Atmospheres (ATM) vs. ATA: Understanding Pressure in Chemistry and Scuba Diving

Question
We are using atm in our chemistry calculations and a student asked why they use ata in scuba diving and I use atm.  Since I am clueless concerning scuba diving, I couldn't answer her, but possibly you can.  What is the difference between atmospheres and ata?   What does ata stand for?  Thanks much,  kandy

Answer
Hi Kandy,

It's like gage vs absolute pressure wrt the diver.  So, a diver goes down 10m (33') and experiences 1 ATM of pressure from the sea water.  The ATA for the diver is 2 ATA, because of the additional ATM at the surface (they are cumulative).  

It is also well-explained here:
http://www.mild-hyperbaric-oxygen-therapy.com/ata-atm-psi-psig.html

hope that helps,
James