# Calculating Underwater Pressure

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Ordinarily people live at the bottom of an "ocean" of air. A vertical column of air 1cm2 extending from sea level to the top of the Earth’s atmosphere would weigh about 1 kilogram. A column 1in2 would weigh 14.7 pounds. This value, normal air pressure at sea level, is referred to as 1 atmosphere or 1 bar. If you climb a high mountain or fly at high altitude in a plane, the depth of the atmosphere is less, resulting in lower air pressure. However, most people live most of their lives with the pressure of 1 atmosphere pressing in on them.

We don’t notice this enormous force because our bodies are constructed to live in those conditions. Most of our body is solid or fluids that do not compress. The air in our lungs and body cavities is generally connected to the outside, so it is at the same pressure as the air that surrounds us. So it is only when we go very high or very low that we notice the effects of air pressure changes.

When we go underwater it is very different. A vertical column of air is less dense than water and therefore weighs much less than a vertical column water. The weight of several hundred or several thousand feet of water pressing down is much greater than the weight of miles of air. Calculating the underwater pressure that results from those differences can mean the difference between life and death for divers, submariners, researchers and others whose work takes them deep underwater. The salts and other minerals dissolved in salt water also make it heavier than freshwater. Ocean divers must adjust for slightly greater pressures than freshwater divers. The deeper the dive, the greater this difference becomes. Pure water weighs 1kilogram per liter or 64 pounds per cubic foot. Can you calculate how deep a column of water would need to be to equal 1 atmosphere

Instructions Define the depth for which you wish to calculate the pressure. Determine whether the water is salt water or is it fresh water. Multiply the depth in feet by 0.432 to calculate the underwater pressure in fresh water, in pounds per square inch (psi). If the depth of fresh water is 100 feet, then: 0.432 x 100 = 43.2 pounds per square inch. Multiply the depth in feet by 0.445 to calculate the underwater pressure in salt water, in pounds per square inch. If the depth of fresh water is 100 feet, then: 0.445 x 100 = 44.5 pounds per square inch.

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