Rock Bottom Air Reserve for Ascent from 100'

I'm curious as to reserve gas level used by other divers.

Specifically, what rock bottom reserve do you maintain at various depths. Of particular interest to me are your gas turnpoints for ascent from 100'.

I'm assuming unobstructed free acess to the surface.

I end up with about 1100 psi (28 cu ft) being my rock bottom ascent point, using a couple different independent limits.

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#1. One limit assumes sharing air with an excited buddy. 3cfm, but with relatively fast ascent rate of 50fpm. Added to this is 1 minute of getting our act together on the bottom. If my buddy is someone I know and trust, I may drop this down to 2cfm. There is no "psi" fudge factor in this limit. Hopefully, 3 cfm has enough fudge.

#2. The other one reflects my desired gradual ascent. 20-30fpm, a couple of stops for a couple minutes in 40-50 and 25-35' ranges, then a safety stop that start 15-20' and transitions to 8-10' by the end of 4 minutes. All told, from 60' to surface this requires the same gas as would 20 minutes on the surface.
For this I use an estimate of 0.6cu ft/min, a bit more if cold, or if I've been using more gas than normal so far in the dive.

Rule #1 at 100' requires 12cf at the bottom and 15 for ascent. 1100psi on an AL80. (700psi with trusted buddy)

Rule #2 requires about 5cu ft for ascent (0.75cfm) and 12 for all stops. I expect to use about 700psi, add in 300psi for SPG 0 and it also ends up at 1000psi for ascent.

An old and basic rule of thumb for a diver on a single tank no decompression dive would be 100 psi for each 10' ft of depth. So for 100 ft. you would want no less than 1000 psi. when you begin your ascent. This comes out pretty close to what you have figured as "rock bottom". If you are smart, you will add a reserve to this - 500 PSI for recreational diving. This means you should begin your ascent with no less than 1500 psi showing on the SPG.

A lot can be said for having a larger reserve of 1/3rd your air supply. Plan on using 1/3 of your air for the dive up to the time you begin your ascent, plan on using another 1/3 for the ascent and plan for reaching the surface with 1/3rd in reserve. It's conservative and you'll finish the dive with about half your air supply left in most cases, but you never have to sweat running short.

For any deep dive, I'd also use a redundant air system with enough air to make a normal ascent from any point in the dive and then not include that air in your plan. At a absolute minimum you should have a 30 cu ft pony for a 100' dive. A 19 cu ft pony offers less than the volume corresponding to the "rock bottom" figure of 1000 psi in an aluminum 80, so you need a 30 cu ft pony at a minimum.

And sooner or later it hits you that, with regard to deep diving, there is nothing a pony can do that a properly configured set of doubles can't do better and safer.

Well, I can tell you from experience that 500# is the bare minimum from 80-85' in a reasonably strong current when you really really want to stay close to the flag line against the drift. In the same situation I'd definitely leave earlier the next time. I barely had time for a 3 min safety stop at 20'. What I had left over I used quickly when I got tired waiting for a pickup, fighting the current, trying to stay close to the flag, and stuck my face in the water to paddle after the flagline. I didn't dare grab the flag rope as I had been warned that I would be keelhauled if I did that with divers still down at 80'.
One other thing: Check the size of yer buddies' tanks before the dive. How was I supposed to know that they had 100's? I thought I wuz doin' okay staying down fer 55 minutes on an al80 (Nitrox) at 80-85', but they just weren't ready to leave yet.
Now I rent 100's when I dive with those airhogs.

JF

DA Aquamaster once bubbled...
An old and basic rule of thumb for a diver on a single tank no decompression dive would be 100 psi for each 10' ft of depth. So for 100 ft. you would want no less than 1000 psi. when you begin your ascent.

Click to expand...

You might want to calculate the actual volume of gas needed than rely on an old rule of thumb.

Calculating rock bottom, you should calculate how many cubic feet (or litres for metric folks) both you AND your buddy require to end the dive from the deepest point, with a conservative SAC of 1.0 or higher. Then take this cubic foot measurement, and convert it to a pressure relative to the tanks you are using. Simply stating 1000psi could be either insufficient or too conservative - look at what that would yield in these common tanks:

AL63 (63cf@3000psi) - 1000psi = 21cf
HP80 (80cf@3500psi) - 1000psi = 22.9cf
AL80 (77.4cf@3000psi) - 1000psi = 25.8cf
HP100 (100cf@3500psi) - 1000psi = 28.6cf
LP95 (95cf@2640psi) - 1000psi = 36cf
LP120 (120cf@2640psi) - 1000psi = 45.5cf (ok, not so common )

Assuming an ascent from 100', at 30fpm, you're looking at 3.33 minutes to the surface, with an average depth of 50'. With a SAC of 1 (assuming you're stressed because of the OOA), that's 50/33 + 1 = 2.52 ATA, which is 2.52 * 3.33 * 1 = 8.39cf to ascend to the surface. Throw in a 3 minute safety stop at around 15-20', that's (15/33 + 1) * 3 * 1 = 4.36cf for a safety stop. Say 12.75cf to exit. Now double it for you and your buddy - that's 25.5cf to safely ascend from 100' while sharing air doing a safety stop. Figure out how many PSI 25.5cf is for your particular tank - THAT's the rock bottom psi. Make sure if you're diving different tanks, you know each other's rock bottom PSI - the first person to reach it turns the dive.

In the above example, say I'm using an AL80, and my buddy's on a HP80 - if we had to surface on his tank, we'd run out somewhere during the safety stop, whereas with my tank we would have been ok - even though we're both diving "80" cf tanks, and he actually started the dive with 2.6cf more gas than me.

That's with immediate ascent as well - every minute at 100' at those rates is going to burn another 5cfm between you.

Hope I got that right. Do your own calculations, don't use mine.

Seacur once bubbled...
Well, I can tell you from experience that 500# is the bare minimum from 80-85' in a reasonably strong current when you really really want to stay close to the flag line against the drift. ...
One other thing: Check the size of yer buddies' tanks before the dive. How was I supposed to know that they had 100's? I thought I wuz doin' okay staying down fer 55 minutes on an al80 (Nitrox) at 80-85', but they just weren't ready to leave yet.
Now I rent 100's when I dive with those airhogs.

Click to expand...

I thought that older, wiser divers didn't try so hard to avoid being first up that they stayed beyond safe turnpoints.

Since Gilboa doesn't have much current, it sounds suspiciously like a dive off Boynton with ESG.

I find 21 cu ft, about 800psi on an AL80 is a good number for departure from 80'.

I thought surely those bozoes would start to ascend soon. But when I hit 500# I swam over to ESG and signalled 500#. He just gave me a bye-bye wave of his Mickey Mouse garden glove and retruned to the business at hand. Lee Bell had his head stuck in a bughole as usual and didn't even know I had departed.

When I expressed my shame at running out of air so soon, Lee was kind enuf to remind me that he and ESG were using 100's, which gave them something of an advantage over my puny little 80.

He might have said something before I nearly bust a gut trying to stay with them. 8)

And who said older meant wiser? But yer right. 800# woulda been nice.

JF