Best way to Calculate RMV?

Drmike

that just doesnt make any sense whats so ever

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

mountain diver

here is a simple cash flow program:
if you die with cash in the bank
you have miscalculated.

DanDunfee

The discussions above seem to substantially UNDERESTIMATE expectable RMVs (and O2 consumption) which might occurr in a bailout situation, particularly one which might involve some panic, and especially if augmented by a CO2 or PPCO2 hit ( which might have caused the bailout in the first place.)

Heres some common RMV and O2 Consumption benchmarks:
Extracted / Interpolated From NOAA Diving Manual, 2001:
Work
Level...................................... Typical Swim....Typical O2
Description__ _____RMV, lpm__ __Speed,Knots_ _Cons., Lpm__
Light..................... 22.5........ ...... 0.6............... 1.1
Moderate............... 40.0 ...............1.0............... 1.8
Moderately Heavy.... 62.5............... 1.2............... 2.5
Heavy.................... 75.0............... 1.4............... 3.0
Extremely Heavy...... 90.0............... 1.6 ...............3.5

USN data are close enough to the NOAA data, to warrant not repeating here.

EC Spec. EN 14143 Test Criteria For Rebreathers. 2003:
Moderate.................40.0 ....................................1.8
Upper Test Criteria... 75.0......................................3.3 (Specifically selected to cover statistical probabilities for emergencies.
Still, less than the 90.0 and 3.5 actuals documented in the NOAA Manual above)

Remember that RMVs are typically quoted in STPD SE (Surface Equivalent) units. as they are above. For OC diving, they're the basis for at-depth, gas-demand estimates, as illustrated in Marks example above. For both OC and Rebreather diving the Physical, En-Situ RMVs are
not strongly influenced by depth. They are very strongly influenced by the divers physiological and psycological variability, and especially in heavy-work-load or hi-stress environments. Bailouts, especially those involving 'panic' and ' O2 or PPCO2 hit' elements, are especially threatening.

In this, and other, similar Rebreather World threads dealing with bailout, many of the statements focus on 'normal values' , rather than 'emergency values' of breathing and consumption. Some divers exhude confidence, and others actually assert that they can exercise control and restore breathing and consumptions to normal levels veeery soon after an emergency incident, even those involving panic,.. so they don't need all that much bailout reserves. IMHO, this represents extreme naivete based on inadequate appreciation for how REALLY BAD a critical emergency bailout, with a panic element, potentially augmented by a PPO2 or PPCO2 hit, can get to be .

Its true that not every bailout will represent a panicky, critical, emergency situation...or not to every individual...and when they occurr, the critical, emergency conditions will typically not last throughout all the remaining dive period. .. HOWEVER..

Based on a substantial body (vs a few personal incidents) of RMV and O2 consumption data, and major studies of the effects of panic and confusion on divers highly-variable performance during critical emergencies,,,, it is quite reasonable that, under critical emergency / bailout conditions RMVs and O2 consumption can approach,... and potentially exceed ....SEs of 75 lpm and 3 lpm respectively for a significant portion of the remaining bailout period. !!! For OC bailouts the SE RMVs then need to be corrected to at-depth pressures per Mikes example, to estimate total OC gas consumption. Yes, the resultant numbers are large, but must not be naively ignored or dismissed by wishful thinking !!

It's sometimes not practicable to carry bailout reserves for every possible combination of worst-on worst-on worst case scenarios. This is especially true for bailouts near the end of deep, long dives ... therefore, significant compromises must sometimes be made, and with that comes increased probablistic risk. However the tradeoff estimates should, at least, start with the virtual certanty that emergency / bailout RMVs and gas consumptions will shoot up to levels well above 'normal' values. The critical questions, whose answers must be embedded in the pre-dive planning are....how high will they go??....and for how long ?? You, must choose those estimates for yourself..and assume the risks that go with the choices.

While one certainly needs to 'practice' switching off from primary to bailout gas sources to develop it as an intuitive / instinctive emergency procedure...this practice doesnt, and can't, simulate how bad it can become under real critical emergency conditions. IMHO it's extreme naivete to assume that it does for estimating your RMV and consumption in critical emergency conditions. I specifically disagree with use of a SE RMV of 40 as an initial emergency bailout estimate. It should be more like 75.

It remains then, for each diver to first ensure the thoroughness of his own training and education, then understand the implications and risks of each option,.. then make the benefit / risk tradeoff decisions, and imbed them in comprehensive pre-dive planning. I, personally, am a Conservative (Chicken , if you choose), and whenever practicable, I try hard to carry / provide sufficient bailout reserves to get myself (and hopefully my buddy) all the way back home. Failing that, I try hard to deduce the most probablistic risks based on: dive conditions & profile; dive buddy(s); and surface & underwater suport factors; make the best choices I know how at the time, and leave the rest to God.

Its a ' take-your-own-chances and pay-Davy Jones-his due" game. My basic philosophy is in my signature line below

__________________
There are 'Bold' divers and old divers, but NO old, ' Bold ' divers

DanDunfee

I've extracted the text from a recent msg I sent out offline from Rebreather World...and posted it below as info. It's one example of a planned Bail Out profile, integrated into overall diving gas management planning. Note that it's for planned ND dives with a small, recreational Rebreather rig, not a deep diving technical rig.

I hope this helps some.
===========================================prior msg text below

Thanks for feedback. Your and others feedback helped a lot . It turns out that there's some reasonable consistency among the more seasoned, frequent divers that 'average out' at about 200 l , or about 1 l x 200 Bar, or 6.5 cu ft (SE) , for a 'typical 1 hr dive', including Loop, Dil & BC functions (excludes BO). Obviously theres substantial variation, driven particularly by how 'plateaued' the dive profile is .... But I'll use 200 l / 6.5 cu ft (SE) as my 'average design point'.

As i said before, my philosophy for rec diving is about 'Semi-Siege' vs 'Alpine',, so I'm leaning toward the following design points for the first iteration, of the little RGU 2 mCCR that I'm working up.

RGU Scrubber w/ 2 Tandem Design Factors of Safety (FS)
= 'Back on the Boat' with 20% FS + 20% FS for Efficiency & Ullage.
1 kg, 8-12 mesh SofnoLime, 140 l CO2
= 66 minutes SE @ 70 * F
= ~ 45 min ND dive, with ABT = 30 min @ 99 ft & 70 *F,


O2 Supply w/ 2 Tandem FSs = 2 repetitive boat dives, ~1 hr ea as above, each avg. 200 l, 400 l tot
= 2 l x 200 Bar, or 13 cu ft.

Nitrox Supply w/ 2 Tandem FSs
= 200 l, or 6.5 cu ft per dive or
400 l, or 13 cu ft tot for 2 ND dives
++ Plus ++
Reserve for Bailout from incident near the end of the second 99 ft, 30 min ABT ND dive
Bailout profile model=
1.0 min @ 99 ft @ semi-panic RMV of ...........63 lpm (SE) to asess emergency, go on OC, & start ascent
1.1 min transit, 99 to 66 ft, @ avg. stabilizing 50 RMV
1.1 min transit, 66 to 33 ft, @ avg. stabilizing 45 RMV
1.1 min transit, 33 to 0 ft, .@ avg. .stabilized 40 RMV (moderate work load)
Tot Bailout Reserve (SE) ................ 600 l or 21 cu ft.
=
Total Dil + BC + BO Supply (SE)..... 1,000 l or 35 cu ft


This gas supply config provides for 2 nominal repetitive boat dives as defined above without recharging or swapping out tanks...... only one swap-out of prefilled refillable RGU scrubbers. I like the notion of fresh scrubbers for each ~1 hr dive.

I have to confess to a secondary objective in a 1,000 l or 35 cu ft Nitrox supply for the little rec rig. I also intend to do an SCR variant of the RGU 2, particularly since it provides a very good pairing with the intended ~ 1 - 1.25 kg class scrubbers. Either CCR or SCR dives are limited at-depth by the intentionally smaller scrubbers. ..... and my ole body is very satisfied, but not worn out by ~ 1 hr dives ;-)

Just to keep / remember perspectives, this little rig will provide about the same performance & safety factors for comparative 99 ft dives as a pair of 110 cu ft OC tanks per dive. Not too shabby ;-)

I'd be interested in your comments.

Best Regards

Dan

__________________
There are 'Bold' divers and old divers, but NO old, ' Bold ' divers