RMV Spinoff from Accident & Incident Discussion - Northernone - aka Cameron Donaldson

[johndiver999]

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A ScubaBoard Staff Message...

The very interesting subject about RMV has been moved from the Accident & Incident Discussion - Northernone - aka Cameron Donaldson

Please continue the RMV discussion here.

Thank you!

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Can someone provide some input about the gas management procedures or constraints that would
arise from a dive with 30 minutes of deco and a max
Depth of around 150 - which seems ???? To be the relevant profile.

More specifically what kind of bottom time and air
Consumption rate and volume would be expected for a diver to accrue 30 or more minutes of deco for a dive that was presumably shallow at the start with an excursion to deep over the wall and perhaps back onto and over the shallower reef.

I’m trying to envision the time at depth (and depth) required to get that much deco for a non repetitive dive and what kind of emergency reserves of gas supply would be necessary and available with 2 80’s?

I don’t have the software to run the numbers myself for a potential profile and don’t have a good intuitive feel for the amount of gas it takes to get that much of a hang and how much it takes to clear it and how much reserve needs to be available to handle a tank/regulator failure.

I know using a scooter could reduce exertion levels and air use tremendously.

I looked at a navy table and did some arithmetic on an envelope with a straight down and up profile, and it looks really tight to me with respect to potential air reserves that kind of setup would provide for a dive to 150 that yields 30 minute of deco.

I’m sure the math is not challenging for tech divers who know how to use software. Does the profile being described as usual make sense?

 

[scubadada]

As we don't know Cameron's dive profile, his usual RMV, or his computer settings, it is not possible to begin to estimate his possible gas use.

It is quite simple to calculate the gas requirement for a hypothetical air dive to 150 feet, with a RMV of 0.5, that results in about 30 min of deco when a computer is set at 50/85. Any of the variables can be easily changed. In this example, the 21 minute dive would have a gas requirement that is well below that available in two AL80s.

 

[scubafanatic]

No access to a deco program at moment, and many many years since diving either air or open circuit, but off the top of my head if diving air and accruing 30 mins of deco at max depth 150ft, then I would think two 80's (of air) would not be sufficient gas to complete said dive / deco (although a scooter should - all things being equal - alleviate the workload / RMV).

But let's see what transpires when someone cuts an actual table for the dive you are proposing.

I'm a very experienced recreational diver (dabbled a little in tech many years ago as a new diver but never pursued it seriously due to the $ and training requirements) but offhand I'd agree that (2) Al 80's isn't enough gas to safely do "accruing 30 mins of deco at max depth 150ft" type dives. I also do carry quite an abundance of safety/signaling/PLB devices as my preferred trips are deep, open ocean, very remote, 3rd world liveaboard style diving. (marine radio/McMurdo Fastfind MaxG/(2) lights/Jotron NATO-grade strobe/mirror/whistle/air horn/(3) SMB's/(1)safety 'streamer'/Halcyon blunt tip titanium knife/BCD shoulder pad 'reflectors'/reef hook/spare mask/(3) computers + SPG). I'm thinking of adding in a sat phone, and a practical way to carry some energy bars/water as well.

 

[scubafanatic]

As we don't know Cameron's dive profile, his usual RMV, or his computer settings, it is not possible to begin to estimate his possible gas use.

It is quite simple to calculate the gas requirement for a hypothetical air dive to 150 feet, with a RMV of 0.5, that results in about 30 min of deco when a computer is set at 50/85. Any of the variables can be easily changed. In this example, the 21 minute dive would have a gas requirement that is well below that available in two AL80s.
View attachment 510878

Does that accommodate contingencies (downdrafts/fighting currents/etc.) ?

 

[scubadada]

Does that accommodate contingencies (downdrafts/fighting currents/etc.) ?

Of course not, it is the straightforward profile I described. A change in the profile, an increase in the RMV, etc., would alter the result.

 

[Kay Dee]

Of course not, it is the straightforward profile I described. A change in the profile, an increase in the RMV, etc., would alter the result.

0.5? IMO, a generous RMV given the supposed conditions, scooter or not.

Be that as it may, seemingly by those calcs, uping it to even 0.8 would have still made it (with not a lot to spare, admitedly). I'm somewhat surprsed. Shows how long ago I had to calc an OC dive I suppose.

 

[scubafanatic]

Of course not, it is the straightforward profile I described. A change in the profile, an increase in the RMV, etc., would alter the result.

'Alter the result' is putting it mildly.

 

[rjack321]

Does that accommodate contingencies (downdrafts/fighting currents/etc.) ?

An RMV of 0.5 seems pretty unrealistic at this site on this day. Especially in light of other dives in the neighborhood and "clawing back up to 70ft" kinds of reports. That said, I don't think 10mins on air at 150' in Coz with 2x al80s is totally crazy talk. It's not for me, and I think the risk profile for deep-ish air isn't worth it, but not insanity.

Looking at the 6.6g/L density of air at 150ft gives me the shivers. The air and the deco were no doubt accentuating the risks, but like most incidents there's rarely just one thing to point to as a conclusive flaw. Right now we have solo, very high currents, dense narcotic gas, depth, likely deco, and perhaps a personal tendency to turn around "late" all pitted against relative youth and extensive site experience. That's a lot in the "against" column and not much in the "asset" column.

 

[scubadada]

0.5? IMO, a generous RMV given the supposed conditions, scooter or not.

'Alter the result' is putting it mildly.

Just a simple, hypothetical example, with the many caveats I pointed out. You can easily change any of the variables you'd like, feel free

 

[Kay Dee]

Looking at the 6.6g/L density of air at 150ft gives me the shivers.

While I am a big advocate of diving gas, and with ALL due respect, I am constantly surprised to hear that people would consider air - especially in warm clear water - at 45m / 150ft so thick, or 'cold' - in the above context - to give them the shivers, if you get my drift. That is, air below what, 40m (or even 30m for GUE, no?) is to be avoided almost no matter what? No offence meant, as this is a general comment i.e. not directed at the poster - but methinks way too much "cool-aide" has been drunk since helium became popular.