Dive called with 1000psi left?

[BrianOrange]

Hey guys -- finally getting some time be back in the water down here. Diving The Keys all this week.

Was pretty surprised yesterday that our 2 dives ended around 1000psi. Is this normal? I'm a newbie so I am not sure. It was a guided tour with a DM so we were kind of I guess forced to do what he wanted and surface when he wanted. Plus before the first dive he mentioned both dives would be capped at 40 min in order to stay warm for the 2nd dive. Kind of weird I thought but I am newbie so what do i know.

Anyway, I thought it was weird we had to go up with 1000psi. Esp since we were doing shallow dives right by the boat. I'm almost thinking he was sort of mailing it in cause he was cold both dives and it is a free dive guide so we're not paying him.

 

[Jersey Chris]

If you are on a guided tour thats pretty common. There might have been a few other divers in the 500 psi range that needed to start their assent soon. Its hard in large groups when all the divers consume air differently. Hope you had fun at least

 

[BabyDuck]

for many, that's even late in the game to end the dive. assuming you start at 3000psi, you should be back on the boat (or dealing with an emergency) by 1000psi. for many, 'be back on the boat by 500psi' isn't well thought out.

so it was fine! and i hope you had some great dives.

 

[diver 85]

yep, happens all the time....If you were that interested, you should have asked all the other divers what their gauges read, bet you would have found 1 or 2 much below your reading......Check it out today & see what the 'group' has........

 

[Garrobo]

What everyone above said. I imagine that the waters are a little cold down there right now and anyone who lives down there all the time will be having trouble staying warm. These DM's dive all day every day and they lose their core heat so I imagine they would like to make the dives as short as possible. 40 minutes is a pretty good time for a newbie considering that you had a thousand left over. A lot of times the captain will tell everyone to be back in 45 minutes so as to keep a schedule. I wouldn't complain. Just be glad you are there and not here.

 

[SkimFisher]

On a charter, that doesn't surprise me. What was your bottom time and max depth?

 

[Reg Braithwaite]

You may find this article on Rock Bottom and Gas Management for Recreational Divers interesting. "Rock Bottom" is a strategy for deciding when to start ascending from a dive by estimating how much gas you would need if your buddy had an emergency and you need to solve the problem then make a normal ascent together, including safety stop(s). The interesting thing is that the deeper you dive, the more air you need to keep in reserve for you and your buddy since it will take you longer to return to the surface together.

The basic rules of thumb for AL80 tanks are that you need 1,000 psi for a dive to 60 feet, 1,300 psi for a dive to 100 feet, and 1,800 psi for a dive to 130 feet, the typical recreational deep diving max.

If either of your dives were deeper than sixty feet, many divers would actually end the deep portion long before hitting 1,000 psi.

p.s. You didn't say whether these were square profile or multi-level dives. On a multi-level wall dive you might dive to 90' on a full tank, ascend to 50' when you hit 1,500 psi, then ascend to the top of the reef at 35' when you hit 1,000 psi. Ascending to a shallower depth lowers the amount of gas you need to keep in reserve.

 

[SkimFisher]

You may find this article on Rock Bottom and Gas Management for Recreational Divers interesting. "Rock Bottom" is a strategy for deciding when to start ascending from a dive by estimating how much gas you would need if your buddy had an emergency and you need to solve the problem then make a normal ascent together, including safety stop(s). The interesting thing is that the deeper you dive, the more air you need to keep in reserve for you and your buddy since it will take you longer to return to the surface together.

The basic rules of thumb for AL80 tanks are that you need 1,000 psi for a dive to 60 feet, 1,300 psi for a dive to 100 feet, and 1,800 psi for a dive to 130 feet, the typical recreational deep diving max.

If either of your dives were deeper than sixty feet, many divers would actually end the deep portion long before hitting 1,000 psi.

p.s. You didn't say whether these were square profile or multi-level dives. On a multi-level wall dive you might dive to 90' on a full tank, ascend to 50' when you hit 1,500 psi, then ascend to the top of the reef at 35' when you hit 1,000 psi. Ascending to a shallower depth lowers the amount of gas you need to keep in reserve.

Yeah, that's where I was about to go...

 

[BrianOrange]

On a charter, that doesn't surprise me. What was your bottom time and max depth?

For both dives, we'll say 40 min and 34 feet.

Thanks for all the help guys!

 

[GrumpyOldGuy]

Anytime someone states a fixed threshold to end a dive without regards to depth or SAC rates, they are slacking, no matter if the number is 500, 1000 or 1800. Boyles law combined with variances in your (and your buddies) SAC rates change your needed reserve anywhere from 300-2000psi. If you resort to just picking a number you might as well go to Vegas as the odds in both cases are stacked against you.

In all fairness, the DM might have calculated rock bottom for the divers, guesstimating worst case SAC rates. At 34' I would calculate 700lbs for typical divers, a bit more if you have a newbie / hoover.

Take the time to calculate you rock bottom as Reg and others suggested at several different depths. Update it as you improve on your SAC and you will have reasonable numbers to work with. Dive with these numbers confident that you have enough air to safely ascend for you and your buddy. If you have a DM suggesting grossly different numbers, share your data with him. He might have valid reasons why his are different due to local conditions in which case you learn something. He might also modify his plan to suit you if possible.