Matching tanks and gas and dives: What's your recommendation?

[RJP]

Scenario:

On an upcoming trip to Hawaii I will be doing two charters in one day - one in the morning and one in the afternoon - for a total of four dives. The dive profiles on each trip will be "a deeper dive followed by a shallow reef dive."

The dive op has one set of 100s and the rest 80s. The 100s are air only, the 80's are nitrox.

Question:

To maximize my bottom time and safety how should I mix and match tanks and gas?

Assuming sufficient surface intervals between dives 1-2 and 3-4 (and a longer interval between dives 2 and 3 as the boat will be heading back and then out again for the afternoon charter) I'm thinking that I should dive the 100's in the AM with air and then the 80s with Nitrox in the PM. Figuring I'll be more tired in the afternoon, that if my dive is shortened by gas supply I'm not that worried about it.

Thoughts?

 

[The Kraken]

Well given the long S/I between the two sets of dives I'd probably dive 80/EAN32 - 100 on each set of dives.

Let's just say you have a SAC rate of 0.50 and each of the two sets of dives will be something around 100' for the first dive and 60' for the second dive.

This analysis is based on a residual gas pressure of 500 psi at the end of the dive.

At 100' you'll have about 30' on the bottom with an Al 80. A 100 cf tank would yield approximately 42 minutes of bottom time. The NDL for an EAN32 dive to 100' is about 27 minutes (EAD = 85'). The 100 cf tank would garner you nothing, especially since you would have to be diving on air, which would limit your NDL to 20 minutes.

Diving the AL 80 with EAN32 would put you in the "Q" pressure group at the end of the dive.

Now if your surface interval was say, 1:45, that would put you in pressure group "B".

If your second dive was to 60', that would give you an NDL of about 45 minutes . With a SAC rate of 0.50, an AL 80 will give you approximately 43 minutes bottom time at 60'.

The 100 cf tank would give you gas to spare.

But that's just the way I'd probably do it.

the K

 

[jbd]

I had a similar dive day in Mackinac Straits. I had nitrox avaliable for all four dives though. I dove the lower FO2 on the first dive of each set followed by the higher FO2 for the second dive of each set. I felt as though that gave me the least N2 loading prior to the longer lunch break SI.

 

[Meng_Tze]

Well given the long S/I between the two sets of dives I'd probably dive 80/EAN32 - 100 on each set of dives.

Let's just say you have a SAC rate of 0.50 and each of the two sets of dives will be something around 100' for the first dive and 60' for the second dive.

This analysis is based on a residual gas pressure of 500 psi at the end of the dive.

At 100' you'll have about 30' on the bottom with an Al 80. A 100 cf tank would yield approximately 42 minutes of bottom time. The NDL for an EAN32 dive to 100' is about 27 minutes (EAD = 85'). The 100 cf tank would garner you nothing, especially since you would have to be diving on air, which would limit your NDL to 20 minutes.

Diving the AL 80 with EAN32 would put you in the "Q" pressure group at the end of the dive.

Now if your surface interval was say, 1:45, that would put you in pressure group "B".

If your second dive was to 60', that would give you an NDL of about 45 minutes . With a SAC rate of 0.50, an AL 80 will give you approximately 43 minutes bottom time at 60'.

The 100 cf tank would give you gas to spare.

But that's just the way I'd probably do it.

the K

What he said....

Focus the tank/gas selection of what you need.....based on depth, consumption rate and exposure (N2 loading, OTU's, CNS, PO2, etc).

 

[MikeFerrara]

I haven't seen any mention of gas planning or prudent reserves...the 500 psi thing don't count. I didn't see any mention of how deep the "deeper" dives are but there's a good chance that I'd be using the larger gas supply for the deeper dives just because of contingency gas requirements. I also didn't see any mention on the type of dives and the relative need to have enough gas to get all the way back underwater vs just getting to the surface. Remember you need reserve gas for your buddy too.

If the choice of equipment is limited, you might consider choosing sites that will allow you to do the kind of dive that you was using that equipment.

 

[TSandM]

For me, doing four dives in a day, I'd dive 32% for all of them, and match the profiles to the gas supply.

 

[The Kraken]

TS&M,

He can't do all 4 dives on nitrox. Only the 80 cubic feet tanks can be used for EAN.

But that's a good point. Why not just ask for 4 80's ???

Really don't feel like doing the math again, but it just may work out better.

the K

 

[akscuba]

TS&M,

He can't do all 4 dives on nitrox. Only the 80 cubic feet tanks can be used for EAN.

But that's a good point. Why not just ask for 4 80's ???

Really don't feel like doing the math again, but it just may work out better.

the K

Where are you reading he has to use the 100s??? I'd just use 80s with 32%...

 

[MikeFerrara]

TS&M,

He can't do all 4 dives on nitrox. Only the 80 cubic feet tanks can be used for EAN.

But that's a good point. Why not just ask for 4 80's ???

Really don't feel like doing the math again, but it just may work out better.

the K

I think she is saying that she would stick with the 32% and plan her dives within her gas supply.

LOL, let the gumball heads who can't figure out how to get nitrox into the bigger tanks dive with them.:11doh: in the time it took me to type this, they could have just about had those tanks cleaned, inspected and ready for a nitrox fill.

 

[TSandM]

Yes, I meant do all four dives on Al80s with Nitrox, and adjust profiles accordingly.