A SAC question

[mala]

You're confusing your definition of SAC with the definition of SAC. I fully agree that it makes a whole lot more sense to measure SAC in terms of volume per minute consumed, but that doesn't change the fact that some people understand SAC as pressure change per minute. You can ignore that all you want, it doesn't change reality.

the definition of sac was given in post 16.(not by me)

sac is at the surface and is not related to pressure.- thats what the s in sac is for.



this is reality.

 

[dfx]

the definition of sac was given in post 16.(not by me)

sac is at the surface and is not related to pressure.- thats what the s in sac is for.



this is reality.

You're confusing ambient pressure with tank pressure. Two very different things.

 

[mala]

You're confusing ambient pressure with tank pressure. Two very different things.

ambient is the pressure exerted on your body - whatever planet you are on.

there are 2 tank pressures.

one is what you calculate is in the tank by knowing your sac

and one is what the spg reads.

would you trust your life on the latter ?

 

[ffhamm]

FWIW my average gas consumption per minute is .40-.46 cuft/min. I haven't tried to figure out SAC based on depth etc. This is based on the 56 dives I have logged since using SmartTrak and my Galileo air integrated computer.

I am usually among the last in whatever group I am in to run low on air. (I mostly dive with vacation type divers) Not sure how this rates with frequent divers, but as a new diver, maybe this is something you can use to compare.

Jeff

EDIT Changed units to Liters as that seems to be the main discusion on this particular thread. Warm water appears to haver around 11.4 l a minute and cold h2o (Mid to high 50's is cold to me) 13 or so LPM

 

[dfx]

ambient is the pressure exerted on your body - whatever planet you are on.

there are 2 tank pressures.

one is what you calculate is in the tank by knowing your sac

and one is what the spg reads.

would you trust your life on the latter ?

No idea what you're on about. There's only one tank pressure, it's what your SPG shows (or should show) and it's directly indicative of how much air you have (for a particular type of tank, neglecting differences caused by temperature variation). It has nothing to do with ambient pressure. That's why it's perfectly suited for calculating and expressing your SAC. You just have to take your tank size and type into account.

Anything calculated from your SAC are just theoretical values. But that's not what we're talking about.

 

[Kevrumbo]

FWIW my average gas consumption per minute is .40-.46 cuft/min. I haven't tried to figure out SAC based on depth etc. This is based on the 56 dives I have logged since using SmartTrak and my Galileo air integrated computer.

I am usually among the last in whatever group I am in to run low on air. (I mostly dive with vacation type divers) Not sure how this rates with frequent divers, but as a new diver, maybe this is something you can use to compare.

Jeff

EDIT Changed units to Liters as that seems to be the main discusion on this particular thread. Warm water appears to haver around 11.4 l a minute and cold h2o (Mid to high 50's is cold to me) 13 or so LPM

That's excellent!

So with an AL80 (11 litre/bar) tank in warm water, your pressure SCR (Surface Consumption Rate) is approx 1 bar/min --see how much easier a value of "1" is to use than 14.5 psi/min in US Imperial Units???

Your depth multiplying factor is equal to how deep you are in ATA, and that's easily figured out from meters by simply dividing-by-10 and adding one. So for example, 30 meters depth equals 4 ATA and so your pressure SCR of 1 bar/min multiplied by 4 is now a Depth Consumption Rate (DCR) of 4 bar/min at 30 meters depth. So in a 10 minute interval at 30 meters depth, you would expect to nominally consume 40 bar of gas.

See how much easier & faster it is to use metric units than the cumbersome US Imperial System of feet & PSI for gas planning, especially when you can use your volumetric SCR to match up with a particular tank's metric rating . . . ? (Switch to metric and ebay that AI transmitter).

 

[ffhamm]

That's excellent!


See how much easier & faster it is to use metric units than the cumbersome US Imperial System of feet & PSI for gas planning, especially when you can use your volumetric SCR to match up with a particular tank's metric rating . . . ? (Switch to metric and ebay that AI transmitter).

You might get me to think in Metric, but I doubt you'll get me to abandon my AI transmitter.. I embrace technology, and my diving is recreational so I can always get to the surface. I've seen nearly as many mechanical gauges fail as I have seen quality transducers. (general industry, not scuba)

Jeff

 

[mala]

That's excellent!

So with an AL80 (11 litre/bar) tank in warm water, your pressure SCR (Surface Consumption Rate) is approx 1 bar/min --see how much easier a value of "1" is to use than 14.5 psi/min in US Imperial Units???

Your depth multiplying factor is equal to how deep you are in ATA, and that's easily figured out from meters by simply dividing-by-10 and adding one. So for example, 30 meters depth equals 4 ATA and so your pressure SCR of 1 bar/min multiplied by 4 is now a Depth Consumption Rate (DCR) of 4 bar/min at 30 meters depth. So in a 10 minute interval at 30 meters depth, you would expect to nominally consume 40 bar of gas.

See how much easier & faster it is to use metric units than the cumbersome US Imperial System of feet & PSI for gas planning, especially when you can use your volumetric SCR to match up with a particular tank's metric rating . . . ? (Switch to metric and ebay that AI transmitter).

sanity returns to the thread.

 

[RTee]

My warm water air consumption average .35 - .40 cft/min (3mm, min weight) while my cold water set-up (drysuit) with doubles and stage(s) will be .60 - .65 cft/min range

 

[Kevrumbo]

My warm water air consumption average .35 - .40 cft/min (3mm, min weight) while my cold water set-up (drysuit) with doubles and stage(s) will be .60 - .65 cft/min range

0.35 cf/min (roughly 10 ltrs/min) is my personal best for tech diving, in the warm calm waters & deep wrecks of Chuuk Lagoon.

Using double 11 litre tanks (double AL80's) for a total metric tank rating of 22 litres/bar, my pressure SCR turned out to be 0.5 bar/min (that's 10 divided-by-22 ~round it to 0.5). Going to 60 meters depth on the Oite Destroyer then is equal to 7 ATA, so my pressure DCR is 7 times 0.5 which is 3.5 bar/min. In 10 minutes at 60 meters (7 ATA) I used up 35 bar of backgas (3.5 times 10 is 35). My planned bottom time at 60 meters was 20 minutes maximum, so I expected my SPG to be down by two 35 bar segments for a total delta of 70 bar of backgas consumed. My Rock Bottom/minimum emergency backgas reserve to get me and a buddy from 60 meters to our 21 meter eanx50 deco stop was 90 bar, so consuming 70 bar of backgas out of 180 bar usable left good margin.

Now to get a somewhat similar pressure SCR and gas plan for cold water at 0.65 cf/min (approx 18 litres/min) which is the best I've ever done here in the temperate waters of Southern Calif, I would have to use larger steel double HP130's (double 16 litre tanks for a total metric rating of 32 litres/bar); 18 divided-by-32 is 0.57 ~round it to 0.6 bar/min.

(I don't use the "personal best" SCR values above for pre-dive gas planning, but more conservative rates like 15 litres/min for warm tropical waters, and 22 litres/min for my home cold waters here in SoCal.)

Anyway the point of the above exercise is to show how to use your SCR/RMV with a particular tank(s) set-up to gauge your gas planning and actual consumption during the dive --all made much simpler and easier to understand by using the Metric System.