I’ve been playing around with vacuum the last couple of weeks. While I was waiting for more parts and a proper vacuum gauge arrive from eBay, I was impatient to get at least some measurements of how good the vacuum was in the system.

I started thinking of ways to improvise a pressure sensor.

Marshmallows provided a great visual indication that, yes, there was vacuum aplenty. But it was a bit less quantitive than I hoped.

I played around a bit with Pirani gauges, which are a way to use resistors as pressure sensors. That wasn’t too bad, and I’ll detail those in a later post when I write up the chamber itself. But it seemed to me that there should be some easier way of getting an indicator.

I could always see how high water could be drawn up into a tube. Although for a decent vacuum, that would mean about 10m high of water, which was taller than any building I had access to. You can make the necessary height shorter by using different fluids (mercury would require less than a meter tall gauge), but I couldn’t work out any combinations that would be both safe and practical.

I eventually came up with the idea of a Bourdon tube using plastic I had laying around. This seemed to worked pretty nicely:

Without another gauge to calibrate it against, I could only test it by bleeding air into and out of the system slowly using the valves. It seems surprisingly linear, though.

You can even make a direct digital readout by driving the tube onto an electric scale!