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Gas Theory

Pressure and Gases

It has long been known that air has weight. The weight of the atmosphere exerts a pressure of14.7 pounds per square inch of force at sea level. This is to say that a 1 inch column of air as tallas the atmosphere, would weigh 14.7 pounds. We commonly call this 1 Atmosphere of pressure,or 1 ATM.

Water weighs considerably more than air does, so it can exert much more pressure. It only takesa 1 inch column of sea water 33 feet tall to weigh 14.7 pounds. This means that at a depth of 33feet deep in the ocean, there is a total pressure of 29.4 pounds per square inch (psi). This wouldbe 2 ATMs of pressure. One ATM from the water, + one ATM from the water. We call this theambient pressure or absolute pressure.

Every additional 33 feet of sea water, will add another 14.7 pounds of pressure, or another ATM. At 66 feet we have two ATMs of water pressure plus our 1 ATM of air pressure for an absolutepressure of 3 ATM. At 99 feet our pressure is 4 ATMs etc. (see figure 1.1)

Absolute pressure differs from gauge pressure. Gauge pressure would be the pressure that wouldshow up on a gauge at this depth. A pressure gauge would start at 0 at the surface and show 14.7psi at our depth of 33 feet. Gauge pressure would ignore the 14.7 psi of atmospheric pressure.

And now for a little math!

With a little math, we can determine the ambient pressure at any given depth. Knowing that 33feet of sea water exerts a pressure of 14.7 lbs, it is simply a matter of dividing 14.7 by 33 todetermine that sea water exerts a pressure of .445 lbs per foot of depth. Fresh water beingslightly lighter, requires a depth of 34 feet to equal 1 ATM, so fresh water exerts a pressure of14.7 divided by 34, or .432 lbs per foot of depth.

Let's then find out the absolute pressure for a depth of 50 feet of sea water. To solve this, wemultiply the depth, 50, times the pressure per foot, .445. This gives us an answer of 22.25. Thishowever would be the gauge pressure at a depth of 50 feet. To find the absolute pressure, wemust add in the 14.7 psi of atmospheric pressure. 22.25 + 14.7 = 36.95 Thus the pressure for adepth of 50 feet in sea water is 36.95 pounds per square inch absolute. It is common to refer tothis as "PSIA", pounds per square inch absolute.

Try to figure out the gauge pressure for a depth of 75 feet in fresh water. To solve this wemultiply .432 by 75 to get 32.4 psi. Since we wanted gauge pressure in this example, we do notadd in the atmospheric pressure and show the pressure using psi, not psia.





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