Incentive spirometer

A family member has been presented with an incentive spirometer, to use in training for an upcoming procedure.

It has both a small 'indicator' for air flow rate, and a piston that rises in a cylinder marked off to 4000 ml of air volume.

The idea is you shoot for inhaling at a pretty constant flow rate (keeping the 'indicator' near the middle of its travel), and see the total volume you have inhaled from how high the piston rises.



What seems nifty to me about it is the graduated cylinder is nowhere near 4 liters in size! That would be like gallon-jug sized. This is sized more like a drinking glass. I might guess more on the order of 0.4 l, or a tenth of the indicated volume.

So I'm thinking this gizmo must have some kind of clever proportioning valve that's letting 9/10 of the air you inhale come in directly, while 1/10 is drawn from the cylinder above the piston.

I'm no fluid dynamics guy, but I'd guess it's hard to build such a proportioning valve (especially a cheap and disposable one) and have it be accurate over much of a range of flow rate. Of course, the problem here is simpler, because there's already the flow indicator and part of the game is you try to inhale at a constant rate in the middle of the range. So maybe a proportioning valve that's kinda accurate at that target rate is 'good enough'.

Makes me wonder how far off the volume measurement might be at higher or lower flow rates than the target.

I did a brief search this morning for "incentive spirometer patent" and found several patents on other aspects of designing the things, but still haven't found anything for "here's how we got away with a graduated cylinder that's way smaller than the actual volume being measured".

Edit: or is the trick just that at the targeted flow rate, you're sucking with an absolute pressure near 1/10th atmospheric, and the volume is marked off for standard pressure? My gut tells me that 1/10th atmospheric would feel like a really hard suck, though, and the target rate feels like taking a pretty easy breath.

 

Somehow, though, the idea is that the marks on the grad cylinder do tell you how much air you inhaled ... and when it tells me I've inhaled 3 l of air, I really do feel like I've inhaled 3 l of air, even though I can't have gotten more than 0.3 l from the cylinder above that piston.

And the more I think about my later idea (am I just sucking it down to 1/10 pressure?), the more implausible that sounds. If I could do that, I could suck water up a three-storey straw. I don't think so. (Just think of the possibilities for hickies though.)