My son (a physics student) wrote to me to point me to a crowdsource project for a CPAP machine.
https://www.indiegogo.com/projects/airing-the-first-hoseless-maskless-micro-cpap#/story
My immediate thought upon seeing the mock up was: balony, with a capital 'B'.
It is a tiny thing looking a little like a plastic moustache. I do not see how such a small package could store enough power to run a CPAP device. It is a simple question of moving mass. People breath about 5 liters of air per minute at a low activity level. That’s 2400 liters in an-8 hour period. Air weighs about 1.2 gm/l = 2.88 KG moved in an an 8 hour period. And that illustration did not seem to have much space for a battery. Not to mention space for the magical flapping air pumps derived from CPU coolers!
And they claim a price of $3 per device… And it’s disposable.
They also say they are using a zinc air battery, which has energy density of 470 Wh/kg or .47 Wh/gm. Let’s generously say that they have 10 gm of battery—that’s 5 watt hours. Lets even more generously say that they have 8 watt hours. That’s 1 watt continuously for a night of sleep.
They are saying they can operate a CPAP machine on 1 watt, have it be the sized of a junor tootsie bar, and sell it for $3 per device.
The question I’m asking here is what should we do as engineers? Ethically, I mean. I really hate to see people taken in by such an obvious scam.
The site claims that the device can provide 20 cm H₂O (which is ~1961 Pa). Apparently some hypothetical normal man breathes at a rate of 6 liters (i.e. 0.006 cubic meters) per minute at rest [1]. If you pump a volume V of fluid across a pressure difference of ΔP, you've done work V·ΔP, which is 11.76 Joules per minute or 0.1961 Watts. Keep in mind that this is the work done, not the power consumed, but let's be generous and assume a 100% efficient blower. We'll also assume that no power is harvested during exhalation.
In eight hours, that's 1.56 Watt hours.
Duracell's zinc-air chemistry claims up to 442 Watt hours / kg [2]. That's about 3.5 grams of zinc-air goo per night, ignoring packaging, blower loss, etc.
This seems entirely plausible to me.
[1] http://www.normalbreathing.com
[2] http://media.ww2.duracell.com/media/en-US/pdf/gtcl/Technical...
Edit: In the interest of precision, I should add that the analysis is accurate for an incompressible flow. Air is definitely compressible, so, if you pump air, more volume is going into the pump than is coming out, and you're heating up the air (adiabatically if your pump is any good). So, with a compressible fluid, you have to consider not on the work done moving the fluid across a pressure difference but also the energy you've stored in the fluid by pressurizing it. The pressure difference for CAP is tiny here, though, so this would be a miniscule correction to the math above.