Using SAC rate to determine bottom time at various depths

[Panga Man]

Hey guys, I know my SAC rate and was wondering what the formula is to determine bottom times at different depths.

Thanks

 

[rstofer]

If you know SAC then you know your consumption rate at the surface, 1 ATM. Why wouldn't you expect it to double when you are at 2 ATM - 30' and triple at 3 ATM - 60', etc.

In fact, it will probably increase more than can be accounted for by ambient pressure. The deeper you go, the darker and colder the water. Perhaps there is more current. SAC is useful but it isn't the holy grail of air consumption. External conditions have a huge effect.

Richard

 

[The Kraken]

Rough forumua:

Volume of tank / (SAC rate X depth in ATA) less residual volume for safety stop, etc.

the K

 

[Sloth]

For gas planning it's better to know your SAC range. One day you may be at .6cuft a minute after a long break maybe .7. Plan on the conservative side. So if i am planning with this SAC range i would use .7. When you actually calculate dive time it's usually easier to use 5 mins instead of 1 min as its going to be a ballpark calculation anyways. So on the surface for every 5 mins I use 3.5 cuft with a SAC of .7.

To figure you bottom time multiple 3.5 (SAC of .7 times 5 mins) times the ATA of the target depth then divide into the tanks cuft. So at 99ft or 4 ATA you get 3.5 times 4=14 divide by 80 (assuming an alum80) = 5.7 (call it 5) 5 times 5 (to convert our 5 mins into mins) puts you at roughly 25 mins.

While this tells you how much time that alum 80 will last at depth it doesn't factor in your emergency gas you need for you and your buddy to ascend. So you have to remove that volume of gas from your calculation to determine actual bottom time. So for me I might calculate a emergency gas or min gas of 1200 PSI in an alum 80 for a dive to 99ft. So I need to remove this 1200psi from the calculation. To do that I have to convert PSI to cuft. I am not going to go to much into that here but roughly 1200psi equals 30cuft.

So my actually usable gas for a dive to 99ft is actually only 50cuft (30cuft minus 80cuft). So now when I divide my SAC at 4 ATA of 14cuft into 50cuft equals 3.5 so call it 15mins of bottom time on an AL 80.

 

[Panga Man]

Thanks for the great replies, what I'm trying to figure is how much more bottom time I will get by increasing my tank size to a 119. Should be able to compute it now. Thanks!

 

[LeadTurn_SD]

Here is a neat SAC (and RMV) calculator:

SAC Calculator

Be sure to use the true volume of your tank (for example, an AL80 is actually about 77.4 cu. ft.)

Best wishes.

 

[GrumpyOldGuy]

I use the spreadsheet instead of calculating. I believe Vondo published it some time ago. Anyway, using rock bottom, you can plan you max bottom times easy with both you and your buddies SAC.


http://www.scubaboard.com/forums/at...k-bottom-spreadsheet-rock_bottom_imperial.xls

 

[TSandM]

The newest issue of Alert Diver has an excellent article on gas planning using SAC rates, or you can find the information on line HERE.

 

[Panga Man]

That is excellent, thank you.

 

[jhayes75]

The newest issue of Alert Diver has an excellent article on gas planning using SAC rates, or you can find the information on line HERE.

Ditto