Big Heavy tanks!

[scubadada]

So, RMV can be very helpful for recreational dives, maybe this is convenience or maybe this is safety. I have a home in SE Florida and have about 1300 dives in Boynton Beach, Palm Beach, and Jupiter. I take visitors and sometimes I take new divers on dives. I know the profiles very well.

An easy example is routine reef dives in Jupiter. The person I'm diving with must choose between an AL80 or a HP steel 100 or 120. I use an AL80 with my average RMV of 0.36 cu ft/min. So, take an average dive at Spadefish Point, 53 min at an avg depth of 71 ft. I use 60 cu ft of gas from my AL80. If your RMV is more average at 0.50 cu ft/min, you need 84 cu ft and a steel HP 100. If your RMV is 0.65 cu ft/min, you need 109 cu ft and a steel HP 120.

@berndo will probably not accept this as an example of the value of knowing one's RMV, but I do.

 

[steinbil]

Bob is the experienced local diver. Bob's SAC rate is 7.25 L / min, measured at home watching TV

they measure Alice's SAC rate while streaming an episode of their favorite show, and find 6 L /min which is significantly less than Bob's 7.25 L / min. Based on this…

Does anyone actually measure their SAC rate on dry land? Other than out of curiosity, or non-SCUBA related reasons, I don‘t see the point. And making dive plans based on it seems crazy to me. Everything changes when you’re in the water. And different conditions affect your SAC-rate greatly. And all that is without even mentioning comfort in the water, mental state, nerves. If you’re going to use your actual SAC-rate in any type of gas planning, you better have tracked it for a long time in a wide variety of different environments and situations (and hopefully a couple where something went wrong) to make any realistic predictions.

Still, I would agree that tracking your SAC-rate is very helpful, and that every diver should practice predicting what their actual gas consumption is during any dive, by guesstimating remaining gas before checking their SPG every time (and do it frequently).

 

[scubadada]

All of my RMVs are dives, n=1872, mean 0.36 cu ft/min std dev 0.04 cu ft/min. This represents my avg RMV for all the dives I have done during this period, 2010 onward. A very diverse collection of dives under all kinds of different conditions and environments.

 

[Wibble]

Is someone’s SAC constant during a dive? Generally for planning you’d use a "working" SAC meaning when finning on the bottom phase. You wouldn’t use your resting SAC, for example on a safety stop. But for planning you’d use a higher SAC for being stressed and you’d add time to resolve the problem and start your ascent.

Do recreational divers work out how much gas they need at the end of the dive to surface under stress and do a safety stop and leave a sensible margin of gas in the tank? Is that converted into the minimum gas, the point where the dive is over? And what about ascending whilst sharing gas?

 

[scubadada]

Is someone’s SAC constant during a dive? Generally for planning you’d use a "working" SAC meaning when finning on the bottom phase. You wouldn’t use your resting SAC, for example on a safety stop. But for planning you’d use a higher SAC for being stressed and you’d add time to resolve the problem and start your ascent.

Do recreational divers work out how much gas they need at the end of the dive to surface under stress and do a safety stop and leave a sensible margin of gas in the tank? Is that converted into the minimum gas, the point where the dive is over? And what about ascending whilst sharing gas?

That's why average RMV over a wide variety of dives is valuable. I use my average RMV + 1 std deviation for routine planning. I use twice my avg RMV for emergency planning, that is a lot of gas.

 

[gordonscuba]

SAC, as measured at rest on land, is roughly constant, for a given diver, from one year to the next, and by definition does not depend on the type of dive. The fact that it is a constant could be useful. The SDI solo diver course presents one possible scheme for predicting surface RMV which is to measure the SAC on land at rest, and then multiplying by a "dive factor" to predict your surface RMV of an actual dive, depending on the conditions of the dive. The dive factors suggested by SDI are, I believe, 1.5 for an easy tropical dive in warm water, up to 3.0 or higher for a difficult, cold water dive in a drysuit. This only makes sense for a diver whose finning technique, trim, equipment has stabilized, so probably not in your first 50-100 dives.

 

[Underwater Tourist]

I had roughly 500-600 recreational dives in Croatia, Portugal, Canada, Dominican, Mexico and several LOBs in Egypt. So a fairly diverse range of places meeting a fairly diverse range of people.

Not once have I heard the word sac being used and for most of my recreational life I had no idea what it is, and I am willing to bet that neither did the 99% of the people I was diving with. Nobody cares about sac in recreational world.. And those who do they are the exception to the rule, not the rule. 99% of my dives were just jump in the water, follow the guide until the first guy in group reaches 100bar and then we turn around and start making our way back.

Thats how the absolute vast majority of recreational dives are done in the world. Sure there are some places that are exceptions, but they are just that.. exceptions.

 

[Wibble]

SAC, as measured at rest on land, is roughly constant, for a given diver, from one year to the next, and by definition does not depend on the type of dive. The fact that it is a constant could be useful. The SDI solo diver course presents one possible scheme for predicting surface RMV which is to measure the SAC on land at rest, and then multiplying by a "dive factor" to predict your surface RMV of an actual dive, depending on the conditions of the dive. The dive factors suggested by SDI are, I believe, 1.5 for an easy tropical dive in warm water, up to 3.0 or higher for a difficult, cold water dive in a drysuit. This only makes sense for a diver whose finning technique, trim, equipment has stabilized, so probably not in your first 50-100 dives.

SAC obviously has different meanings. It's got nothing to do with air consumption on land; it's your underwater consumption.

SAC & RMV are essentially the same things. The amount of gas consumed over a minute. This is expressed either in ISO numbers, litres per minute, or cubic feet per minute adjusted to surface pressure.

It is FAR more simple to use ISO units as they massively simplify the arithmetic.

Throughout our diving 'careers' we will constantly return to working out how much gas we need to do a particular dive. This could be a simple NDL 18m/60ft dive all the way through to a 100m/330ft multi-gas bailout and beyond. It's the same problem: get out alive.

You end up with similar figures: your working SAC which you measure over a time period (e.g. 10 mins at a constant depth, noting the start or end pressures), or even using AI (air integration) transmitters which you can grab the pressures at any two points to compare.

Your SAC will generally be higher if you're a novice and will improve with experience and skills.

SAC increases dramatically when you're stressed or working hard, e.g. finning into a current.

You will adjust your own SAC for stress.

Once you know your stressed SAC, you use this for calculating how much gas you need to ascend from the bottom (adding a couple of minutes to sort your stuff out) and sum up the per-minute consumption from the bottom to the top AND include a safety stop. You will add to that the 50 bar or 500 PSI needed to bring back to the boat (for both contingency and SPG errors).

The other thing that a high SAC rate would imply is bringing more gas. People don't find it funny when someone's always low on gas and wanting to suck on someone else's tank to extend their dive. These people can be dangerous as they could empty another's tank.

 

[gordonscuba]

SAC obviously has different meanings. It's got nothing to do with air consumption on land; it's your underwater consumption.

SAC & RMV are essentially the same things.

I'm not asserting that either one is more "correct" as there appears not to be a consensus, but I observe that Wibble's definition is not the same as SDI's definition -- I was using the one from SDI. It seems that experienced divers disagree on the exact method of gas planning -- e.g. berndo suggests determining that the buddy has 40-50 min of time with a 12l tank at roughly the same depth, from conversation. The assumption, then, is that the buddy's reply is based on his own past dives, so this is similar to estimating RMV from past data, except that it estimates the total consumption at depth, rather than the rate of consumption. It seems that scubadada has analyzed data from a large set of past dives.

 

[_sgm_]

Does anyone actually measure their SAC rate on dry land? Other than out of curiosity, or non-SCUBA related reasons, I don‘t see the point. And making dive plans based on it seems crazy to me. Everything changes when you’re in the water. And different conditions affect your SAC-rate greatly. And all that is without even mentioning comfort in the water, mental state, nerves. If you’re going to use your actual SAC-rate in any type of gas planning, you better have tracked it for a long time in a wide variety of different environments and situations (and hopefully a couple where something went wrong) to make any realistic predictions.

Still, I would agree that tracking your SAC-rate is very helpful, and that every diver should practice predicting what their actual gas consumption is during any dive, by guesstimating remaining gas before checking their SPG every time (and do it frequently).

I check my sac every dive... it has become a need to know variable vs a nice to know... in my opinion this should be stuff a beginner diver should have taught so as they progress they have the knowledge and application becomes 2nd nature when they plan or even decide on doing a impromptu dive with friends or with others on vacation. One less variable to have a gotcha moment with.