average sac rate

[Atomic_Diver]

0.231 was my cu ft pr min on a dive for 50 mins in my pool....that may be the best i could get it at that day

 

[WD8CDH]

Try sleeping underwater in a hot tub.

 

[LeadTurn_SD]

My RMV is always a bit less when solo.

For me, there is always a subtle increase in "stress" level when diving with a buddy, thus a slightly higher breathing rate. Another factor is when diving with a buddy I tend to be swimming/exploring the reef more... when solo, I might "camp out" at an interesting spot for several minutes.

Best wishes.

 

[MXGratefulDiver]

As for the SAC v. RMV debate, SAC is expressed in psi/min while RMV is expressed in cf/min. SAC is cylinder dependent. RMV is cylinder independent. SAC is usually expressed in whole numbers while RMV is expressed in decimals.

That is also how I use the nomenclature.

If there's anyone on ScubaBoard who's not seen it yet, this article may help you understand the topic a bit better ...

... Bob (Grateful Diver)

 

[Sas]

Yea my air consumption is often lower when solo diving but I attribute it to the fact I go *a lot* slower when solo diving, rather than a decrease in stress levels. Buddy diving is generally not very stressful to me. The main thing is that my regular buddy is quite easy going and happy to go at whatever pace if I am leaving but I just don't feel right making him wait for 10mins whilst I watch a nudibranch or something like that

On solo dives my air consumption can get down to 0.31 cf but is usually 0.35. Buddy diving it tends to be more 0.38-0.42 cu ft.

 

[Puffer Fish]

I would say mine is better solo diving... around .4

On a side note, it is really obvious that most divers are not engineers or have a science background. Technically, SAC (surface air consumption) would be in units that air comes in at one atmosphere (the surface)...which would be in Liters or cubic ft or some other volume measure.

If one wants to use psi, they are talking about SPC (surface pressure consumption).

Somehow, someone got this wrong and it has became some sort of religious doctrine to use the wrong term to refer to it.

With each one refering someone earlier that got it wrong... The letters are supposed to actually stand for something.

 

[derwoodwithasherwood]

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.

 

[divengolf]

According to the SDI Solo Diving Manual (2007 edition), both SAC and RMV are expressed in volume (cubic feet or liters) per minute. In the manual SAC is defined as air consumption rate on the surface when the diver is at rest and is a "constant" for any diver at a point in time. The manual also uses the term "Surface RMV", but only defines it via inference as an air consumption rate calculated from actual dives, but adjusted to the surface (1 ata). The SRMV accounts for varying conditions of the dive, temp., workload, current, etc., so you'd have a different SRMV for each set of dive conditions excluding depth since it's normalized to 1 ata.

The manual doesn't make much use of term RMV and doesn't clearly define it. But since SRMV is volume per minute, then it follows that RMV should also be meaured this way. I was always taught that RMV is adjusted to 1 ATA so maybe in the SDI world it's the same as SRMV. There is no mention of psi/min in either SAC or SRMV since that meaure is tank dependent.

But in the examples in the manual, it uses a SAC rate and adjusts it for conditions and depth as one normally does.

It appears that SDI made this overly confusing. Maybe the SDI instructors can address this.

 

[MXGratefulDiver]

According to the SDI Solo Diving Manual (2007 edition), both SAC and RMV are expressed in volume (cubic feet or liters) per minute. In the manual SAC is defined as air consumption rate on the surface when the diver is at rest and is a "constant" for any diver at a point in time. The manual also uses the term "Surface RMV", but only defines it via inference as an air consumption rate calculated from actual dives, but adjusted to the surface (1 ata). The SRMV accounts for varying conditions of the dive, temp., workload, current, etc., so you'd have a different SRMV for each set of dive conditions excluding depth since it's normalized to 1 ata.

The manual doesn't make much use of term RMV and doesn't clearly define it. But since SRMV is volume per minute, then it follows that RMV should also be meaured this way. I was always taught that RMV is adjusted to 1 ATA so maybe in the SDI world it's the same as SRMV. There is no mention of psi/min in either SAC or SRMV since that meaure is tank dependent.

But in the examples in the manual, it uses a SAC rate and adjusts it for conditions and depth as one normally does.

It appears that SDI made this overly confusing. Maybe the SDI instructors can address this.

I'll say ... what you just wrote confuses me ... and I teach this stuff.

FWIW - I use the terms the way I do for a reason.

When you're planning your dive, you know the volume of your cylinder. So thinking about your gas consumption in volume/minute makes sense.

And when you're executing your dive, you monitor your gas supply by looking at your pressure gauge. So thinking about your gas consumption in terms of psi/minute makes sense.

So for dive planning, volume/minute (RMV) is more useful ... for dive execution, psi/minute (SAC) is more useful. Since they are representations of the same metric expressed in different ways, understand the concepts of how to convert from one to the other, and why it matters. Then whatever you choose to call them really isn't important.

... Bob (Grateful Diver)

 

[derwoodwithasherwood]

The manual doesn't make much use of term RMV and doesn't clearly define it. But since SRMV is volume per minute, then it follows that RMV should also be meaured this way. I was always taught that RMV is adjusted to 1 ATA so maybe in the SDI world it's the same as SRMV. There is no mention of psi/min in either SAC or SRMV since that meaure is tank dependent.

I'm not an SDI instructor (or an instructor period) but may be able to shed some light on this:

Technically, depth has no effect on your RMV. In that sense, it is a constant. It's not a true constant because it varies by rate -- you inhale X cu. in. of air per breath and Y number of breaths per minute, so your RMV is z cu. ft. per minute at any depth.

Of course, a lungful at depth has far greater mass than a lungful at the surface, and therein lies the problem: (in North America anyway) tank sizes are commonly referred to by the equivalent volume of air at one atm. Someone might get confused and think that if they have an RMV of 1 then they can stay down 80 minutes on an 80 cu. ft. tank regardless of depth.

So the term SRMV was coined simply as a reminder that the specified tank volume can only be tied to RMV at the surface. Other than that, there is no real difference between RMV and SRMV.

SAC can be expressed in pressure/minute, as some agencies teach or in volume/minute, as others teach. I personally favour psi/minute for exactly the reason given by NWGratefulDiver: when I'm diving, I have a pressure gauge in my hand.

I actually go one further than he does -- I utilize psi/min when planning dives too. That way I know what PSI I should have at any point in the dive without thinking about it and can quickly adjust if required. I have a chart in my log book converting RMV to SAC in psi/minute for a number of common tank sizes so I don't even have to do the math if I'm using a rented cylinder ;-)