Question about bottom time, NDC times between dives

[grey2112]

Considering doing a dive on either 21% or 32% to about 110 feet (maybe 120) - going to be spearfishing.

AL 80 tanks

What sort of bottom time (keep in mind I'll be spearfishing, so maybe a little more exertion than just slowly taking in the sights) should I expect considering I'll want to do a safety stop. Also what sort of time before my next dive should I expect?

Been a while since I've crunched numbers for this sort of thing.

Is there an online calculator to figure these things out, or an app?

 

[tursiops]

Are you a certified diver? if so are you nitrox certified?

 

[ontdiver]

This is one of those questions that is going to generate a number of pointed comments, most of which are justified. Cutting to the chase, as it were - and following from tursiops - you really need to look at your (PADI or other) Recreational Dive Planner (RDP) to figure out what is appropriate for 32%.

 

[scubastingray]

What is your sac rate? Some would barely last 15 minutes at depth, and others could probably eek out an hour (on the extreme end). Depends entirely upon your consumption.

 

[grey2112]

What is your sac rate? Some would barely last 15 minutes at depth, and others could probably eek out an hour (on the extreme end). Depends entirely upon your consumption.

Roger that - SAC rate is normally about 65-68 psi/min on 32%

 

[Peter_C]

Single or double AL80's? Solo or with a buddy? Gas supply redundancy?

You do know what the maximum operating depth of 32% is right?

This one has a clear chart on gas management.
www.nwgratefuldiver.com/articles/gas4.html

There is a calculator at the bottom once you are done reading. There are also three more links to read on min gas.
DiveNerd - About the Rock Bottom Calculator for Scuba Gas Management

 

[tursiops]

At a SAC of 65 psi/min on an AL80, you won't have any problems with NDLs or surface intervals. At 100 ft, you'll be blowing through the tank in about 10 minutes without the spearfishing, and probably 3-5 minutes with it.
A ten-minute dive to 100 ft, even on air, won't accumulate much nitrogen, and after a 45-60min SI you'll be OK to do it again. Even 30 mins won't be too bad.
On nitrox you are even better off, of course; the EAD for 32% at 100 ft is about 80 ft.
But don't go to 120 ft on the 32% and exert yourself with spearfishing, but I'm sure you know that. I doubt even 110 ft is a good idea. The CO2 buildup appears to be one of the triggers for O2 toxicity.

 

[grey2112]

Single or double AL80's? Solo or with a buddy? Gas supply redundancy?

You do know what the maximum operating depth of 32% is right?

111 ft, correct? I guess that means I should probably nix the nitrox since our depths will be between 90-130 max.


This one has a clear chart on gas management.
www.nwgratefuldiver.com/articles/gas4.html

There is a calculator at the bottom once you are done reading. There are also three more links to read on min gas.
DiveNerd - About the Rock Bottom Calculator for Scuba Gas Management

Thanks for the links - after looking over my SSI dive charts, it looks like 10 minutes is about the max I can stay at that depth, then do a 1 hour 30 minute rest up top, then another dive at those depths for 10 minutes or less on the bottom.

I better bring my "A" game with regards to my shooting

---------- Post added July 26th, 2014 at 02:58 PM ----------

At a SAC of 65 psi/min on an AL80, you won't have any problems with NDLs or surface intervals. At 100 ft, you'll be blowing through the tank in about 10 minutes without the spearfishing, and probably 3-5 minutes with it.
A ten-minute dive to 100 ft, even on air, won't accumulate much nitrogen, and after a 45-60min SI you'll be OK to do it again. Even 30 mins won't be too bad.
On nitrox you are even better off, of course; the EAD for 32% at 100 ft is about 80 ft.
But don't go to 120 ft on the 32% and exert yourself with spearfishing, but I'm sure you know that. I doubt even 110 ft is a good idea. The CO2 buildup appears to be one of the triggers for O2 toxicity.

Looks like the old rust is coming off the brain - I got some pretty close numbers to yours when I remembered how to do all this - LOL. I normally do all my spearfishing at 75 feet or under on Nitrox 32-34. But I have an opportunity to go out to 100-130 feet to the Middle Grounds and was trying to figure out what my parameters were going to be.

Wow - not a lot of time down there - would really suck to drop down on a promising location and find nothing there.

 

[tomfcrist]

Or not do the dive on an AL80....or better yet, drastically improve your sac rate prior to doing any more deep diving.

 

[tursiops]

What you are describing is, frankly, a dangerous dive, even (especially?) on air.
(1) the gas will go away really fast
(2) you will be focusing on hunting, not gauges
(3) you have no redundancy
(4) the inevitable CO2 buildup will exacerbate any tendencies for narcosis, which will..
(5) ...make (2) even more important