I have an oxygen system for my sailplane that provides a short burst of
oxygen at the start of each inspiration cycle.  The oxygen burst width
varies with altitude and by firing the oxygen in at the start of the breath
intake, the oxygen gets right through the lungs.  The over all system is
extremely frugal with its oxygen use.  The system uses no mask, it has two
tiny cannula that poke 10 mm into the nostrils.

The pressure sensor is a capacitor fabricated on the PCB as a 50 mm
diameter disk of vacuum aluminised plastic (looks like wine cask
material).  This is vented to atmosphere one side.  When you inhale the
small pressure drop in the cannula moves the capacitor plate and a '555
(yep - it should be a PIC) changes frequency. This is sensed and the oxygen
valve is opened for the required milliseconds.

It all runs on one 9v PP9 battery and works to over 25,000 feet.

Hope this helps.

Cheers
Brian

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