# Air pressure at water depth and buoyancy

1. Jun 10, 2012

### malek340

I'm doing a project that requires a platform that is able to float up and down a water depth of 0-100m. It'll be pulled down by a fishing line. I'm currently using air as buoyant. This air will be trapped in a container. However there is air pressure at a certain water depth. The air will certainly be compressed at a 100m water depth. What volume of the air should i use? Is it the volume of air needed to float the platform at atmospheric pressure(sea level) or at the 100 water depth.

If i were to use the volume of air to float the platform at 100m, wouldn't the net upward force(buoyancy force against the weight of object) acting on the platform be greater?

2. Jun 10, 2012

### the_emi_guy

Is your container compressible like a balloon or is it rigid?

3. Jun 11, 2012

### malek340

Yup it's quite rigid.

4. Jun 11, 2012

### HallsofIvy

Staff Emeritus
Then the bouyancy force is the weight of the container minus the weight of the water displaced (the volume of the container times the weight density of water).

5. Jun 11, 2012

### LURCH

If it's rigid, then you can use the air needed to float the platform at sea level. In a rigid container, the air will not compress.

However, you must make the container quite strong. Pressure at 100m is about 160psi, or 1.10 Mpa. There's a good chance your container will just implode.

Alternatively, you might consider filling the container with gas or oil. These are lighter than water and will provide bouyancy, but are incopressable and will not get crushed or change volume. Or you could just use wood for floatation.