# Q & A: Water Pressure

Q:
How deep do you have to go to get 80 bar
- Catherine Considine
A:
Hi Catherine,

The pressure in a fluid at a depth h is due to the weight of all the fluid on top, plus the pressure of the earth's atmosphere on top of that fluid. The pressure is equal to rho*g*h + 1 atmosphere, where rho is the density of the fluid, and g is Newton's gravitational constant (9.8 meters/sec**2). Be sure the units all match up (I recommend SI units. rho is in kilograms/cubic meter, and h is in meters, and you'll get the pressure in Pascals. But I'll express the results in the usual, clumsy units below).

80 bars is about 79 atmospheres. Accounting for the 1 atmosphere due to the atmosphere, you only need 78 from the fluid. In seawater (you'll need the ocean for this I think), the pressure increase is about one atmosphere for every 10 meters. So you need to go down 780 meters below the surface, or about half a mile. You'll need more depth in fresh water because it is less dense (no salt in it).

Tom

(published on 10/22/2007)