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During this covid-19, I was thinking about designing a low cost simple ventilator using a blower. There is a limit of the inspiratory and expiratory pressures, the inspiratory pressure being higher. Can anyone tell me how can I adjust the pressures? I also don't know how to control the fraction of the inhaled oxygen (FiO2). Will an oxygen concentrator work in this case? If anyone wishes to look into the plan, it is in this link.

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    $\begingroup$ I think you are out of your depth. Your best bet, if you want to help without killing somebody, is to join one of the thousands of projects already in progress. Pick a good one that has qualified people involved. $\endgroup$
    – Transistor
    Commented Mar 28, 2020 at 9:42
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    $\begingroup$ Medical respirators are multi-tens-of-thousand dollars devices. You can come up with hundreds of ways to push air into someone's lungs via valves, dampers, etc., but none would meet the actual needs, unless your objective is to recreate a device from the 1700s. A forge-style bellows would work to move air (think Princess Bride Mighty Max); saving a life may be harder. $\endgroup$
    – Tiger Guy
    Commented Mar 29, 2020 at 6:33
  • $\begingroup$ First, they are right. Second, all (great) inventors receive such comments and responses. That is because (great) inventors and innovators are a bit crazy. You have to determine if they are right or you can overcome, if you are crazy fail, or crazy succeed. $\endgroup$
    – Ack
    Commented Mar 29, 2020 at 19:40

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I am going to answer the question on lowering the pressure, but I will not encourage you to use it on any patient due to the fact that professional medical equipment requires rigorous tests, using a homemade medical device would pose a large hazard on the patient.

Since your goal is to build a low cost ventilator, I believe a simple adjustable valve can lower the pressure effectively by draining part of the supplied gas away.

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  • $\begingroup$ That is generally true, but many (most?) such valves, especially inexpensive ones, use a spring to hold a plug onto the outlet. At the low pressures needed for medical ventilation (typically under 0.5 psi), such valves may have troublesome behaviors. For example, the weight of the plug itself becomes a much bigger factor, so the orientation of the valve starts to matter. If you make a ventilator that tests fine upright, but fails if someone tips it over, that's kind of a big problem. $\endgroup$
    – TextGeek
    Commented Apr 23, 2020 at 19:14
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There are a number of ways to approach this in a low cost, home build, way.

The first thing to address is that you need a vent. Air flowing into the patient (via a face mask of some sort) and then the air flowing out of the mask. You need the outlet flow to be high enough to flush all the exhaled air away. This then makes your pressure control much easier. The simplest way this is done in current products is just to have a bunch of holes in the face mask to vent the air directly. Vent through a HEPA filter if this is for patients with covid or other infectious diseases.

The ways I would look at for pressure control are:

  1. Vary the speed of your compressor. I assume this is some sort of rotary fan compressor? Just speeding up and slowing down the fan will change the output pressure. A small centrifugal fan sized to produce the flows for human breathing is easily able to adjust speed within a fraction of a second, suitable for matching human breathing. Even if you're using a different sort of compressor, the balance between airflow in and the exhaust airflow out will mean that slowing the compressor will drop the pressure.
  2. Have a constant airflow from the compressor, and have a big vent valve to bleed off enough flow to drop the pressure. I've seen this done with a few different sorts of valve, the simplest one was highly effective, they just used a speaker cone from a big speaker. Driving the speaker coil with a simple DC current would move the cone back and forth, opening and closing a vent. This allows very precise control of the pressure using a simple electronic feedback loop.

And I will emphasize that it is critical that you do not have pressures that are too high. For obvious reasons the lungs and airway should never be subject to dangerous pressure. I would stay well below 200 mmH2O water pressure, probably 100 mmH2O pressure (1000 Pa). This is perfectly effective to assist breathing and is something that keeps you within the pressure ranges of a common CPAP device. It is also vital that no foreign matter (dust, plastic etc) is in the airstream. Major medical companies have got into big trouble for this.

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