I'm playing with MEMS pressure sensors which detect air pressure. If I put the mems sensor in a water proof container and put it 50meters under water what 'pressure' will the sensor see (preferably in millibars)? Does it depend on the container? I mean, if the container is plastic won't the sensor have air pressure exerted on it? If the container is steel would the pressure be less? If the container is filled with water then I suppose the pressure would equal the mass of 50m of water.. Thanks.

If you seal the container at normal atmospheric pressure into a rigid container, then the air pressure remains at 1 atmosphere (which equals 1 bar, or 1000 millibars) regardless of how deep in the water you put the container. Think of a submarine - the air presssure inside remains relatively constant as the submarine dives. Otherwise the sailors would experience the "bends" when the submarine comes to the surface.

But if the container is flexible (like a balloon) or not completely sealed so that water can enter, then the pressure is proportional to the depth of the sensor in the water. For every 10.2 meters below the surface you get an additional 1 atmosphere of pressure. The formula for total pressure (due to atmosphere + water) is:

P (bars) = 1 + (0.0981)h

where h = depth in water of the sensor measured in meters. So at 50 meters the expected pressure is 1+ 0.981 x 50 = 5.9 bars = 5900 millibars.

I was in the canal 1 time with an empty glass, turned it upside down, dove down , and the air compressed to about half the volume, then filled back up with air at depth, and as I slowly surfaced, bubbles would pour out the side. Its basic, but interesting to see, once.