Maximum SAC for Tech diving?

[BeijaFlor]

Ok so now that the rebreather side is dealt with, back to the original question....

SAC (or RMV) is a funny thing. Generally, it does not matter in and of itself what your SAC rate is. What is important is that you know what it is and plan accordingly.

Lets say you have a RMV of 1.0 cuft / min (that is quite high). Your buddy is 0.5 (low)

All it means is that for a given dive, you will need to carry more gas, and your buddy will have to carry more for you in case of OOG. This will lead to running out of ability to carry tanks quite soon, thus limiting the dives you can do. It will also limit your buddies, which becomes problematic when the dives start to get expensive (Helium etc)

Either go shallower, go shorter, or bring the SAC down. Generally, high SAC is a symptom not a problem, in and of itself. Usually, high SAC can be attributed to one or more of the following:

Equipment issues (drag, streamlining etc)
Technique issues (overweighting, inefficient propulsion, unnatural breathing pattern etc)
Physical issues (overweight, unfit etc)
Mental issues (discomfort / unease in the water etc)

Addressing the first two, ie getting rid of drag, getting weighting correct and using efficient propulsion, will often give a dramatic drop in SAC. That will in turn help with comfort in the water etc.

Out of interest, do you have any dives where you can get average depth, dive time and remember how much you had left on surfacing? That will give us an idea of your rough RMV and from there we can judge the scale of the problem.

Thank you, Brendan, for pointing out the important factor -- knowing what my RMV is, so I can use it to plan my dive.

And thanks to everyone else for your advice ... I started this with Brendan's post, because he made so much sense and cued me in to the most important factor, knowing my gas-consumption rate so I can take it into account when I'm planning and doing my dives.

I have air-consumption figures for a few dives, and I come up with an average RMV of around 20 litres per minute, or about 0.7 cfm. Before I changed to side-mount doubles, though, I was generally the first diver in the group to get low on air ... probably part of that is that I wasn't doing much diving, and certainly part of it is that I am overweight and frankly unfit. (Not too bad for a man in his sixties, but certainly not as fit as I was in my forties when I learned to dive.) But I have no doubts that the best thing I could do for my gas-consumption rate is to get off my fat ass and exercise ... both to get more fit, and to get the fat OFF my ass.

Maybe when I get back to Subic Bay (probably in March or April), I'll be in better shape and better able to do a beginning tech course. I'll be working on it.

 

[BeijaFlor]

I don't recommend doing long bottom time at depths more than 40m that requires decompression unless you are trained for decompression. Anyhow; to stay longer at bottom two factors must be considered:
1. Best mix
2. Decompression procedure.

Again, if you are not trained for deco pro, please don't follow my comments!

if you want to make it deep to 43m (the Japanese wreck), let's first calculate your Best Mix ...

Next you need to calculate your SAC rate in order to calculate your gas requirements....

Again I want to emphasize that this is a deco dive, i.e. overhead dive. You can't surface immediately under any reason. You must stick to the plan and do all necessary deco stops as planned. Also your position and buoyancy is critical so you don't violate the planned deco stops. It is very critical business man and better to make it after you are deco qualified.

Roger wilco on that ... and mea culpa, I have transgressed into inadvertent-deco territory a couple of times in my diving; it's easy to do with a Suunto Zoop. The only thing I could do was to wait out the clock, under the boat, and ride out the embarrassment when I got back topside.

Planning for deco is another thing altogether! It's not something I'm going to do on my own, half-cocked. That's why I'm interested in tech training, to learn it right (which may be more than just getting "qualified") and to do it right. It's a higher level of awareness, discipline, and commitment ... and my SAC-rate question is more one of "am I fit enough to go tech?"

It's going to be a few months before my next dive trip. I'm going to work on my cardio and my weight, going forward ... and when I get there, I'll make sure I record my start/end pressures and work out my air-consumption rate for every dive. Then, maybe I'll have a better handle on "am I fit enough?"

Thanks again, everyone.

 

[Dave Bevan]

Adding another post:

Extreme SAC rates may be a symptom of some other issues but that data point alone is fairly unhelpful as a screening tool.

Carrying more gas or decreasing bottom time works just as well at any depth.

SAC rate is useful to know, but not a disqualification for deco/deeper diving.

Enjoy,
Cameron

This is the answer. SAC rate is rather irrelevant to be honest. As when you do your proper decompression course you will be taught how to plan your dive taking into account depth, time and SAC rate. So having a high SAC rate will mean you carry more gas but if your SAC rate is high because you are of a large frame then this shouldn't be a problem.

However if it is high because your trim is bad or you aren't streamlined or you are stressed etc then it's still irrelevant because an instructor worth his salt won't pass you until these issues are sorted. The best thing about technical diving for me was gas planning and actually forgetting about my SAC and just breathing.

Good luck!

 

[Searcaigh]

Maybe when I get back to Subic Bay (probably in March or April), I'll be in better shape and better able to do a beginning tech course. I'll be working on it.

My advice is that before your return to Subic Bay, contact Andy Davies (aka Devon Diver) and make some plans with him.

My buddy and I had a great dive on the Tabby last year as well as the Skyraider you can check it here unfortunately the photos are no longer linked though.

 

[Colliam7]

My question is -- what SAC rate is acceptable, for tech-diving training?

There really isn't a good answer for the question, unfortunately. Different people have different SAC / RMV rates. I know, sounds like a 'pablum' answer, but it is true. Compared to many people I dive with, I have a higher air consumption rate. I have been diving for a while. I have accumulated a few dives. I am very comfortable underwater, have reasonably good trim, am reasonably streamlined, use low intensity propulsion techniques, am in decent shape, etc., etc., etc. But, my 'working' SAC is 0.7 cfm, my deco SAC is 0.55 cfm. It has been that way for years, and doesn't get any better, or worse (and I periodically measure it just for grins). My wife, with much less experience (but equal comfort in the water, I must say) has a RMV of ~0.5 cfm. I dive with some folks who are ~0.45, one of them much bigger than me, with more mass to propel through the water. I just carry more backgas.

I recognize that I'm an air-hog, but I haven't done much (or rather, quite enough) to establish my SAC rate. Some dives, I've recorded my SPG pressures at the beginning and end ... other dives, I've neglected it. Admittedly, my next project on my next diving vacation is to record it, to calculate it, to nail it down.

That is a good plan. When you say that you 'recognize' that you are an air-hog, presumably that means that you seem to use air faster than your dive buddies. I encourage you to follow through with the idea of actually measuring your air consumption. You might be surprised.

Ultimately, you have to a) carry the gas you need to make the dive you want to make, and b) adjust the dive you make to accommodate the gas you are carrying. It is as simple as that. Do all the things that may help improve your air consumption (optimal weighting, trim, streamlining, etc.) But, don't fret too much about your SAC / RMV. Measure it. Track it over time. Adjust to it. And, plan your dives around it.

 

[divezonescuba]

I don't think anyone has mentioned this yet.

During TEC training your sac/rmv is likely to be much higher than it would be during a recreational dive or a TEC dive after some experience.

The instructor should be conducting drills etc that along with the unfamiliarity of the gear and skills would result in a higher consumption rate. Under normal circumstances, If the instructor does not push you to be proficient and if you are not constantly learning then you are not getting your money's worth.

So don't be alarmed if it is much higher during a course than an uneventful dive.

 

[Germie]

What is your consumption on a normal relaxed technical dive? If you can do only 15-17 minutes at 60m/200 ft on a twin12 liter, then you can think about taking a bigger twinset to get 25 minutes bottomtime. Mostly that is an option if you use 2 decogases. If such a diver wants to go to 80m or 100m? Taking a bigger twinset to get longer bottomtimes is not always an option. Why? You have to do deco too. And that means more gas needed for the high rmv diver.
Of course you can think about safety divers that bring gases, but that is not always an option.
CCR means oc bailout in case of problems. So still oc. And after a CO2 hit even the lowest gas consumption diver will have a huge consumption.

 

[Scuba Legends]

Dear Beijaflor,

I think you should worry a bit less about your SAC. I mean, it is good that you monitor it and learn. The more you learn about yourself the better it is. But if you are considering to take a tec course, well, that is why is a course. Choose a god instructor and allow him train you.

I am sure that with the ammount of dives you have you are a good candidate for a tec course. And tec diving is not only about your SAC. Not by far.

Happy Bubbles

Gery
www.scuba-legends.com

 

[rjack321]

At least here in Puget Sound, 0.7 cf/min is pretty normal for males with modest in water experience. 0.5 is on the low end, below 0.5 isn't rare but considered atypical. 0.9+ would be "high" I wouldn't agonize about yours if the 0.7 is in a drysuit in cool to cold water.

 

[Centrals]

Maybe when I get back to Subic Bay (probably in March or April), I'll be in better shape and better able to do a beginning tech course. I'll be working on it.

Good thing about diving in Subic Bay is the lack of current above or under water.
Physical fitness is really important because you will be swimming with a more bulky volume.
Get a proper tec log book and record everything including calculation, deco profile etc etc.
Take it easy and enjoy the hospitality of the country.