Safe tank pressure at which to begin accent.

[Professor Nemo]

Thank you to everyone who has contributed to this discussion. I will attempt to answer a few questions that have appeared from various members:

· I have dove before at one of the dive locations that I will be visiting. However, when I last dove there I only had Open Water Certification and thus maxed out my depth at 65 ft. On this dive I do not know if I will spend a great deal of time at the bottom. Much of the wreck can be seen between 80 and 50 ft. I simply felt that I should establish my deepest point at the beginning of the dive and slowly work upward.

· I would prefer to dive steel 100s but I am not sure that the dive charter I am booked with rents out these size tanks. I intend to call and inquire about this tomorrow. I know that their standard load out for any diver is Al80s. I, unfortunately, do not own my own tanks. I own all of my other equipment and have not decided if purchasing tanks is a legitimate and prudent investment considering how much I dive. I have read posts concerning this very topic on Scubaboard which I have found most informative.

The rule of depth (FSW) multiplied by 10 and adding 300 makes a great deal of sense and would be a calculation even I would be capable of. Thank you for that.

 

[seeker242]

That's a total of 32 cubic feet for a bad situation at 99 FSW. So if your buddy goes OOA at 99 feet, you should be able to bring that buddy to the surface with a safety stop if you start the ascent with about 45 cubic feet left in your AL 80.

So you recommend beginning ascent with 32 cf or 45 cf?

 

[tbone1004]

standard rock bottom calculation from here
DiveNerd - Rock Bottom Calculator, Imperial Units
2 divers, 1cfm SAC rate each, 77cf for tank size at 3000psi, 200psi buffer, so 1600psi to make the ascent, and a 200psi buffer to make sure the regs still work.

Now, the vast majority of divers can't actually do a 10fpm ascent rate, and this doesn't include a stop. This is my general rule of thumb for OW diving in terms of requiring 20cf/diver to get up from 100ft and why I don't believe in AL19's as pony bottles. If you are much shallower than 100ft, you don't really need one, and if you are deeper than about 80ft, you should at least have a 30 to give you some sort of buffer instead of carrying the bare minimum

 

[Neilwood]

A rough open water (no deco) rule if you're diving with a buddy and may need to share gas, is to have 10* (depth in feet) + 300 to 500 psi. So at 90ft: 900+ 300 to 500, or 1200 to 1400 psi, depending on the size of tank and air consumption. I find this rule more useful than more precise calculations that you just can't do under water. It should be enough for a safe shared ascent with a safety stop.

Not seen this rule of thumb before but handy for quick calcs on the fly. Guess for use here it would be best converted to metric which, if I am not wrong, should be 3* (depth in M) + 30 to 50 bar.

 

[stuartv]

standard rock bottom calculation from here

I did not realize that the standard rock bottom calculation was to be done using an ascent rate of 10 ft per minute for the last half of the ascent, plus a safety stop.

That seems excessive, to me.

I plan my rock bottom assuming 33 ft per minute ascent the whole way, plus omit the optional safety stop.

In the example given, the calculator assumes 8 minutes to ascend from 100' to the surface. Personally, I would allow 1 minute for assessment and 3 minutes for the ascent. If the OOA emergency happens precisely when I hit my RB pressure, then yep, my plan would require a controlled ascent directly to the surface, not making an optional safety stop. If it happened any other time, I would have more gas and would do as much of a safety stop as I could.

Planning for 4 minutes, instead of 8, cuts the required gas in half and leaves you with a RB pressure of around 1000 psi, instead of 1800. I would probably pad that a little bit more and call it 1200. That would give me 200 psi to keep the regs working and another 200 psi in case the combined SAC ended up being more than the estimated 1.0 cu-ft/min per diver.

 

[Macan]

Not seen this rule of thumb before but handy for quick calcs on the fly. Guess for use here it would be best converted to metric which, if I am not wrong, should be 3* (depth in M) + 30 to 50 bar.

For Metric, use:

Depth in Meters x 2 + 30 bar. This will closely approximate the Imperial version.

For added conservatism, use:

Depth in Meters x 2 + 40 bar.

For added simplicity, do depth in increments of 5 meters. Actual depth round off to the next deeper depth divisible by 5. Then apply same rule.

Depth x 2 + 30 or 40.

 

[tbone1004]

no safety stop, just the slow ascent rate. It roughly correlates to a normal ascent rate and a 3 minute stop from those depths. 8 minutes from 100ft, is about 30fpm ascent rate with a 5 minute stop.

Depending on the dive and what is going on, I typically plan for 2 minutes at the bottom, 30fpm to 20ft, 5 minutes there, and a 1cfm SAC rate for all involved.
16cf for calm down
12cf for the full ascent
10cf for the 5 minute safety stop *assuming that people have calmed down by now, or 3 minute at 1cfm
That is 35cf, and in an AL80 is 1480psi with no pressure buffer. 1700psi with the same 200psi buffer that the calculator spits out, and that is close enough to the 1800psi ascend pressure. On a wreck that means when I see 1800psi I would head back to the anchor line and begin my ascent, not try to be at the anchor line at 1800psi. 200psi in an AL80 at 100ft goes quick, so you are splitting hairs.

Now, with some more useful information, we have what will likely be a depth of call it 66ft for some easy math
2 minutes at 3ata is 12cf for calm down
2 minutes to make the ascent for 6cf
Same 10cf for the safety stop
28cf total, is 1100psi in an AL80 or 1300psi with the 200psi buffer so you aren't sucking the tank dry, or in my case having my second stages freeflow the rest of my gas.

With this in mind, my rock bottom is still between 1700-1800psi because that is the worst case, however if I am at the anchor line at 50-60ft, I can hang out there until I am around 1200psi, then make the ascent. I have reserved 28cf for emergencies, and I will plan on using 3 of them, and come out of the water with somewhere around 1000psi if all goes well in the world.

 

[boulderjohn]

standard rock bottom calculation from here
DiveNerd - Rock Bottom Calculator, Imperial Units
2 divers, 1cfm SAC rate each, 77cf for tank size at 3000psi, 200psi buffer, so 1600psi to make the ascent, and a 200psi buffer to make sure the regs still work.

Now, the vast majority of divers can't actually do a 10fpm ascent rate, and this doesn't include a stop. This is my general rule of thumb for OW diving in terms of requiring 20cf/diver to get up from 100ft and why I don't believe in AL19's as pony bottles. If you are much shallower than 100ft, you don't really need one, and if you are deeper than about 80ft, you should at least have a 30 to give you some sort of buffer instead of carrying the bare minimum

The standards seem to be every changing.

The description I gave was based upon an article on rock bottom calculations written by Lamont, the administrator of the DIR Practitioners forum. I never head anyone criticize him for it.

I learned rock bottom using the supposed standard approach when I learned decompression diving, and I was required by my instructor to make those calculations before every dive. The purpose was to make sure we had enough gas to reach the decompression stop on which we would do our first gas switch, at which point the supposed OOA buddy could switch to his or her own decompression gas. There is a big difference between decompression diving (with its increased risk of DCS and the increased importance of completing the ascent as dictated by decompression procedures) and recreational (sport) diving (where an air sharing buddy team should be able to ascend directly to the surface, eschewing a safety stop, at 60 FPM without consequences.) I would certainly not advocate planning for the latter approach, but I do recognize that it can be done if needed.

 

[boulderjohn]

So you recommend beginning ascent with 32 cf or 45 cf?

I miswrote it--thanks for spotting it.

You should calculate what you will need to make a safe ascent with your buddy and begin your ascent no later than the point at which you reach that level.

 

[KevinNM]

The standards seem to be every changing.

GUE has recently simplified their minimum gas calculations to avoid the whole iterative process for min deco dives. So there are a few different approaches out there that are roughly comparable.