover both the mouth and nose ( half face industrial respirator would work as well)

  2. I think you are looking at what is basically a PAPR, or Positive Air Pressure Respirator.

  3. There are "car air purifiers" that can do a lot of the legwork for this, providing a 12V/USB powered fan, fan control, and filter housing. I'm not sure I'd trust the quality of this, but chaining a few HEPA filters together would seem to help for a PAPR

  4. after more review, I think a cordless HEPA hand-held vacuum is a better option than a car air purifier for supplying clear pressurized air to the PAPR mask.


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