SAC rate & RMV

[Tortuga68]

Your breathing rate is likely to be the primary reason for variation, no surprise

What would the secondary reason be? Global warming??

 

[Rhone Man]

Respiratory Minute Volume - the volume of air a diver consumes per minute (while diving)

It's measured by taking SPG readings over time, at constant depths, under different conditions (rest, high load etc)

Then (depth in metres / 10) +1 X RMV = gas usage/requirement

SAC (Surface Air Consumption) is just a reference point - you will breath differently under water. DO NOT use you SAC to calculate gas requirements for diving, despite what seems to have been implied in ealier replies

To make a safe dive plan you should calculate based on your maximum RMV, not your 'without exertion' RMV - everyone knows you can do better, but when it becomes critical will be when you're stressed

Thanks. Again, apologies for my lack of knowledge, but whilst I agree you breathe differently underwater, but I'm confused by the implication that SAC rate is calculated at the surface and RMV at depth. Presumably if your SAC rate is calculated by your computer at the end of your dive based upon consumption and depths, you should end up with (over time) an equally accurate range of figures to base gas requirement calculations on? I read through a copy of TDI's Extended Range manual once, and I swear they never mentioned the concept of RMVs in gas calculation requirements.

 

[Tortuga68]

Thanks. Again, apologies for my lack of knowledge, but whilst I agree you breathe differently underwater, but I'm confused by the implication that SAC rate is calculated at the surface and RMV at depth. Presumably if your SAC rate is calculated by your computer at the end of your dive based upon consumption and depths, you should end up with (over time) an equally accurate range of figures to base gas requirement calculations on? I read through a copy of TDI's Extended Range manual once, and I swear they never mentioned the concept of RMVs in gas calculation requirements.

There's no need to apologize for asking questions, we're not in the DIR forum

It's not an implication that SAC is calculated at the surface (otherwise it wouldn't be called Surface Air Consumption)

Therefore, your dive computer doesn't calculate SAC during a dive, since by definition you're not at the surface when diving

Dives should be planned on max consumption anyway, not 'average' or 'ideal'

RMV is part of TDI's decompression procedures (pre-requisite for Extended Range), so maybe it's not reproduced in ER - I don't know because I haven't done ER yet

On a side note, you don't use a dive computer for tech diving - you use run time and a bottom timer with a backup

 

[MXGratefulDiver]

Hi everybody

I've just been calculating my SAC & RMV and it appears that there is a wide variation. I have just 51 dives and am looking towards the futurehopefully wreck & deep diving certs. At this point how do I apply this info when it seems fairly inconsistent-- or do I just need more time to develop my skills? any input is greatly appreciated.

First off you need to understand that air consumption rates underwater will vary from dive to dive ... your SAC rate isn't a number, except as it relates to one specific dive. Although the variation between a high-end and low-end number will decrease with experience, there will always be some variability.

When you're on the surface, your breathing rate isn't always the same. It depends on exertion, state of mind, physical well-being, and a host of other factors that can change from day to day or moment to moment. It's no different when you dive ... each dive presents its own challenges (current, cold, excitement, narcosis, etc) that can affect your breathing. Tracking your consumption rate over a period of dives provides useful information for dive planning, no matter what your experience level.

For the ignorant among us: what is RMV?

RMV is just a different way to express your consumption rate. There is no real standardization, but many people use SAC to express their consumption rate in PSI ... which is useful for dive execution ... and RMV to express their consumption rate in Cubic Feet (or Liters) ... which is useful for dive planning.

For a more thorough explanation, I have an article on my website that you may find useful ... NWGratefulDiver.com

... Bob (Grateful Diver)

 

[nereas]

There's no need to apologize for asking questions, we're not in the DIR forum...

Best comment of the year!!!

 

[nereas]

...

RMV is just a different way to express your consumption rate. There is no real standardization, but many people use SAC to express their consumption rate in PSI ... which is useful for dive execution ... and RMV to express their consumption rate in Cubic Feet (or Liters) ... which is useful for dive planning...

Yes, exactly.

I prefer RMV in cu ft per min, and use this to plan my gas volumes.

I find SAC to be fairly useless. Once the dive is planned, it is planned, and then we always stick to the plan.

Anyone who wants to go off on their own is on their own. I always stick to the plan.

With my DPV, my RMV is consistently about 0.5 cu ft per min. Without it, the RMV becomes 0.75 or worse depending on conditions and current, etc.

 

[nereas]

...

On a side note, you don't use a dive computer for tech diving - you use run time and a bottom timer with a backup

Actually, after LEARNING tech diving, which is indeed done without a dive computer, I and many others DO USE tech/helium dive computers to EXECUTE tech-deco dives. The VR3 and the Nitek HE work quite well for this.

You still need to do your ordinary tech dive planning in the usual fashion, and then memorize your plan and your alternative contingencies. But once in the water, the helium tech computers work quite nicely. Your backup in gauge mode is there, together with wrist slates [if you cannot memorize well], in case your primary computer fails.

Remember, we are not in the DIR forum, therefore tech diving computers are ok.

 

[nereas]

...
From what I've read, mammalian reflex only comes into effect if the water temp is below 21C/70F...

Only an Auzzie would call attention to this. The rest of us around the globe are accustomed to waters being colder than 70F/21C.

...Regardless, the primary effect (lowered heart rate) would still be triggered when you went below the surface - I don't believe there's any advantage to surface submersion of the face

Believe it because it works like a charm.

 

[nereas]

What would the secondary reason be? Global warming??

The secondary reason would be poor buoyancy control with continuous repeated inflation, deflation, and re-inflation of your B/C-wing when not using an argon bottle.

[I know, in Oz you don't use argon bottles anyway, and buoyancy control is pretty EZ in the warm waters there. You are a lucky mate, mate.]

 

[shoredivr]

Only an Auzzie would call attention to this. The rest of us around the globe are accustomed to waters being colder than 70F/21C.

Once again nereas demonstrates his inability to read. All Australian waters are not tropical.

Tortuga lives in Melbourne, Australia, it's temperate water diving. Heck, Saspotato recently posted about her drysuit for local Melbourne dives. Hobart water's even colder. Don't let reality get in the way of your preconceptions.

http://www.diveoz.com.au/temperatures/default.asp