Harry Wyeth asked in USMA 12537: >It is interesting that for scuba, divers must know that pressure increases >one atmosphere every 33 feet (as taught in the US), which is, of course, 10 >m. The usual safe diving limit is around 100 feet, which is rounded off to >around 30 m. Not being a physicist, I wonder what relation there is >between air/water pressure doubling at exactly 10 m depth, and adding >another atmosphere at 20 m. Anyone know? Why 10 m and not 9 or 11? The pressure, above atmospheric presssure, in fresh water at a depth of 10 m and a teperature of 20�C is 99,820 kPa. This is at 45� latitude and sea level. The standard air pressure there is 101,325 kPa. In ifp the weight of a column of fresh water at 68�F of 33 feet height and 1 in2 cross section is 14,28 pounds-force per in2, and standard air pressure is 14.70 pounds-force per in2. At Denver or Quito all these figures change. You can see that these figures do not indicate an *exact* doubling of pressure at a depth of 10 m. These ratios are properties of the earth and not of the metric system, which is universal and does not depend on any features peculiar to the earth. Joseph B. Reid 17 Glebe Road West Toronto M5P 1C8 Tel. 416 486-6071
