Breath Pressure Sensors

I want to be able to read data based on airflow, more specifically someone blowing. I read somewhere that a pressure sensor might work if you're blowing into it (which wouldn't be a problem). Has anyone here done something like this before?

Anne Ogborn replies:

1) What's the range? Lung pressures are actually rather low, but you'd be in better shape if you measured your own pressure to get an idea what pressure you're talking about.

2) How much resistance should the test subject blow into? It's a different act to blow into a plugged tube (like blowing into a commercial sensor) vs blowing up a balloon vs. blowing into free space.

3) the biggy - what sort of accuracy do you need? I'm always suprised by the number of people who build gizmos to measure accurately, then only use it for:

if (pressure > THRESHOLD_PRESSURE) {
     do something...
} else {
     do something else....
}

If that's what you're doing, you can get away with a simpler system.

4) How fast do you need to take measurements? Are we dynamicly measuring lung action, or just measuring 'how hard can you blow'?

5) What precautions do you need to use to protect subject from sensor, and vice versa?

ok, as for the actual sensor:

low pressure pressure gages are of three types - a manometer is just a tube with a liquid in it. You measure the change in height of the liquid. This could be as simple as a resistive element dipping into a conductive solution, like a graphite pencil lead hanging in a salt water solution in a U tube.

You can increase the sensitivity by tilting one arm of the U tube til it's almost flat, so a small rise in the level of the liquid means a large change in the position of the meniscus

Another common type is a venturi type. Make a conical hole in a plastic block, like a narrow funnel. Put air into lower end, drop a lightweight ball into the top, ball rises further with more pressure. Good idea to put a crosspin in at top to retain ball.

Finally, you can use a diaphram - some sort of rubber/plastic/metal disk in the end of a container. As the pressure increases the diaphram moves a bit. Usually you have to mechanically amplify this - this could be as simple as a lever arm arrangement. To measure the position of the end of the arm, use a card with a slit of varying width, and a photodetector.

Most commercial gauges use a strain gage permanently bonded to a diaphram. This is a bit hard to interface to. Making your own sensor is easier.

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