Rock Bottom

[c555]

Hi, I know this has been discussed before, but I have a question about rock bottom. I understand the whole concept, but in this article http://www.direxplorers.com/new-dir-articles/1376-rock-bottom.html, (sorry about it not being a link, you may need to copy+paste) it says that rock bottom, the amount of gas needed to got 2 divers to the surface safely, should always stay in your tank. In the first example it says that for a 60 ft dive on Al80's, the rock bottom is 400psi and they need to be on the boat with 400psi left. My previous understanding, which may be wrong, is that you could begin your ascent with 400 psi. My thinking is if you have a failure at 60 ft and you still have 400 psi in your tank, that is still enough to get two divers to the surface safely based on the calculations in the article. If nothing goes wrong and the dive goes as planned, then you should only end up using less than 200 psi for the ascent. If you end the dive with rock bottom in your tank, you could have a much shorter dive and reserve air that wouldn't be used, even in an emergency. I just want to know what way is right. Thanks

 

[cool_hardware52]

What I was taught:

Rock Bottom is never less than 500 psi. This is based on the accuarcy of SPG's at the low end of the range, and the fact that first stage regs need more than zero tank pressure to work.

You need to leave the bottom and begin your ascent when (or before) any team member reaches RB.

Moving shallower will reduce your required rock bottom, i.e. 100 ft on an AL80 will typically require a minimum of 1000 psi. 60 ft will typically require 700 psi. If you hit 1000 psi at 100 ft and reach 60 ft with 900 psi remaining you can stay at 60 until you reach 700.

If you have correctly calculated RB, and do leave the bottom at or before reaching RB, and do a proper incident free ascent you will reach the surface with substantial gas in your tanks.

 

[Charlie99]

See http://www.scubaboard.com/showpost.php?p=401869&postcount=65 for a better explanation. The DIR Explorers calculations are in error.

For example

Two divers with a “SAC” rate of 0.5 at two “ATAs” for two minutes would consume two cubic feet of gas (2 divers * 0.5 SAC * 2 ATAs = 2 cuft).

obviously is off by a factor of 2, even assuming 0.5cfm SAC during the airshare.

2 divers x 0.5cf/min * 2ATA avg * 2 MINUTES is 4 cubic feet.

His calculation also omits any stops.

It also omits any SPG error factor and the 150psi or so needed for regs to properly work.

Even the crude "100psi per 10', but never less than 500psi" rule is better than relying upon incorrect calculations.

p.s. His calculations that result in Rock Bottom of 600psi at 100' on an AL80 is also signficantly less than either the 1000psi of the 100psi per 10' rule, or the Rock Bottom calculations of the SB post linked above.

 

[c555]

See http://www.scubaboard.com/showpost.php?p=401869&postcount=65 for a better explanation. The DIR Explorers calculations are in error.

For example obviously is off by a factor of 2, even assuming 0.5cfm SAC during the airshare.

2 divers x 0.5cf/min * 2ATA avg * 2 MINUTES is 4 cubic feet.

His calculation also omits any stops.

It also omits any SPG error factor and the 150psi or so needed for regs to properly work.

Even the crude "100psi per 10', but never less than 500psi" rule is better than relying upon incorrect calculations.

p.s. His calculations that result in Rock Bottom of 600psi at 100' on an AL80 also has errors.

I wasn't trying to get at what specific rock bottoms are, I was/am trying to figure out if you begin your ascent when you hit rock bottom or if you and the dive with rock bottom still in your tanks. Thanks for the help though.

 

[Charlie99]

I wasn't trying to get at what specific rock bottoms are, I was/am trying to figure out if you begin your ascent when you hit rock bottom or if you and the dive with rock bottom still in your tanks. Thanks for the help though.

You always have rock bottom in your tank, but the rock bottom gets smaller as you ascend. When you get down to around 1000 psi, you should be above 100'. By the time you get down to 700psi, you should be shallower than 60'.

 

[cool_hardware52]

You always have rock bottom in your tank, but the rock bottom gets smaller as you ascend. When you get down to around 1000 psi, you should be above 100'. By the time you get down to 700psi, you should be shallower than 60'.

It's worthwhile to remember that this example is for an AL80. Different tanks will have different RB's for the same depths.

1000 psi in an AL80 is about 25 cuft. In a LP 80 25 cuft would be about 825 psi, and in a HP 130 it would be about 660 psi.

I find it helpful to consider RB requirements in terms of Cubic Feet. That makes converting to various tanks, doubles etc simple, at least for me.


Tobin

 

[jonnythan]

The rock bottom pressure for a depth is the absolute minimum amount of gas you should have in your tanks while at that depth, because it represents the amount needed to get you and your buddy to the surface safely from that depth.

So, you can certainly end a dive before hitting rock bottom.. but you *must* begin your ascent when you do hit it. If my rock bottom for a 90 foot dive is 1000 psi, I must begin my ascent at or before I hit 1000 psi at 90 feet.

 

[c555]

Thanks for the help eveyone. It seems like contrary to what the article said you begin your ascent when you reach rock bottom. Thanks for the clarification.

 

[TSandM]

I'd have to go back and read the article, but I'm sure what was meant was that you MUST begin your ascent when anyone in the team reaches rock bottom for the current depth -- and that's a direct ascent. If a direct ascent is not feasible, you use a different gas management strategy and begin your exit far earlier. You never continue a dive once a team member is at rock bottom.

If you think about it, rock bottom is a generously calculated MINIMUM amount of gas to get you and your buddy to the surface. If you are AT rock bottom, then that ascent might theoretically use ALL the gas in your tank (the generous padding makes that unlikely) and gauges are uncertain at the low end, and regulators need some pressure to function at all. Better to plan the dive so you never actually get there.

 

[jonnythan]

"it says that rock bottom, the amount of gas needed to got 2 divers to the surface safely, should always stay in your tank."

Just for clarification, this means "stay in the tank.... during the dive." In other words, while at depth. It would have been more clear to say "should always stay in the tank until you begin your egress, whether this consists of heading back to the cave entrance or simply ascending directly to the surface. Rock Bottom must be calculated with the entire egress in mind."