Pressure in a Flotation System

[spggodd]

Hi,

A friend is trying to work out the pressure in a sea water flotation system.
What would the pressure be when a 1m^3 bag of air at atmospheric pressure has a 1000Kg weight applied to the top?

General temperature would be approx 20 DegC.

Could you show me the workings of your calculations if possible?

Thanks
Steve

 

[LittleInch]

Pressure = Force / Area.

You've given us the Force (1 tonne), but not the area. Your 1m3 bag could be 1m x 1m x 1m or 10m x 1m x 0.1m etc etc. The greater the area the lower the pressure for the same force(weigth in this instance). Hence this is why air bags can lift trucks with only rel low pressure, but you need large pressure to lift it with an hydraulic ram instead - you get the same force, but with different presusres and areas.

See hover craft design for the same idea....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[spggodd]

This is a theoretical problem at the moment, but we were assuming an area of 1m^2, i.e. a cube.

The trouble is the calculations and the answer I got doesn't seem logical.

F=MA, 1000kg x 9.81 = 9810 N

Then P=F/A

9810/1 = 9810 Pa = 0.0981 Bar

Surely being subject to a pressure the cube would experience a pressure greater than initial pressure? (ie. > 1 bar)?
The only other thing I could think of was that this was pressure increase and I would have to add this to the initlal pressure (1.0981 bar)?

 

[LittleInch]

You've got it. Your answer is in barg, i.e. atmospheric pressure is already included.

If you work it backwards and perhpas in other units - 0.09 barg is about 1.5 psig. if you r bag was flat and then you pumped in air at 1.5 psig, your area is 1,500 in2, force = 2,350 lbs = 1 tonne

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way