Gas Math - Help

[raider11]

I´ve been reading the forum and other websites and I want to confirm if I´m doing my calculations correctly. Just to be sure I´m doing things right before getting in the water.

I don´t have to much experience, but this is a typical profile for me:

Al80 (Serv.Pres.3000)
PSI used: 2000
Total time: 45min
Depth: 33f (2ATM)

SAC: 0,59 ((2000*80)/3000)/45/2


So, if I go on a dive trip and the boat have Steel 100 with a Service Pressure of 2400 PSI, knowing that my SAC is 0,60 ft2 and the depth 100f / 4ATM (I´m not AOW but just an example), using the rule of thirds I should be on my way up when I hit 800PSI and my bottom time should be arround 28 min.


Steel 1000 (Serv.Pres.2400)
Depth: 100f

Gas at 1 ATM 100*0,6 = 168,75 min
Gas at 4 ATM 100*0,6/4 = 42,18 min
2/3 to go down and dive = 28 min at 100f (ideal, but less in reality)
1/3 to ascend

Is this OK? Should I plan for a 20 min at the max depth just to be on the safe side?

I´m a newbe trying to learn to be safe, so feel free to give me any advise you want. I know I´ll learn this in future courses, but I read about it in the forum and think it´s something we should learn in OW, not tec courses.

Thanks a lot and excuse my english, I don´t practice too much my writting skills.

 

[willgr1]

you have to use the rated tank volume...an AL80 @ 3000psi is not really 80cf, it's 78.8cf

so your SAC would be .58.....not much difference, and your calculation is more conservative, but it could make a difference at some point.

the correct use of the rule of 1/3rd would be to turn your dive when you've used 1/3 your air supply and be at the surface with no less than 1/3 left...so if your using a LP100 filled to 2400 psi you should be on the surface with atleast 800psi.

so working backwards in the gas calculations I got a bottom time of 27.XXX minutes using your .6 SAC and using a total of 1600psi on the entire dive

 

[crpntr133]

Also the LP100 isn't a 100 when filled to 2400 but rather 2640. 3.7 cubic feet/100 psi. It ends up being 90.9.

 

[Gombessa]

the correct use of the rule of 1/3rd would be to turn your dive when you've used 1/3 your air supply and be at the surface with no less than 1/3 left...so if your using a LP100 filled to 2400 psi you should be on the surface with atleast 800psi.

Thirds are calculated AFTER you subtract your rock bottom, it's not a replacement. The way I understood it, if your rock bottom for two stressed divers at 100ft is, say, 1000psi, then your thirds calculation is reduced to 1640psi usable gas, so you have roughly 550psi in, 550psi out, 550 in reserve, and 1000psi for RB.

 

[willgr1]

the concept of thirds could differ between individuals, but in Cavern diving I was taught you use 1/3 your air supply going in, turn @ the 1/3, have 1/3 for your exit, and have 1/3 left for your buddy in case of emergency. Your thirds would be calculated after S-drills and such. I've never heard of starting with reduction of your air supply with a "rock bottom" amount before calculating thirds. Where is this type of gas calculation taught/used?

 

[Gombessa]

the concept of thirds could differ between individuals, but in Cavern diving I was taught you use 1/3 your air supply going in, turn @ the 1/3, have 1/3 for your exit, and have 1/3 left for your buddy in case of emergency. Your thirds would be calculated after S-drills and such. I've never heard of starting with reduction of your air supply with a "rock bottom" amount before calculating thirds. Where is this type of gas calculation taught/used?

According to AG, thirds without RB doesn't work for ocean diving. Since this was a "basic scuba discussion" and the OP didn't mention caves/caverns (but does mention ascent, decent), it was reasonable to assume that the type of "thirds" discussed was more or less ocean diving. Thirds gets you in and back with reserve (say, from the start of the wreck), but then you must still have enough to get from depth to the surface with two stressed divers making required stops. I imagine thirds without RB doesn't help very much if your buddy has a problem inside the wreck and you use most of your reserve to extricate yourselves, only to be stuck with no gas 100ft down.

I imagine the assumption with caverns/caves is that the entrance is close to the surface, or that you've got any required bottles waiting for you at the mouth?

 

[rongoodman]

If you finish your cavern dive in a spring or at the surface you wouldn't really need to worry about reserving Rock Bottom gas before calculating your thirds. OTOH, if you were penetrating a wreck at 120', using your extra third for your buddy in an emergency wouldn't help much if when you got out of the wreck you still had to get to the surface with no gas reserved to do so.

 

[willgr1]

ok, that all makes sense

 

[ronbeau]

I´ve been reading the forum and other websites and I want to confirm if I´m doing my calculations correctly. Just to be sure I´m doing things right before getting in the water.

I don´t have to much experience, but this is a typical profile for me:

Al80 (Serv.Pres.3000)
PSI used: 2000
Total time: 45min
Depth: 33f (2ATM)

SAC: 0,59 ((2000*80)/3000)/45/2


So, if I go on a dive trip and the boat have Steel 100 with a Service Pressure of 2400 PSI, knowing that my SAC is 0,60 ft2 and the depth 100f / 4ATM (I´m not AOW but just an example), using the rule of thirds I should be on my way up when I hit 800PSI and my bottom time should be around 28 min.


Steel 1000 (Serv.Pres.2400)
Depth: 100f

Gas at 1 ATM 100*0,6 = 168,75 min
Gas at 4 ATM 100*0,6/4 = 42,18 min
2/3 to go down and dive = 28 min at 100f (ideal, but less in reality)
1/3 to ascend

Is this OK? Should I plan for a 20 min at the max depth just to be on the safe side?

I´m a newbie trying to learn to be safe, so feel free to give me any advise you want. I know I´ll learn this in future courses, but I read about it in the forum and think it´s something we should learn in OW, not tec courses.

Thanks a lot and excuse my English, I don´t practice too much my writing skills.

I noticed one error. An aluminum 80 really only holds 77.4 CF at 3000 PSI. In other words, replace the 80 with 77.4 CF.

Don't worry about your English writing skills. You do better than some people in the USA who use English as their first language.

 

[ucfdiver]

the concept of thirds could differ between individuals, but in Cavern diving I was taught you use 1/3 your air supply going in, turn @ the 1/3, have 1/3 for your exit, and have 1/3 left for your buddy in case of emergency. Your thirds would be calculated after S-drills and such. I've never heard of starting with reduction of your air supply with a "rock bottom" amount before calculating thirds. Where is this type of gas calculation taught/used?

What happens if you hit deco obligation on a dive that you share air on the exit?....Takes over 2/3 to get you out AND do deco. Other factors play a role too that I'm not thinking of, I'm sure.