3 Bottle dives

[rjack321]

OK so you are planning a dive in the 220ft range. Top is at 200ft, bottom 240ft. A wreck, but open water, no penetration. Its moderately cold water, 48F. Vis is typically 20ft at best and 8ft at worst. There's a ton of current and slack is barely predictable. You have a scooter. You are thinking 25min BT. Might be 20, on a bad day might get dragged out to 30mins.

Personally I am bringing the following:
doubles 119 or 130s of 15/55
3 AL40s, O2, 50% and 35/25

I have done a similar dive with smaller doubles and an Al80 stage of backgas, and only O2 and 50% for deco. I thought the deco was clean enough, but man was that AL80 alot of drag.

Toss out your ideas on how you'd plan this dive both volume and gas type wise (and of course reasoning on what works or doesn't for you).

 

[nadwidny]

Toss out your ideas on how you'd plan this dive both volume and gas type wise (and of course reasoning on what works or doesn't for you).

My idea would be to shut down this stupid DIR Practitioners thing.

 

[coldsmoke]

Why the 35/25?

Remember I only have the training for tech 1 level dives.

Is it necessary for deco, is it added for rock bottom issues, or is it due to ICD?

I assume the switch is at 100'. How long are you on it?

Hunter

 

[rjack321]

My idea would be to shut down this stupid DIR Practitioners thing.

I don't have the magic folding snorkel key.

 

[RTodd]

My idea would be to shut down this stupid DIR Practitioners thing.

You don't want to do that. There are certain people, you know who they are, who can be a real PITA. This section keeps them from posting into "real" discussions.

 

[rjack321]

Why the 35/25?

Remember I only have the training for tech 1 level dives.

Is it necessary for deco, is it added for rock bottom issues, or is it due to ICD?

I assume the switch is at 100'. How long are you on it?

Hunter

35/25 is 120ft deco gas
for this kind of dive probably: 3,2,2,2,2 on backgas, 70ft switch.
Ignore ICD its a ruse
Its not necessarily required for deco you can easy do a 200 dive on 50% and O2, 240 probably not, 220 is at the edge.
130s have enough gas for 25mins + rock bottom, 100s definitely don't, 119s are in between (depends on what stressed SAC you use and what your average ends up being)

If you want more gas, you can bring alot of bottom gas (al80) or a smaller amount of a deep deco gas (al40). I would like to hear people's preferences.

Or we can go back to discussing whether folding or straight snorkels are DIR

 

[RTodd]

Why don't you post your proposed deco profile for worst case and expected case and then we can discuss some gas usage assumptions that will make it easier for people to follow.

Deco on the 35/25 would start at 120. Adding a little helium seems to help the deco slightly but frankly I used to do it mostly because I had the helium around when filling everything else so I would put it in the 120 and the 70 bottle.

I don't do this type of dive very often anymore since I don't drive to FL regularly now. The logistics of doing them when you fly everywhere you dive aren't worth the hassle to me now. But, happy to chime in from the peanut gallery.

 

[Doc Intrepid]

I would use the same gasses as you selected.

I own PST E8-130s, so would use those.

The only decision for me (for the dive parameters you identified) would be items like predicted weather and location of the wreck.

Under nearly ideal circumstances I would prefer to take a stage 80 of bottom mix and breath it first, switching to backgas when it empties (and sending it up the anchor line). Then deco out with a 40 of 50% and a 40 of O2. Rationale is it provides a larger backgas reserve in case of issues.

But - though it has never happened to a boat I've been on - have heard reports frequently enough of storms blowing up at sea forcing charter captains to cut deco short to head back in to shore, that if that were a possibility I'd prefer what you selected: "3 AL40s, O2, 50% and 35/25". Allows going onto a higher O2 mix deeper, and puts us in a slightly better position if forced to cut the final deco stop short by 10 minutes or so.

[I don't have any idea how likely that is. But I've talked to (North Carolina) captains who told me the waves can get real high real fast, and that would be one option they'd consider if the alternative was to beat up the boat and the divers on it trying to race the storm back to port.]

Since we're not discussing snorkels...

 

[Doc Intrepid]

Just for your interest -

Here's a dive report of a wreck dive to the battleship USS Virginia, in 390 fsw off
Cape Hatteras, NC

http://uwex.us/DOPUSSVA.pdf

(Adobe reader) It's by Mike Barnette, on this board as AUEMike and Barney on TDS.

They had deco issues complicated by weather and boat issues. Interesting reading...

 

[rjack321]

Ok here's a profile, assuming you can keep to the 220ft average:

220ft -25
2 up to deep stops
160-1
150-1
140-1
130-1
120-3
110-2
100-2
90-2
80-2 backgas + prep for 50%
70-8
60-8
50-4
40-4
30-6 on backgas
20-10 (or 12 its not that important, whatever your fancy), 5 off, 10 on, 5 off, 5 on, 5 up

The O2 time is padded cause I am easily bendable, so I do ~5+ mins more than typical.

Gas usage:
rock bottom: 75cf (0.75cf/min stressed SAC)
15/55: 140cf (0.6cf/min SAC) + adding in a little bit for backgas breaks that I didn't try to calculate precisely
35/25: 20cf (0.5 deco SAC)
50%: 32cf
O2: 24cf

The biggest issues I have with AL80s of backgas (or 50%) is: 1) drag (even with a scooter) and 2) they require you do screw around at depth and waste precious bottom time. If I had bare hands it would be no big deal, scootering in dry gloves I don't wanna waste BT messing with stages if I can help it. I did a couple dives with just 50%+O2 and using a bottom stage and while it was fine it wasn't as clean as I'd like.

(feel free to double check all my numbers, I literally did them on an envelope)