Thursday, 13 June 2019

I was in London a little 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. 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.

The first graph was from ground straight up to the 18th floor.

The second one was 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.

Here is a non-stop trip from the 18th floor down to the ground floor

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.

The cabin of an aeroplane is also an interesting place to keep an eye on air pressure. These two were air pressure measured inside the cabin of the plane during take-off and landing (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.

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