Without bashing me too hard, I would like to get some comments on an incident on my last trip family dive trip as education for future trips. ... looking for insight or wisdon. Always trying to learn.
Okay, here are some (hopefully) useful comments.
Were instructed not to surface but to get back to anchor line.
Regardless of what the DM says, you are responsible for your own safety. When you're getting low and air, sometimes you have to head for the surface. Better to arrive alive on the surface, than to run out of air at depth on the anchor line (as you almost did).
When there is heavy current, as you mentioned, it is best to be prepared with signalling devices in case you do end up drifitng away. Such devices included SMBs, whistles, signals mirrors, and strobes. I even carry a personal rescue beacon (such as the McMurdo EPLB) when the conditions are predicted to be harsh.
Of course, it is best to avoid a low-air situation in the first place. This can be avoided by some basic gas planning.
Basic gas planning starts with "rock bottom" gas estimations. You can do a search here on Scubaboard for that topic. Basically, you should start ascending at the end of the dive with enough gas to get you the surface in case of emergency. You should know this number before your dive starts. For instance, many people would use a "rock bottom" gas of 800 p.s.i. for an AL80 tank and a depth of 60 feet. This "rock bottom" number varies depending on depth, the type of tank being used, and how much gas you want when you arrive back at the boat.
So... you should have been
at the anchor line and starting your ascent from 60 feet with
a minimum of 800 p.s.i. in your tank.
If you hit 800 p.s.i. and you're still a long way from the anchor line... then start ascending in open water. Forget about the anchor line, you blew it.
Sounds easy enough, but how do you make sure than you arrive back at the anchor line with 800 p.s.i.?
In heavy current, people commonly use "thirds." Use 1/3 of your gas to explore the wreck, use 1/3 to get back to the anchor line, and save 1/3 for emergencies (such as gas sharing). I should specify that it is 1/3 of
available gas, after you've subtracted out you "rock bottom" gas. For instance, using your situation:
Available gas = starting tank pressure - rock bottom = 3,000 p.s.i. - 800 p.s.i. = 2,200 p.s.i.
So "thirds" is 1/3 of 2,200 p.s.i., or 2,200/3 = approx 700 p.s.i.
So you should have explored the wreck until you used 700 p.s.i. and got down to 2,300 p.s.i. (3,000 - 700 = 2,300). Then you should turn around and headed back to the anchor line.
You should arrive back at the anchor line after using another 700 p.s.i., so your pressure on the SPG should read 2,300-700 = 1,600 p.s.i. at the anchor line.
You don't have to ascend up the anchor line until you hit 800 p.s.i., so you can hang around
near the anchor line until you drop to 800 p.s.i., at which point you ascend.
In your particular case, you would had to make an adjustment because your starting pressure on the 2nd dive was not 3,000 p.s.i.:
...dive group went down, heavy current moved boat, anchor was not near wreck and had to resurface, take off equipment, reposition boat, re-enter and do second decent.
So you didn't start the 2nd dive with 3,000 p.s.i. Let's say that you started the 2nd dive with only 2,300 p.s.i.:
Available gas = starting pressure - rock bottom = 2,300 - 800 = 1,500 p.s.i.
Thirds = 1/3 of 1,500, or 1,500/3 = 500 p.s.i.
So explore the wreck until you hit 1,800 p.s.i. (2,300-500=1,800) then turn around and head back to the anchor line. Arrive back at anchor line at 1,300 ps.i. (1,800-500=1,300). Explore wreck near anchor line until you hit 800 p.s.i. (rock bottom gas) and then ascend.
If there isn't any current, then you can use "halves" instead of "thirds." This is because,
without current, you can always surface anywhere and swim back to the dive boat on the surface. (When there's current, you have to use the anchor line or you will drift away.)
If you are being guided and hit you turn pressures, signal the DM that you need to turn around, or leave the DM and go back to the anchor line yourself.
NEVER LET ANYONE DRAG YOU AROUND PAST YOUR GAS LIMITS. If you reach your gas limits and the DM won't turn around with you, then wave good-bye and take charge of your situation yourself.
Good luck!
