SAC in the water or out?

[knotical]

The first three-quarters of this post is all preface:

There are several good ScubaBoard threads discussing SAC, including a couple of recent ones started by Jax:
http://www.scubaboard.com/forums/basic-scuba-discussions/347007-sac-vs-surface-rmv.html
http://www.scubaboard.com/forums/sdi-tdi-erdi/347113-sac-vs-rmv-what-standard.html

Reading thorough them, It's clear that different folks use different methods. One difference is SAC derived arithmetically from dives vs. SAC measured on the surface.

But there's one thing I didn't see mentioned (maybe I missed it).

If one measures SAC on the surface, you can measure it out of the water while sitting, (as recommended in at least some of the SDI literature including the Solo Diving Manual); or you can measure it while floating on the surface. < You could also measure it lying down out of the water, but I haven't tried this – yet. >

The last quarter of this post is the point:

I was surprised to find that my "SAC" while resting on the surface in the water was dramatically better (40%) than while resting in a chair out of the water. Try it yourself and see how much yours changes. I suspect the reasons are mostly physiological, so I shouldn't be the only one seeing a big difference.

Thoughts?

 

[String]

2 things, floating on the surface means no extra effort required by heart to move blood UP to the head opposed to sitting and if the face is immersed i'd guess the mammalian diving reflex kicks in as well. Both of these would reduce the workload required in particular by the heart so lower the SAC. I can certainly hold my breathe far longer face down on the surface than i can on a chair.

Personally i see measuring SAC out of the water for diving to be completely pointless. There are so many factors that affect it such as equipment mass, suit tightness, temperature, muscular actions swimming vs sitting etc i class an out of water figure as pointless.

Im also not keen on inventing some magic guesstime "depth factor" to multiply by to get an RMV. Now you're taking a very inaccurate out of water figure and multiplying it by an unknown guessed figure - this can lead to massive inaccuracies.

To me, the only sensible way to get this to effectively get a baseless "RMV corrected for surface". In other words do a dive, take *accurate* air readings and time readings and swim at an EXACT depth for 10 or 20 mins at normal dive pace then take an accurate end reading. The smaller the tank the more accurate this method will be.
Or better yet as most dive computers will give an average depth provided you aren't gas switching just do 3 or 4 normal dives and calculated the figure from the average depth the computer supples (accurate gas readings needed again !).

Now you have an "RMV at surface" which is based on real diving in the normal gear and normal conditions. Its just a case of adjusting for depth then to work out consumptions and reserves etc.

 

[waterpirate]

This is where data is very helpfull. Where you collect it is the only issue. while surface calculations are helpfull in the beginning, real dive data is when the tire hits the road. Be honest with yourself about the dives on which you collect data, exertion,current,stressed,hung over,depth and the usefull information about gas usage will become crystal clear and make planning future dives very easy.
Eric

 

[knotical]

... most dive computers will give an average depth ...

Does anyone know how consistent the averaging algorithms are among computer brands?

 

[knotical]

... surface calculations are helpful in the beginning ...

A key point, and probably one reason why SDI (a training agency, after all) uses them.

 

[abyss_scuba]

When do we want students thinking about gas consumption? After they have 100 dives or when they are new? A new diver wether their floating on the surface or diving at depth probably has enouh to think about, they are just getting really comfortable wiht their gear, bouyancy, everything else that is going on. So now you want them to do an accurate recording of air consumption (BTW do they have an analog or digital spg?) over a period of 10 or 20 minute. How about static vs dynamic readings? Currents? Cold? How about if we are teaching SURFACE AIR CONSUMPTION we do it on the SURFACE in a controlled environment? Like figuring out how many MPG you get in your vehicle, it is a guesstamite and refined with each dive. The more you dive the better your guesstimate becomes.

 

[String]

In my view air consumption rates calculated out of the water are worthless. Too inaccurate to be of any use at all.
No matter what level course or dive being done its not exactly hard to note depth, time and gas or download off a computer to get average if you do it that way.

Doing an SAC dry is like trying to guess the MPG of your car by looking at a different brand a neighbour owns - its a completely different situation.

 

[TSandM]

It's an interesting question. One would not think the muscular effort involved in sitting quietly would be that great. I may try this experiment myself, and see how it comes out.

 

[waterpirate]

I think teaching sac is an important "gateway tool". It illustrates to the new diver that there can be a lot more planning put into your diving then simply swimming around untill your reg breathes hard, then pull your j valve and surface.
some divers begin to collect tools early on, some never do. Introducing sac helps teach new divers which one they will be.
Eric

 

[BoltSnap]

Teaching SAC calculations in a basic course is a waste of time and effort. I believe that we need to spend the precious time in class and water to emphasize more basic skills and more critical environmental factors.