Make:

Andy shows you how to make a ake a beer tin barometer -

...There are simpler designs for barometers (though not much simpler it must be said). But this I think is effective, although quite vulnerable to temperature fluctuations, but if you keep it indoors it should be okay.

Atmospheric pressure exerts a force on all bodies within it that are at a different pressure. Therefore, if we seal off part of the atmosphere, by, for example taking an open ended container and turning it upside down into water, then we effectively preserve that (atmospheric) pressure level in the container.

The pressure is equalised by the can floating at a relevant depth to keep the pressure inside the can the same as out (remember that it is not truly sealed, so water enters/exist the can to make up the pressure difference, thus altering buoyancy).

The level at which the can floats is then altered by the atmospheric pressure because it exerts more or less force on the water that the can is floating in causing more or less water to be present within the can an causing the water to be slightly more or less dense thus effecting buoyancy.

These changes can then be measured as changes in buoyancy, namely as rises and falls in the height that the can floats.

Beer Tin Barometer - Link.