How to Measure Pressure With a Phone and a Baggie

This says pressure is just force (F) per unit of area (A). If the force is measured in newtons and the area is in square meters, then the pressure would be in units of N/m2, which is called a pascal. But you can use other units too, like pounds per square inch or atmospheres (1 atm is about 105 pascals).

As you can see, if the mass on top of the bag stays the same, so that the gravitational force F is unchanged, but the area decreases, then the pressure would have to increase. Is that legit? That's what's so great about this experiment—you can check the results yourself.

Could You Use It As a Scale?

Now how about something more complicated? Given the relationship between the mass on top and the air pressure inside, could you use this barometer-in-a-bag setup to measure the mass of something? I think it will work. (Technically it would measure the weight of the object, not the mass.) Here's what I'm going to do:

  • Start with just the Lego platform on top and record the pressure.
  • Place a new object, of unknown weight, on top.
  • Record the new pressure. It's this change in pressure we want.
  • Next, make the Lego platform smaller and repeat.

Maybe it will help if I rewrite the pressure equation like this:

Illustration: Rhett Allain

If I plot pressure on the vertical axis and 1/A on the horizontal axis, then this should look just like the equation for a line with the force as the slope. OK—let's do it. Oh wait! What about the area of the Lego platform? In this case, I used multiple bricks of the same size (0.96 x 3.18 cm) so that I could just remove a brick and reduce the surface area by a known amount. Yes, Lego pieces are very uniform in size.

Hmm ... that was my plan, but it didn't work. The pressure in the bag should increase when I swap in the smaller Lego platform, but it didn't. I actually think my bag might be leaking. No matter, this experiment is for you to do at home. See if you can make it work.

April 18, 2020