average sac rate

[Guy Alcala]

I have noticed a similar effect. When solo I don't have to focus on anyone else, turn to keep them in sight, or swim at any speed other than the one I am most comforable and efficient with.

That adds up to a lot less muscle movement particularly the large muscles oin the trunk of the body.

When solo, you can also relax to a much greater extent.

I also have noted that when I am cave diving and using lung volume for precise buoyancy control, my SAC also goes up compared to OW where the bottom may be flat and where frequent bouyancy changes are not needed and/or an extreme degree of control is not needed.

When you add it all up, it equates to a much lower sac rate in solo OW situations.

Same/same, except for the cave comments which I have no experience with yet. I find there's another advantage of diving solo: less severe neck rash from the latex neck seal on my dry suit (or the skin-in neck seal on my wetsuit), because I'm not constantly rotating my head to check on my buddy. Aside from the discomfort while diving, if I do a lot of dives on a weekend it's nice to not look as if I'd survived a hanging for the rest of the week

Guy

 

[TomP]

my sac, rmv, or however you'd like to characterize consumption rate is just plain bad. always has been. i'd be thrilled to be .6ish. that said, it is less bad solo.

 

[carrielsal]

my sac, rmv, or however you'd like to characterize consumption rate is just plain bad. always has been. i'd be thrilled to be .6ish. that said, it is less bad solo.

Could you be overweighted?

Carrie

 

[jridg]

Define 'bad' - My SAC rate is between .55 and .7 - has been for years. This is why I dive a HP120. I am in excellent condition, I simply take deep, slow breaths, never skip-breathe, and deal with what comes along. I nearly always run out of bottom time before I run low on gas. My average rate hovers around 0.65 solo or with others it tends to be the same. When planning for emergencies, I tend to plan with a 1.0 rate as I feel that is realistic for working/dealing with a problem.

I would love to have a rate of .3-.5 - just don't see it happening, ever.

 

[TomP]

I'm generally about .7 + or -. My level of fitness has varied over the years with no real corresponding changes in sac. My buoyancy and trim are both fine.

I only care because I'd like to get 2 nice long dives (100 - 130 ft range) out of my double 119's.

 

[cruiser]

My SAC is always lower when solo-ing.

 

[mikemill]

Been working on my drysuit which has actually caused my SAC rate to increase while I regain my comfort level with it.
Two buddy dives before this last Saturday: 0.85, 0.82
Two solo dives this last Saturday: 0.72, 0.61
One buddy dive on Sunday: 0.81

Hmmm yeah, a bit of a difference.

 

[dkktsunami]

My solo RMV is always less than my non-solo RMV. Usually, when I'm solo, I'm in the .3-.4 range.

Same experience for me and it does not matter who the buddy is. Could be a world-class scuba diver and I would consume more gas. It is the implicit responsibility in the back of my mind and the little bit of extra motion looking and checking.........I am most at peace in the water when I am alone.

 

[dadediver]

SAC doesn't specify volume, pressure or mass for that matter. It's an imprecise term. I think pressure is a fair measure of consumption, and one that divers traditionally utilize (eg: Q."How much air did you use?" A."1200psi") -- provided the cylinder size is constant.

The term RMV (Respiratory Minute Volume) is very clear and precise. It does not need conversion when changing cylinder sizes or types.

But isnt pressure only part of the math. A 19cuft pony and an 80cuft tank both holds 3000 psi, however they have different amounts of volume to breath from. So you can really use just psi for gas monitoring. Not gas planning only.

What I was taught in my solo class was:

To find your SAC rate:

volume/pressure
80cuft tank/3000 = .02666cuft. This is your tank baseline

Now watch tv or whatever with your reg in your mouth and mask on.
Breath on it for a period of time. Then write down the pressure pressure used.
So I started at 2900 psi in my tank and ended at 2600 psi. I used 300 psi for 22 minutes

.02666 / 300psi = 7.99 cuft

7.99 cuft / 22 minutes = .363 which is my "sitting" SAC rate.

I heard of some divers actually excercise doing this a second time and that will be their working SAC rate.

If I recall correctly, SDI uses a 1 through 4 number for the stressors. So if you are diving in currents that is stronger than you are used to, you will multiply this stressor to your sitting SAC rate. .363 x 2 = .726 is my new SAC rate to use to find my required gas volume needed. Now say I am diving in these strong currents and am diving in cold water that I am not used to, I may multiply my stressor by 2.5 so now
.907 is my new sitting SAC rate.


This is what I carry in my dive kit to help me remember the formula's:

SAC Rate per tank size Averaging SAC Rate
80 cuft tank baseline. Base line tank volume divide volume used
80 cuft/3000 psi = .02666 cuft .02666/300psi = 7.99 cuft

100 cuft tank baseline Cuft/ minutes of gas used
100 cuft/3000 psi = .03333 cuft 7.99 cuft / 22 minutes = .036 SAC RATE

19 cuft tank baseline
19 cuft/3000 psi = .0063


Require Gas volume needed

(Depth/ 33) + 1 = ata
80’ / 33 +1 = 3.42 ata

Ata x sac rate = cuft per minute
3.42 x .5 = 1.71 cuft per minute

Dive factors of 1 2 3 4. of stressor factors

Dive factor of 2 x calculated cuft per minute
1.71 x 2 = 3.42 cuft per minute

Cubic feet per minute x planned dive duration
3.42 cuft per minute x 20 minutes = 68.4 cuft total dive time

And this profile exceeds the diving 3rds on a 80cuft tank.

 

[derwoodwithasherwood]

But isnt pressure only part of the math. A 19cuft pony and an 80cuft tank both holds 3000 psi, however they have different amounts of volume to breath from.

Absolutely correct.

One could liken it to driving a car, and watching the gas gauge. If you are driving your own car, you can glance at the gauge and decide if you have enough gas left to make a particular journey based on your knowledge of that particular car and how many miles you get to a tank. You don't have to do all the math to be pretty accurate in your assessment. That would not necessarily apply to a different car, with different fuel consumption and a different size tank.

So you can really use just psi for gas monitoring. Not gas planning only.

I disagree -- partially. Again, if you are driving your own car, you know that a full tank will get you X number of miles. You can look at the gas gauge and decide if you will make a particular journey without refueling. Now, if you are driving a different vehicle that you are unfamiliar with, the story changes. In that case you need to find out how much the tank holds and the typical mileage of that particular model and do some math.

This is roughly analogous to switching scuba cylinder sizes. If you always dive single AL80s, your direct experience can be used to plan future dives based solely on pressure. However, this approach is invalid if you suddenly find yourself with an HP100 or a steel 72 on your back. In that case, you must determine your RMV and do the math to properly plan the dive.

If I recall correctly, SDI uses a 1 through 4 number for the stressors. So if you are diving in currents that is stronger than you are used to, you will multiply this stressor to your sitting SAC rate.

Absolutely. Again, back to the car analogy: if you are tooling along in the slow lane at a steady 55mph, you will get better mileage than if you are working the gas pedal harder.

But again, it doesn't matter if you are basing your calculations on volume or pressure -- more is more either way. You can still plan your dive based on pressure calculations provided your original data is based on (or has been converted to) the particular cylinder size you will be using.

What matters here is that you plan the dive one way or the other, and some agencies teach it one way, some teach it the other. If you use a pressure-based approach, then you must convert the pressures to volumes when switching cylinder sizes. If you use a volume-based approach, then you must convert to pressures when planning and from pressures to record your consumption after the dive. Both approaches are correct and ultimately achieve the same goal.

I would suggest that the pressure-based approach is probably more convenient for someone who generally uses the same cylinder size over and over, but that the volume-base approach is easier for someone who commonly uses a variety of cylinder sizes.