Maximum SAC for Tech diving?

[RainPilot]

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.

 

[elmo]

Rebreathers are great and are really efficient with gas. But they come with risks and take a lot of training and practice to dive well. if you're worried about your gas consumption and want to do a little bit of deco diving sensibly there are two obvious and very good options. 1 dive more and your sac will improve. 2 use larger tanks. It isn't more complicated than that.

 

[atdotde]

SAC to a large degree depends on how calm and relaxed you are during a dive. Just a little bit of nervousness can likely be spotten in your SAC when compared to other dives. And for this, only experience helps. There is a baseline SAC for each diver that can be approached this way (and at some point you will achieve this in all your happy bunny dives) and that in my experience is to very good approximation proportional to your total body weight (as it is dictated by your body's metabolism). Check it for yourself: For easy, relaxed dives compare the SAC to your dive buddies after diving by body weight and you will roughly find a constant.

 

[Kevrumbo]

I'm certified as a recreational diver, NAUI advanced with SSI Nitrox and PADI Sidemount/Wreck/Deep. . .

. . .I'd like to have the option of spending more time on the deep wrecks, for example the Japanese DC-3 "Tabby" wreck that lies at 140 feet (43 meters) in Subic Bay. . .

I recognize that I'm an air-hog, but I haven't done much (or rather, quite enough) to establish my SAC rate... but let me return to my question:

What SAC rate is acceptable, for tech-diving training?

Here's a representative Open Circuit gas consumption and tank(s) table range. You will find that most nominal pressure SAC rates fall between 1 to 2 bar/min per ATA with a particular cylinder or manifolded twinset of cylinders.

Below are some example pressure Surface Consumption Rate (SCR) values for a variety of common cylinders with an arbitrary volume SCR (also known as volume SAC rate or RMV):

Given a arbitrary nominal volume SCR of 22 liters/min per ATA (that's 0.78 cuft/min per ATA in US Imperial Units, a reasonable & achievable volume SCR for most novice divers, and exercise level for fit advanced divers with sustained active finning):

Cylinder Size | Pressure SCR
11L/bar tank (AL80): 2bar/min per ATA;
12L/bar tank (Steel HP100): 1.8bar/min per ATA;
13L/bar tank (AL100): 1.7bar/min per ATA;
15L/bar tank (Steel HP119): 1.5bar/min per ATA;
16L/bar tank (Steel HP130): 1.4bar/min per ATA;
11L Twins (Double AL80's): 1bar/min per ATA;
12L Twins (Double HP100's): 0.9bar/min per ATA;
16L Twins (Double HP130's): 0.7bar/min per ATA.

Given a arbitrary nominal volume SCR of 15 liters/min per ATA (0.53 cuft/min per ATA in US Imperial Units, relaxed with minimal finning for advanced divers):

11L/bar tank (AL80): 1.4bar/min per ATA;
12L/bar tank (Steel HP100): 1.3bar/min per ATA;
13L/bar tank (AL100): 1.2bar/min per ATA;
15L/bar tank (Steel HP119): 1bar/min per ATA;
16L/bar tank (Steel HP130): 0.9bar/min per ATA;
11L Twins (Double AL80's): 0.7bar/min per ATA;
12L Twins (Double HP100's): 0.6bar/min per ATA;
16L Twins (Double HP130's): 0.5bar/min per ATA.

Given a arbitrary nominal volume SCR of 11 liters/min per ATA (0.39 cuft/min per ATA in US Imperial Units, drift diving floating neutrally buoyant & going with the current):

11L/bar tank (AL80): 1bar/min per ATA;
12L/bar tank (Steel HP100): 0.9bar/min per ATA;
13L/bar tank (AL100): 0.8bar/min per ATA;
15L/bar tank (Steel HP119): 0.73bar/min per ATA;
16L/bar tank (Steel HP130): 0.68bar/min per ATA;
11L Twins (Double AL80's): 0.5bar/min per ATA;
12L Twins (Double HP100's): 0.45bar/min per ATA;
16L Twins (Double HP130's): 0.3bar/min per ATA.

The point is that SAC rate for most nominal activity is going to fall in between 1 bar/min per ATA and 2 bar/min per ATA. (And how easy is it to figure multiplying factors of numbers like 1 and 2?) --All you need now is a convenient time interval like 5 or 10 minutes and your metric depth converted to ATA as multiplying factors, and you will then know what your Depth Consumption in bar will be over that time interval with a particular single sidemount tank or backmount manifolded twinset tanks, and a corresponding level of physical activity. . .

So for example with a max RMV of 22 liters/min, your Subic Bay Wreck at 43 meters deep and using two single 11L/bar (AL80's) sidemount cylinders, using the table above you would expect to use 2 bar/min per ATA pressure SAC rate. The depth of 43 meters is equal to 5.3 ATA pressure. Therefore your Depth Consumption Rate is 2 times 5.3 equals 10.6 bar/min at 43 meters (5.3 ATA).

So checking the SPG of the sidemount 11L tank that you're currently breathing after 5 minutes of bottom time and a Depth Consumption Rate of 10.6 bar/min, you would expect to consume 5 times 10.6 equals 53 bar in that five minute time interval. . . Hence if you started with 190 bar SPG reading, you would expect after five minutes at depth the SPG to show 190 minus 53 equals 137 bar remaining. (Note: if you were breathing manifolded 11L twinset backmount tanks instead, you would be consuming 53 bar in a ten minute interval -makes sense?).

Finally, any increasing RMV going beyond 30 liters/min per ATA (1 cuft/min per ATA in US Imperial units) at depth 40 meters and deeper breathing Air or Nitrox is unsustainable because of work-of-breathing/increased gas density, and a dangerous predicament of over exertion spiraling into a stress condition that can quickly deteriorate into excessive metabolic CO2 retention, extreme narcosis and ultimately unconsciousness. If you cannot recover a slower, full and normal respiratory rate to efficiently expel the excess CO2 generated after hard physical activity at that depth, consider immediately aborting the dive.

CO2 Retention
Advanced Knowledge Series: The Gas Density Conundrum
NOAA Diving Physiology

 

[DevonDiver]

The actual SAC rate isn't as important as WHY the diver has that SAC rate.

On average, my students typically have a SAC between 18 and 24 Lpm. 12-18 is pretty good.

Above 24 and I'm asking WHY.... is it a cardio/fitness issue, or a low level of dive comfort or lack of skill finesse?

 

[Tom Ahola]

Of course, the SAC depends also on your body size and metabolic rate. There is a diver pair diving often in our group and it is quite funny looking odd couple as the small guy has small tanks 2x7l and the big guy has HUGE tanks 2x20l. When they go with rebreathers they are more matched. So I guess you can not set an absolute SAC recommendation. But you can always improve yourself to a certain point. I find it very useful to monitor the SAC during the dive and afterwards from the log. One can pinpoint where during the dive one gets too strained and where one is more relaxed and learn from that. I also experiment with different breathing techniques, and get feedback from the measurements. I dive with Suunto Eon Steel and Eon Core, with the Tank Pod. SAC is shown on screen in the dive computer during the dive and DM5 software can plot the SAC after the dive.

 

[hanymamdouh]

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

Best Mix = 1.4/(5.3) = 0.25 = 25% O2
EAD for 25% = 40m
Using No-Deco Air tables for 40m, your no-deco limit is 8 minutes. This will not increase your bottom time so much.
Your PN2 at 43m = 3.97 which is less than 4, hence narcosis will be manageable.
So, to extended your bottom time, let's say up to 20 minutes, we will use Buhlmann tables for simplicity using EAD 40m, your profile will be like this:

Descend to 43m (rate 20 m/min)
Stay at 43m for 18minutes
ascend to 9m (to start your decompression)
Stay at 9m for 2 minutes
Ascend to 6m
Stay at 6m for 6 minutes
Ascend to 3m
Stay at 3m for 10 minutes

I prefer also to add deep stop for off-gassing at 30m depth for 1 minute.

This plan assumes you will use 25% O2 mix for the whole dive.

Next you need to calculate your SAC rate in order to calculate your gas requirements. If you know your SAC rate simply use it to calc required gas for each level, and for big descend and ascend travel time based on deepest depth.
For example:
if your SAC rate is 20 l/min
Gas req for 43m = 20 x 20 x 5.3 = 2120L
Gas req for 30m deep stop = 20 x 1 x 4 = 80L
....
and so on
then sum all required amounts of gas together

Also I prefer to include the reserve into SAC rate before doing the gas requirements, I mostly use 1.5 factor.

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.

 

[Diver0001]

What SAC rate is acceptable, for tech-diving training?

What you will learn in technical training is that nobody gives a rat's ass what your SAC is as long as you are comfortable and you are not stressed out. The ONLY thing that is important is that you KNOW what your SAC is in different conditions and you need to be able to plan for the dive you're about to make, that's it. Part of the training is to make detailed measurements of your SAC and learn how to account for it.

That said, in my own little circle of divers, the majority of us have SAC's during technical dives in 10-12 l/m range (I think this is like .35 - .40 in imperial measure). If your SAC is .6 or .7 (18-20 l/m) then I really wouldn't care as long as I was convinced that it wasn't stress related. You might not have the range that I do but we'll always find a dive that suits your interests.

That's the way I see it.

R..

 

[northernone]

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

 

[RyanT]

@RainPilot, @Diver0001, and @northernone hit the nail on the head. One of the really important things you'll learn with technical training is how to plan and manage your gas for a particular dive. You learn what your own SAC is and then match your dive profile with your team's available gas (or vice versa). Either way, their should be no surprises.

I know my SAC rate quite well, but I also add in an additional "fudge factor" to ensure I plenty of reserve left. I made a hot drop in a ripping current earlier this year. On the descent, it got dark and I got disoriented with regards to direction and ended up swimming really hard on the descent. The huge exertion pushed my SAC through the roof and it definitely took a little time to bring it down once I quit swimming. Despite that unexpected exertion, my planning still ensured that I had enough gas to finish the dive according to my team's plan. So that's part of the beauty of tech training, you learn how to plan and manage these things.