Air Consumption and Planning

[Naomi S]

I've just started getting my head around the whole dive planning thing and am curious as to what SAC poeple use for their air planning?

I plan for 25 l/min but have just worked out that on my last dive i had a SAC of 16.5 l/min. (Apologies for the blatent she pee waving there but i was quite pleased with that TBH).

Planning for a SAC of 25 obviously gives a factor of safety in case of heavy breathing due to stress/ excitement etc but it seems like quite alot more than i'd need. Would it be realistic to use say 20 l/min instead to get a longer dive and if you end up breathing more than planned just come up sooner?

What do you all use for your planning and does it change for the type of dive you are doing?

[Naomi S]

Hey stop talking old speak i want some serious advice here guys.

I've had to resort to doing a finless and replying to my own thread ;-)

[machiavelli]

For shallow dives I don't plan air - I just know what to expect. For deep dives I plan on a square profile at 25L/min and intend to start the ascent with enough to complete the dive if one cylinder (I use independent twins) is lost. This tends to leave me enough for the second dive.

I don't know my working gas consumption, but it must be less than 25L/min because the above works. Resting (eg deco) it's about 1Bar/min on a 12 litre cylinder.

[big_si]

20 sounds like a reasonable planning value which would give extra safety. (16.5 is good, well done! )


Bit of an aside... Are you using Nitrox? Reason I ask is that depending on what depth you're diving to you'll start to be limited by No Decompression Time rather than available gas.

Dive to 20 meters (based on 16.5 SAC and using 2/3 of a 12L cylinder) gives an available dive time of 37 mins (PADI NDL = 45mins)
Dive to 30 meters (based on 16.5 SAC and using 2/3 of a 12L cylinder) gives an available dive time of 28 mins (PADI NDL = 20mins , PADI EAN 32% tables 30 mins)

Jus thought I'd mention it incase you'd not considered it.

Si


(I plan for 20L/Min which is just over my SAC at the mo. 19 last dive.)

[Naomi S]

Originally Posted by big_si

20 sounds like a reasonable planning value which would give extra safety. (16.5 is good, well done! )


Bit of an aside... Are you using Nitrox? Reason I ask is that depending on what depth you're diving to you'll start to be limited by No Decompression Time rather than available gas.

Dive to 20 meters (based on 16.5 SAC and using 2/3 of a 12L cylinder) gives an available dive time of 37 mins (PADI NDL = 45mins)
Dive to 30 meters (based on 16.5 SAC and using 2/3 of a 12L cylinder) gives an available dive time of 28 mins (PADI NDL = 20mins)

Jus thought I'd mention it incase you'd not considered it.

Si


(I plan for 20L/Min which is just over my SAC at the mo. 19 last dive.)

What's PADI NDL? and why do they estimate more time at shallow and less at deeper do you think?

[machiavelli]

You can recut the plan if things are going better (or worse) than expected of course. Eg ascend at x bar on each side or when y minutes of ascent time are showing. You want to run through this in advance anyway so you know what to do if the electronics packs up.

I forgot to mention - the way I do this any significant gas loss ends the dive. No manifold means I can't get at one side's gas with the other regulator.

[Naomi S]

Originally Posted by machiavelli

You can recut the plan if things are going better (or worse) than expected of course. Eg ascend at x bar on each side or when y minutes of ascent time are showing. You want to run through this in advance anyway so you know what to do if the electronics packs up.

I forgot to mention - the way I do this any significant gas loss ends the dive. No manifold means I can't get at one side's gas with the other regulator.

Sure this is what i have done last puddle dive based on air needed at stops. 70 bar = time to come up

[big_si]

Originally Posted by Naomi S

What's PADI NDL? and why do they estimate more time at shallow and less at deeper do you think?

NDL is No Decompression Limit. It's the amount of time you can dive to that depth before incurring mandatory decompression stops.

At deeper depths the gas you're breathing is inhaled at a higher pressure and so tissues are loaded with gas at a faster rate. so Deeper = less time without HAVING to do deco.

[Stevie H]

Originally Posted by Naomi S

I've just started getting my head around the whole dive planning thing and am curious as to what SAC poeple use for their air planning?

I plan for 25 l/min but have just worked out that on my last dive i had a SAC of 16.5 l/min. (Apologies for the blatent she pee waving there but i was quite pleased with that TBH).

Planning for a SAC of 25 obviously gives a factor of safety in case of heavy breathing due to stress/ excitement etc but it seems like quite alot more than i'd need. Would it be realistic to use say 20 l/min instead to get a longer dive and if you end up breathing more than planned just come up sooner?

What do you all use for your planning and does it change for the type of dive you are doing?

There are two methods I know of, possibly more. Both need a computer.

1. Diligently log air used during a the dive (no rounding), also time, find average depth and then do the maths:

Air used in bar divided by time of dive then multiplied by average depth (in bar).

2. Once at max depth, stay there for 10 mins and log air used during this time and then do the maths:

Air used in bar divided by 10 then multiplied by depth in bar

On fun/work dives using a single cylinder I get 7.5 lpm, but for some reason this shoots up to 12 when Im using twins......

[Paul Oliver]

20 LPM will give you a good buffer and i would say is fine for your planning, once you are happy that you have got your actual rate well sussed out then use that and build in your fudge factor in the totals.

If you are diving thirds then you have it there anyway

Time to go at 70 Bar is plenty and what i used for donkey years and TBH still do now a lot of the time its what my club uses as its standard.