Pressure, Volume and Density Relationships At sea level, the surrounding air pressure remains relatively constant. This pressure is a standard reference called one atmosphere (ata) because it is the weight/pressure of (but of course) the atmosphere. It's also called one bar; there's a slight technical difference between an ata and a bar, but it's so minor that for diving applications, we disregard it."fen metres/33 feet of water (sea water, to be precise) exerts the same pressure as the atmosphere, or one ata/bar.

Therefore, add one ata/bar pressure for every 10 metres/33 feet you descend. At 10 m/33 ft, you're under two ata/bar one from air and one from water. At 20 m/66 ft, you're under three ata/bar, and so on.

If you take an air volume underwater with you in a flexible container or an inverted jar, the volume changes proportionately with pressure. If you descend to 10 m/33 ft, you double the pressure (two ata/bar) and halve the volume. At 20 m/66 ft - three ata/bar - uou have one third the volume, and so on Density also changes proportionately when pressure changes. When you double the pressure and halve the air volume, the volume reduction comes from squeezing the same number of air molecules into half the space. So, the density doubles. When you triple the pressure(20 m/66 ft), you triple the density. Hope you're picking up a pattern here.

Air volume as you descend, you need to add air to the space to keep up with the volume reduction. This is the concept behind equalization (more about this in a moment); the air you need to add is proportional to the pressure increase.As you've probably already figured out, air expands pro-portionately as you ascend and the pressure decreases. If you take an air volume to 30 m/99 ft - four ata/bar - it compresses to one fourth its surface volume. When you return to the surface, the air expands to its original volume.