Do you use your SAC for anything? POLL

[spoolin01]

I have an idea of time passing without looking at a clock. It's not perfect, but way better than my idea of the pressure remaining in a tank.

You dive without perfect knowledge of some parameter? You need more training, complex modeling, and task loading. And probably better equipment. It's a wonder you've survived this long.

spoolin, it's a cross-check on the SPG. If your predicted reading doesn't match the actual one, you have to worry about a stuck gauge (if it's too high) or a leak somewhere that will mean you'll need to end your dive before you expected to.

I can see that tracking expected consumption is the only check you've got on your SPG, yet it would be an extraordinary effort to collect data adequate for more than a gross check (except I think I would hear any leak that was fast enough to shorten my dive materially). For my history so far, the ratio of depth/time instrumentation failure to SPG failure is right around infinity. Given that an adaptive response is almost always adequate to the task - this do be the Basic Scuba forum - it just seems a distraction without significant benefit.

If your dive plan or psyche is simply not amenable to adaptive management, then forecasting and monitoring air consumption is indicated.

I think it's very sad that so many divers see absolutely no need to understand thier breathing rate when diving. How in the hell can you PLAN a dive if you don't know how long your tank will last. "Oh I'll just jump in and come up when I reach 700psi" is not dive planning.

...
Before your dive you should be planning what depths you are going to, how long you can SAFELY stay at that depth, and how much work would be involved in this dive. What happens if your buddies 1st stage dies? Will you have enough air to share (in a now emergency situation) without running out before reaching the surface? Or do you just make every dive praying nothing goes wrong?

Every diver should constantly strive to improve thier diving. Knowledge about yourself and your equipment the priority in improving.

If you plan using such a 'reserve' model, information afforded by forecasting or monitoring within the portion of your tank that you consume is rendered moot by the reserve, isn't it? The only part of your dive that is committed to any particular profile or quantity of air consumption is the ascent. I found it valuable to look at consumption rates as a function of depth (using an estimated SAC, because the interesting part to me is the rapid relative increase with depth, not the absolute rate). Once you know how much air is needed to ascend safely from various depths, you allow for some reserve, and learn from experience how to adjust for varying conditions (long swim under kelp or such). If you're reserving against a consumption rate at 66 ft that is 3X that at the surface, +/- 20% for possible varying SAC conditions along the way just isn't material against that, and certainly not for the majority of air consumption that occurs during the 'adaptively manageable' phase of the dive.

Looking back at all the times I wished I'd done something different during a dive, related to safety or convenience, I can't think of any that would have been materially improved by SAC knowledge either beforehand, or during the dive. If my goal were to breathe my tank just to completion (and it often is), I'd say SAC would help there. If I intend to respect reserves, it's the discipline of that decision, not knowing the consumption trajectory prior to that, that would be the issue.

 

[Kern]

In my training here in the states, learned that SAC is calculated for a particular tank and expressed in PSI/minute.

Found a thread discussing calculating RMV

Yes, I know how gas consumption is planned, & the nomenclature used in the US. It just happens to be different in the metric world.

 

[Kern]

........

 

[fire_diver]

Why?...

If you will NEVER dive with a different tank, you don't need to. If you travel, or borrow tanks, then its much simpler to understand in a more universal term. cf3/minute usage doesn't change whether diving with an LP50, AL80, or HP120. If you only understand XX psi/minute then you have to re-calculate when using a different tank.

 

[Kern]

If you will NEVER dive with a different tank, you don't need to. If you travel, or borrow tanks, then its much simpler to understand in a more universal term. cf3/minute usage doesn't change whether diving with an LP50, AL80, or HP120. If you only understand XX psi/minute then you have to re-calculate when using a different tank.

Yes, I misread the post. You were typing while I was deleting the question.

Is it SAC, is it RMV, I'm soo confused.

 

[mk706]

You'll need to know SAC on one volume of tank to be able to calculate RMV. RMV is expressed as cubic volume gas consumed per minute.

I use v-Planner & Baltic to plan dives. Both programs use RMV as an aid in calculating projected gas requirements.

 

[Kern]

You'll need to know SAC on one volume of tank to be able to calculate RMV. RMV is expressed as cubic volume gas consumed per minute.

I use v-Planner & Baltic to plan dives. Both programs use RMV as an aid in calculating projected gas requirements.

No, all I need to plan my gas requirements for a dive with any tank, is my SAC in lts/minute, the water capacity of the tank/s, & the pressure thats actually in the tank.

 

[mk706]

So Kern, you're saying you need to calculate RMV. Got it. You'll also need to know pressure in ATA at depth.

Gas needed = RMV * ATA * time at depth

 

[Kern]

Yes, I should have added pressure depth & time to my last post.

To figure my SAC, what you call RMV I use this formula;

SAC = (bar used / time / depth pressure) x WC.

I never need to know what you cal SAC in pressure/minute.

PS. I have rewritten this half a dozen times, & due to the variations in terminology, my recently crushed toe, lack of sleep, & the pain medications, have sunk further & further into confusion. I hope the last edit makes some sense.

 

[fire_diver]

I think the big disconnect here is in terminoligy.

Some people are using a definition of SAC = psi/minute which is tank dependant
Others (including myself) are using SAC = volume/minute which is not tank dependant.

Personally I don't know why someone wouldn't do the calculation to know thier volume/minute of consumption. I've never seen anyplace outside of this forum use anything else.