Pressure spikes in aeroplanes

Today I had my first lecture for my MPhil, Digital Signals Processing, and the lecturer ended with a small puzzle: a graph of pressure over time, with a mean value of approximately 775 hPa and a sharp impulse. What physical event is this waveform a measurement of?

Well, pressure at sea level is approximately 1000 hPa, so the air must be thinner here. After some nudging we discover that we’re in an aircraft! Aeroplanes have pressurised cabins to prevent a bunch of physiological issues. The impulse spike in the signal is the sudden drop in pressure as the bathroom toilet is flushed! There’s a valve that briefly allows a pressure differential between the air in the cabin and the outside air to produce a suction force to flush the bowl. The environmental control system is constantly monitoring and stabilising the cabin pressure, hence the spike and gradual tapering off of pressure after the flush.

The lesson is: the barometer in your mobile phone is remarkably sensitive, play around with the various sensors! It’s a marvel of modern engineering that we’ve managed to miniaturise so many components and pack them into such a small form factor. Next time I take a flight I’ll have to capture this on my phone and attach a screenshot.