Sunday, March 2, 2008

The Difference Between An ATM & An ATA

Atmospheric Pressure
Atmospheric Pressure
While studying to become a PADI divemaster...on the road to becoming an PADI instructor, you have to develop a strong understanding of pressure. There are so many terms and formulas related to pressure that I frequently felt confused. I generally calculated the correct pressure, but was always unsure of the ending unit indicator (ATA or ATM). I asked Larry "Harris" Taylor, Ph. D. if he could provide a concrete explanation, and this is what he said.

ATM is a unit of pressure equivalent to the weight of the earth's atmosphere at sea level. Absolute pressure (ATA) is the total ambient pressure on the system being calculated or measured.

ATA is used to indicate that the absolute pressure includes the 1 ATM addition to the read gauge pressure.

So, if water depth is 1 ATA (as read by depth gauge at 33 fsw), the total pressure on the diver at depth is 1 ATA (weight of water) + 1 ATM (weight of atmosphere) = 2 ATA