Monday 1 April 2019

Experiments with a Smartphone Barometer

Modern smartphones have a wealth of sensors that provide all sorts of avenues for amusement for modern geeks.  One of them is a barometric pressure sensor, which is the topic of this post.  I have an app called Phyphox which provides direct access to the data from those sensors.

I was in London a while ago, working on the 18th floor of an office-block. There is an express lift which gets from the ground to the 18th floor (60m up) in 15 seconds non-stop. The locals tell me that when the lifts went in first, they used to be even faster but they hurt peoples' ears and had to be slowed down. I have no trouble believing that.

These graphs were produced using the Phyphox app on my phone, measuring the changes in air pressure with altitude (like the altimeter in a plane) as well as vertical acceleration.  This is from ground straight up to the 18th floor.

This from the 18th down, but with stops on the 17th and 16th (other people wanting to use the lift and messing with my consideration !) - hence the "staircase" effect for the first 45 seconds or so.

A bit later on, I managed to capture a "clean" descent all the way from the 18th down to the ground:-

The cabin of an aeroplane is another interesting place to try this.  These were taken on the flight home during take-off and climb-out:

...and coming in to land:-

(I'm probably on a no-fly list now). You can see the cabin-pressure gradually drop from ground-level pressure to around 760hPa as the plane climbs and gradually increase from that back to ground-level pressure again as the plane comes in to land. The change is made gradually over the course of about 14 minutes each time.
Of course the cabin of the plane is pressurised and outside air-pressure at the cruising altitude of 36,000ft is far lower than 760hPa. The cabin pressure is equivalent to being at an altitude of around 2,300-2,400m (a good bit less than ⅓ of the way up Mount Everest).
The Phyphox app really is an awful lot of fun. It gives you direct access to all of the sensors on a modern smartphone (and there are lots of them) and lets you run experiments like these with them.