RMV math…

[scubadada]

Out of curiosity, what’s the particularity of the Spiegel Grove? Is that next to a shipping lane?

Brisk current on a blue water ascent can move you quite a way, even finning into the current. I would hope you have your DSMB with you too

 

[rsingler]

Out of curiosity, what’s the particularity of the Spiegel Grove? Is that next to a shipping lane or in an area which has strong currents?

Extremely popular wreck site in Key Largo, FL.
Current is indeed unpredictable and more than one trip out to the site has been aborted when the captain sees the mooring buoys underwater.

Thousands of divers annually on a wreck that is very large (300') in 135' of water with nothing else around. Shallowest part of superstructure is 75', and there are lots of cutouts from 85-100' that tempt divers to make penetrations that start out looking like swim-throughs. At 100' with the end of the corridor "sortof" in sight, it's easy to suddenly find yourself low on gas.
DM's on cattle boats that run a conga line thru the wreck often carry a pony. More than one death, including an OOA/lost in the wreck. CCR and Advanced Wreck training and running line was a blast deep in the hull. I love it.
Needless to say, you need to gas plan for that "find my way out" segment before your begin your ascent.

Turn on subtitles/closed captioning - YouTube stripped my sound track for a copyright violation. You can watch me struggle learning to run line.

Then my son made this one on the SG while he was instructing down in FL...
Looking for the Snoopy mascot medallion is just beyond recreational limits (about 145' counting linear run and depth), and folks routinely scare themselves trying to find it. It's the SG "got the t-shirt" challenge.

 

[scubadada]

Ha @rsingler mooring balls underwater, been there done that on the Spiegle Grove, Duane, and Bibb

Once, on the Castor in Boynton Beach, I swam down the line during a very, very brisk south current. I barely got the line secured on the bow. The line had two big balls, both were underwater. The first was quite collapsed, the second was just a little below the surface. Made it a bit difficult for the divers following me to hit the descent line.

 

[rsingler]

Oh dear! Another ScubaBoard off-topic thread wander...

 

[scubadada]

Oh dear! Another ScubaBoard off-topic thread wander...

But a good one, swimming around on the Castor that day took much more effort and gas consumption over the usual. Be prepared. Watch your gas.

 

[Kendall Raine]

I have calculated my sac (though at this point it is all over the place). I think I understand the concepts but can’t put it into practice yet.

Bravo. You're on the right path.

Your SAC will be all over the place depending on circumstances regardless of how many dives you do. Cold, stress, gear package, work load, pre-dive fatigue and technique all influence consumption. After thousands of dives my SAC still varies between .35 and .6 depending on what I'm doing where-swimming multiple steel tanks with deco bottles in deep dark cold water dry is very different from a warm reef dive wet in an 80 is different from a deco hang or a scooter dive. If I live to do another couple thousand dives I doubt very much that will change. With experience your variance will decrease and become easier to predict, but it will naturally remain depending on factors mentioned.

This will be true for your dive buddies, as well, which will factor into your assessment of rock bottom for any given dive. Frankly, the best time to assess consumption is after a screw up/problem. That is, by how much did the mistake/problem change your (or your buddy's) consumption? Accident analysis typically illustrates a cascade of problems where one (initial) problem precipitates perceptual narrowing/breathing rate changes resulting in mental mistakes leading to a bigger problem. Keeping your breathing rate in control during a problem is the basis of resisting/breaking the cascade sequence and working out of the problem. Being aware of/focusing on maintaining an effective slow deep breathing rate helps avoid mental confusion/panic and preserve situational awareness.

 

[scubadada]

...Your SAC will be all over the place depending on circumstances regardless of how many dives you do...

But can still be quite valuable in dive planning

I have my RMV for my last 1,792 dives since mid-2010. I have less variables than @Kendall Raine but these dives cover water temperatures from 47-85 F, current from zero to ripping, unlimited to terrible visibility. All dives are in a wetsuit, single backmount cylinder +/- pony, less than 15 min deco.

So my RMV is 0.36 +/- 0.04 cu ft/min (mean +/- std dev). So 95% of my dives statistically fall between an RMV of 0.28 and 0.44, pretty tight. The real range is 0.24-0.63. Importantly, I know that other than exertion level, being cold is the most important determinant of my gas consumption. Psychological stress plays a role but is difficult to identify and quantitate.

I generally know the approximate conditions I will be diving in and can easily pick an RMV to use in gas planning. I have practiced this quite a lot.

 

[Belzelbub]

Your SAC will be all over the place depending on circumstances regardless of how many dives you do. Cold, stress, gear package, work load, pre-dive fatigue and technique all influence consumption.

Absolutely correct. SAC will vary considerably based on what is going on with the dive. I don't really do cold water dives, so that's not a factor for me. I also rarely do leisurely dives just do dive. I'm almost always doing something else. As such, my range can be quite wide.

For example, since I've been tracking, my min SAC was 0.42 cu ft/min. Max was 1.35. The max was a bit of an outlier. Short dive down to 85' to recover an anchor. Anchor was big and heavy and pulled up by hand along my other anchor line.

More typically, I'm in the 0.5-0.6 range, maybe up to 0.7 when I'm spearfishing. Drops back down to 0.5 or so when I'm diving at an aquarium cleaning the habitats. Worst at the aquarium was 1.08, but that was for a show where we were shooting "ghosts" with bubble guns. Those things use a lot of air.

Even though the rate will vary with conditions, you should be able to get a feel for what it should be and identify any outliers.

 

[60plus]

Absolutely correct. SAC will vary considerably based on what is going on with the dive.

That statement is incorrect, SAC is regarded as a diver specific constant It does not vary with what is going on with the dive. It is the actual gas consumption that matters. This is the main point I was making and why I feel that exact calculations based on SAC are not necessary for recreational / no stop diving (remember this thread started in the basic section). The fact is that after very few dives any diver should have a fair idea approximately how long a given cylinder size will last at a given depth or dive profile. I knew this pretty well after less than 10 dives and after 15 dives (which included several to over 30 m) I knew it as accurately as I would had I done a SAC calculation. So many things can change your gas consumption, a few as follows, being buffeted by waves whilst hanging on a line waiting to descend, delayed descent because someone struggles to equalize, insufficient recovery time before descending after long surface swim, a slight increase in current or slight increase in pace by dive leader can make a huge difference, stress.
An example of where a SAC calculation is a bit pointless is the following. I do 2 dives in a day to about 33m on nitrox 32 with about 1 &1/2 to 2 hours between them. However on the second dive for many of the divers and the dive leader it is the first dive of the day and quite a bit of the dive will be at 30m +. Its not my SAC I am concerned about its my NDL. So on the dive I make sure I am one of the last to descend and hang up a bit on most of the dive group so I am at say 25 - 28 m instead of 33m. I was on a night dive in the sea a couple of months ago, one buddy was a very experienced instructor, the other newly qualified. Whilst still no deeper that 13m we twice encountered nylon monofilament fishing lines. The newly qualified instructor (first night dive) got very nervous, her gas consumption shot up and the experienced instructor called the dive.

 

[Belzelbub]

That statement is incorrect, SAC is regarded as a diver specific constant It does not vary with what is going on with the dive.

Seriously? My SAC/RMV numbers over many dives say otherwise. I posted examples of how they vary based on what I’m doing. Exertion requires more oxygen than a leisurely activity. This is true on land and underwater .

I think I’m now understanding your reluctance to knowing your typical SAC. You have no idea what it is. It is certainly not a diver specific constant that never varies based on activity.