Air use -What is normal?

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Air use -What is normal?

Postby Frosty » Thu Nov 11, 2010 7:01 pm

Hey again Mr 10000 questions.
It seems that in the dive community theres a bit of a thing about air consumption.
So just so I've got some indication could you give me an idea what is normal air use and what is frugal in the following situation.
45 minute drift dive off a boat in warm water at 18-20 m with a safety stop. How much air should you surface with given you start with 200bar
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Re: Air use -What is normal?

Postby ChuckyBob » Thu Nov 11, 2010 7:45 pm

Smart a*** answer. 50 bar.

When you start diving you will use lots. As you relax and get comfortable you will use less.
Air/gas consumption is measured as SAC or Surface Air Consumption ( also known as RSV and some other TLA that I cant recall )
This is how much air you will use at the surface. But due to Boyles law that will be x2 at 10 meters and x3 and 20 meters.

To calculate it you need to know your tank size rated in Water Capacity/WC. Common sizes are 10L,12,L and 15L ( steels) and 11L ( aluminiums).

You then need to know how many BAR you used.

WCxBAR= actual free air in litres.
You then need to know your AVERAGE depth which is very hard to figure out unless your computer tells you. Using your max depth is pointless.
Convert your average depth in to atmospheres. 0 meters= 1 ata 10 meters = 2 ata. 25 meters = 3.5ata

Then use the forumla
WC x Bar used / depth in ata / minutes of dive time.

example
100 bar use from a 12L tank (1200litres) / 10 meter average dive (2 ata)/ 30 minute dive
1200/2/30 = 20 surface litres per minute.

For the record my SAC rate is just less than 20 LPM on average ( but can get much higher).
I know people ( usually women) than can be half that.
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Re: Air use -What is normal?

Postby Andy » Thu Nov 11, 2010 11:01 pm

ChuckyBob wrote:Air/gas consumption is measured as SAC or Surface Air Consumption ( also known as RSV and some other TLA that I cant recall )



RMV - Respiratory Minute Volume


ChuckyBob wrote:For the record my SAC rate is just less than 20 LPM on average ( but can get much higher).
I know people ( usually women) than can be half that.


20 lpm is a good guess for a fairly active diver doing a fairly routine dive. In my experience, most new divers tend to be around the 35-50 lpm mark (often just because they are overweighted!).

The thing to remember is the combined RMV/SAC. Like Greg, I'm somewhere just under 20lpm on average.... the missus is about 12lpm. In theory, that means that she could use a tank half the size of mine to do the same dive. But if I have an equipment failure and have to share air then we'd be screwed.

One of the things I like for (to avoid) in a dive buddy is the "My SAC rate is so good that I only need twin 7s" (or some other tiny volume). That's not enough gas for me, so I don't dive with those people.
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Re: Air use -What is normal?

Postby Frosty » Fri Nov 12, 2010 10:47 am

Ok so using ally 11l resort tanks with 200 bar. Diving in a shorty 3.0mm steamer in perfect conditions and drift diving
Drop off to 8.0m to regroup then over the side of the "cliff" down to 18-20m and other than the safety stop the dive stayed pretty well at that depth till 35 minutes under then we eased up the side of the cliff to the safety stop at 5.0m Came up with 70-75bar still in the tank (according to the gauge)
So lets see if Ive got this right. 11l tank at 200bar means 2200
Ok --and finished at 70 bar means 770
So used is 1430
Ok so allowing an average depth of 15m?
divide by 2.5?
SAC rate would be 12.71l/m ??
Is that calculated right ?
I guess not having a 7mm suit and heavy weights would help a lot
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Re: Air use -What is normal?

Postby ChuckyBob » Fri Nov 12, 2010 11:56 am

Simply put you can not guesstimate your average depth.
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Re: Air use -What is normal?

Postby Frosty » Fri Nov 12, 2010 1:18 pm

so do you just use thee actual air consumed figure then?
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Re: Air use -What is normal?

Postby ChuckyBob » Fri Nov 12, 2010 2:37 pm

Well that will tell you how much air you used. And it will be of some help when planning another similar dive.
But if you keep track of your SAC you can use it to help plan any dive to any depth.

If you are diving with a computer it may have an average depth function. Just about all Suuntos do. I am now using a Shearwater and it doesnt seem to have the feature which is a pity. The upgraded model probably does but Im not forking out $1000 to get it ( yet).

The other way to work out your SAC rate is by simply diving to a constant depth, note the BAR at the start and the BAR at the end. Swim around for 10 minutes with out changing depth. Your average depth will then be your actual depth.
Its not something you are likely to do very often.
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Re: Air use -What is normal?

Postby Frosty » Fri Nov 12, 2010 4:01 pm

arrggg--so in a nutshell am I gettin ok air consumption ?
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Re: Air use -What is normal?

Postby Andy » Fri Nov 12, 2010 4:31 pm

Frosty wrote:arrggg--so in a nutshell am I gettin ok air consumption ?


Easiest way to answer that is to think about your dives - do you find them frustratingly short? If yes, then you want to work on your gas consumption. If no, you're fine.
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Re: Air use -What is normal?

Postby ChuckyBob » Fri Nov 12, 2010 5:50 pm

Frosty wrote:arrggg--so in a nutshell am I gettin ok air consumption ?

You cant tell unless you know what your air consumption is. :lol:

Not that it really matters at this point, what matters is do you turn the dive because you are running low on air or because some one else is running low on air?
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Re: Air use -What is normal?

Postby Frosty » Fri Nov 12, 2010 5:56 pm

Andy wrote:
Frosty wrote:arrggg--so in a nutshell am I gettin ok air consumption ?


Easiest way to answer that is to think about your dives - do you find them frustratingly short? If yes, then you want to work on your gas consumption. If no, you're fine.

well then I need to work on air consumption cos 45 minutes seems like 5 minutes
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Re: Air use -What is normal?

Postby ChuckyBob » Fri Nov 12, 2010 6:22 pm

Frosty wrote: well then I need to work on air consumption cos 45 minutes seems like 5 minutes


Yep, Thats why I brought a rebreather. I now use about 1.5 LPM of O2 regardless of depth.
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Re: Air use -What is normal?

Postby ADAP » Mon Nov 15, 2010 11:38 pm

Frosty wrote:Ok so using ally 11l resort tanks with 200 bar. Diving in a shorty 3.0mm steamer in perfect conditions and drift diving
Drop off to 8.0m to regroup then over the side of the "cliff" down to 18-20m and other than the safety stop the dive stayed pretty well at that depth till 35 minutes under then we eased up the side of the cliff to the safety stop at 5.0m Came up with 70-75bar still in the tank (according to the gauge)



Hi Frosty, my opinion is that based on your trip, your SAC rate probably isn't that bad...At the end of the day, there are just so many different factors that will influence your SAC rate and your breathing, but I would guess that the trip you were on had a Divemaster/Instructor leading the dive, which probably meant that they slowly finished the dive as someone's tank was at around 50 bar...And it wasn't yours!!! :)

If I were you, I wouldn't worry too much about SAC rates....Of course more experienced divers tend to show off that their tank is half full when others' are at 50 bar....But hey, that's why there's DIFFERENT sized tanks!! :lol: (Unfortunately tropical dive resorts will probably have just the one size...)...If your SAC rate is higher, just go for a 12lt. steel, and trust me, you will always end up with more air when you surface compared to 80-95 cu.ft AL tanks...(Which actually isn't that good anyway as you technically end your dive before you have to... :cry: )

Frosty wrote: well then I need to work on air consumption cos 45 minutes seems like 5 minutes


Oh, and in regards to a 45min. dive feeling like 5 mins...?? Trust me, it's like that everytime with me!!!! (I think everybody here will agree...) If you love diving, time will always fly by (or swim by in this case) when you're underwater...."When you feel like you've been 45 min. underwater and you ACTUALLY have been 45 min. underwater, that's when you've lost the joy of diving"...he he he :wink:
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