RMV math…

[Kendall Raine]

My feeling is that knowing your sac or rmv is pretty pointless. There are so many factors that change your consumption below the surface. It is far better to have a general idea of how long a certain size cylinder will last in a given set of circumstances and how that will change as circumstances deviate from expectation.

How do you get away from sac/rmv? Sac/rmv is a consumption calculation. Easy enough to factorize for stress, sharing gas, temp, etc. Your "general idea" is just a consumption proxy.

 

[Wibble]

My feeling is that knowing your sac or rmv is pretty pointless. There are so many factors that change your consumption below the surface. It is far better to have a general idea of how long a certain size cylinder will last in a given set of circumstances and how that will change as circumstances deviate from expectation.

The SAC is how you work out your gas consumption for a dive or any period underwater.

What’s your normal SAC?
Double it for contingency — or use another fraction
Work that for the depth and starting cylinder contents.

It’s much easier with wet volumes using litres, ATA and bar — probably why it’s more difficult when using max gas volumes, PSI and cubic feet

 

[60plus]

This is the basic scuba section so planning a complex dive should not come into it. I do not know my SAC. I could measure it and do the calculations but I don't need to. I use 12 or 15 L tanks and know near enough how long they last me on different dives / depths. I find nearly all the others I dive with use gas faster than me so it is their gas consumption or the NDL that is the limiting factor. Just had a look at at recent dives in my log. 210 bar air on entry for all dives. 32.8m and came out after 57 minutes with 65 bar. 41.9 m and came out after 49 minutes with 50 bar. This dive included 3 minutes deco. 20.9m, 45 minutes came out with 80 bar because buddy was on virtually 0 bar.

 

[tursiops]

I think what 60plus is saying is that he has no need to know how much gasoline his car uses. He knows he can get to the grocery store and home again on a tank.
Well, OK.
His data dump shows he has no understanding of the issues. He apparently thinks his gas usage depends on his max depth (rather than his average depth).
Well, OK.

 

[rsingler]

Give him time to learn. I didn't know anything about RMV at 50 dives. It wasn't taught (or I didn't remember).
He's correct that so far he has no need for that info (because he's got a good SAC, lol!). I'm sure his opinion will change as his dives get into "what if my OOA buddy comes rushing over at 90 feet?" territory.

 

[Kendall Raine]

This is the basic scuba section so planning a complex dive should not come into it.

There is nothing complex about SAC and it is applicable in all types of diving. It’s the foundation of gas management/dive planning/situational awareness. The fact recreational agencies don’t teach it at the Intro level is irrelevant. You don't need to be a surgeon to know your blood type.

I do not know my SAC.

You should.

I could measure it and do the calculations but I don't need to.

Says it all. Over and out.

 

[Wibble]

This is the basic scuba section so planning a complex dive should not come into it. I do not know my SAC. I could measure it and do the calculations but I don't need to. I use 12 or 15 L tanks and know near enough how long they last me on different dives / depths. I find nearly all the others I dive with use gas faster than me so it is their gas consumption or the NDL that is the limiting factor. Just had a look at at recent dives in my log. 210 bar air on entry for all dives. 32.8m and came out after 57 minutes with 65 bar. 41.9 m and came out after 49 minutes with 50 bar. This dive included 3 minutes deco. 20.9m, 45 minutes came out with 80 bar because buddy was on virtually 0 bar.

You need to know your minimum gas; the point where you must leave the bottom with sufficient gas for the ascent from depth, stops, surfacing and plenty of reserve (e.g. at min gas your buddy needs your gas for their ascent).

You could be arbitrary and set it at, for example, 100 bar.

However, it is a good exercise to determine if this is correct, especially if you've someone else sharing, but particularly if you're moving into light decompression after deeper dives.

Just note that at 40m/130ft you're consuming gas at 5X your SAC; you need more minimum gas in reserve.

 

[clownfishsydney]

My feeling is that knowing your sac or rmv is pretty pointless. There are so many factors that change your consumption below the surface. It is far better to have a general idea of how long a certain size cylinder will last in a given set of circumstances and how that will change as circumstances deviate from expectation.

At less than 100 dives, then I can possibly understand why you do not think this is important. However, most divers of your experience will have worked this out and take it into account when planning dives.

 

[larsdennert]

I blame ow instructors in the USA for teaching imperial and handing out an imperial spg. ow students can estimate a yard as easy as a foot. Everything else is new to them anyway.

 

[60plus]

I think what 60plus is saying is that he has no need to know how much gasoline his car uses. He knows he can get to the grocery store and home again on a tank.
Well, OK.
His data dump shows he has no understanding of the issues. He apparently thinks his gas usage depends on his max depth (rather than his average depth).
Well, OK.

I have spent my entire working life working with pressurised gases for industrial use. For over 12 years I was the UK fabrication manager for a major international company that supplied the chemical, petro chemical and nuclear industries, I also had 13 years as a university engineering lecturer so I think it is fair to say I know quite a lot about gases and gas consumption and am well able to carry out calculations for complex dives IF I needed to. But this is the basic scuba section and I when I post I am just stating my opinion as applied to basic recreational diving. If someone wants to measure their SAC and calculate consumption before a dive then I have no issue with that. Knowing your SAC is a bit like knowing your cars typical gasoline consumption. Interesting up to a point but of little value if you can't complete your journey because your tyre blows out and you forgot to check the spare.