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View Full Version : Droptanks: 80% or 100%



Gary
21-05-2013, 08:53 PM
We've always had 80% in our 12L droptank, generally for a couple of reasons

a) a diver needing a droptank may be task loaded and may not stick rigidly to a 6m stop
b) we can get higher pressure 80% (generally 240-250bar) than O2 (200bar)
c) I as a diver used 80% rather than 100% so its a carry-over from that

Having used our droptank a couple of weeks ago via a RescueEAN for an ancillary oxygen supply, I gave it further thought, and I'm wondering if we should change ours to 100%

Thoughts?

The Duck
21-05-2013, 09:03 PM
I like O2 but if it comes to needing the skippers gas I'll take whatever's going and alternate between being very thankful and cursing myself for having f*uked up so bad that I'd blown both my primary & backup plans!

DiverMike
21-05-2013, 09:04 PM
80% for the following reasons
- a 230 bar fill (over 180b in most places for O2) means more gas
- CCR divers bailing from a CO2 hit will be more likely to tox
- there was a 3rd one but I forgot.

if you go 100% putting it on 6.5m of line will limit the damage.

Id also make sure it was clearly marked with mix and mod as no one pays attention to boat briefs (which seats are the fares under?) but when the bottle arrives at 6m it's good to know what it is.

Are their any liability issues with a boat providing a drop tank?

Roy
21-05-2013, 09:14 PM
We've always had 80% in our 12L droptank, generally for a couple of reasons

a) a diver needing a droptank may be task loaded and may not stick rigidly to a 6m stop
b) we can get higher pressure 80% (generally 240-250bar) than O2 (200bar)
c) I as a diver used 80% rather than 100% so its a carry-over from that

Having used our droptank a couple of weeks ago via a RescueEAN for an ancillary oxygen supply, I gave it further thought, and I'm wondering if we should change ours to 100%

Thoughts?

I'd think 80, primarily for reason a.

But as the Duck said, I'd just be grateful for whatever arrived (if clearly marked, of course).

GLOC
21-05-2013, 09:20 PM
What depth Gary?

80% for the following reasons
- a 230 bar fill (over 180b in most places for O2) means more gas
- CCR divers bailing from a CO2 hit will be more likely to tox
- there was a 3rd one but I forgot.

if you go 100% putting it on 6.5m of line will limit the damage.


80% at 6m or 80% at 9m? The latter will cause the same issues for toxing with high pO2.

Regards

Gary
21-05-2013, 09:27 PM
What depth Gary?

6m

notdeadyet
21-05-2013, 09:34 PM
I've always been a fan of oxygen for deco particularly when it's deco in the shit but I can understand the practical aspects of EAN80.

Marginally higher fill pressure but also marginally longer deco compared to O2, I suspect they might cancel each other out.

Barrygoss
21-05-2013, 09:37 PM
We've always had 80% in our 12L droptank, generally for a couple of reasons

a) a diver needing a droptank may be task loaded and may not stick rigidly to a 6m stop
b) we can get higher pressure 80% (generally 240-250bar) than O2 (200bar)
c) I as a diver used 80% rather than 100% so its a carry-over from that

Having used our droptank a couple of weeks ago via a RescueEAN for an ancillary oxygen supply, I gave it further thought, and I'm wondering if we should change ours to 100%

Thoughts?

A drop tank of 80%, because we're really talking a minute or two deco saved.
A 18L of O2 on the boat. Cos that's where I really need it, on the journey back to port.

B

In the man cave piccie from FB, that's a steel 18L in silver in the corner ;)

gobfish1
22-05-2013, 12:32 AM
02 for on the boat , drop tanks should be sorted by the dive team , feek me they will want you to have a diver ready to jump in next .. , :8):

if you want to toss a cylinder over the side o2 , 6m if the diver is going to tox best move to 3m asap , closer to topside the better

dont go there m8.

divers gets a o2 hit on one of your drop tanks , whats going to happen ,, adw time .:shagger

Iain3
22-05-2013, 05:11 AM
I would probably go for 100%.


If task loaded then I hope the diver will be hanging on a line, the line the tank is on if nothing else, and will not go deeper than the line allows.
If administered to a bent diver on the surface O2 is apparently far more valuable to them than 80%, or at least I am sure I read that somewhere.
If they are running a rebreather with low or lost O2 supply they can use it to inject O2 into the loop via the manual adds, if you make sure there is a LP inflator hose on there.


Only downside I see is the added risk of Tox. If they are that close to a tox that 100% triggers it and 80% doesn't then they should probably sit on air for a while or get out the water and breath the O2 up there while waiting for a helicopter.


Having read some of the other posts my answer is assuming there is only room for one extra cylinder on the boat, which is sometimes the case, otherwise I choose onboard manned deco chamber with O2, martinnis and sexy nurses.

JPTaylor
22-05-2013, 05:31 AM
Most of the boat skippers I know use 100%.

Why? Easy to top up from a J.

Mark Chase
22-05-2013, 06:48 AM
We use 80%

1: CNS on CCR dives can be an issue especialy if the diver has bailed out for C02 reasions. A steressed diver breathing heavy on pure 02 slightly too deep after a long dive on 1.3 isnt playing the odds on the safe side.

2: The deco station isn't at 6m Put 6m ropes on a deco station and every one ends up constantly bumping into missed deco on their computers so we set it lower usualy 6.5 -7m. You might not notice this issue on shorter deco but you will if you start having 30mins + at 6m on a bouncy day.

3: You need a good booster to boost 70 bar of 02 up to 235bar in a 10. So blending 80 is easier for good fills and keeping 80 as a standard every one knows where they are (on our boat)

4: Most divers have their computers set at 1.3 for deco so if they have forgoten to change the computer over, 80% is conveniantly 1.28 at 6m. This isnt an advantage over 02 but its not a disadvantage and its a reasion we dont chose 70%


5: ALWAYS on the 9m line because you send for the drop tank when your at the 9m stop you dont wait to 6m and wait another 20mins for the tank to arrive. Having 100% droped to 9m could tempt the stressed diver on to a tox gas.

6: The 50% tin is the one your most likley to empty if you look at bailout plans with abreaviated deep stops. So it may be necessary to get on the 80% at 9m

7: The 6m stop is where your most likley to be with other divers who can provide gas so if you run out of 80 because you used it on the 9m stop there should be plenty of gas about. You might think only being 3m below the other divers is no problem to signal for gas etc but its a mile away in safety terms. Having bailed out you could find your early to the deco bar (so alone) so it stops you haveing to go down to 9m on 100% to get gas from another diver. Or you may be late (the reasion for bailout may have delayed your return to the shot) so your almost out of gas but all the divers ar at 6m.. The 80% drop tank bridges that gap.


But for all our good intentions we have often have to work with what we have got. On one ocasion we actualy needed the drop tank, it had been air toped and ended up at 70%


Any drop tank is better than none but anything planned at less than 80% seems a bit pointless and anything over runs into logistical and safety issues.

Janos on his flooded KISS uses the drop tank Note we are the only divers left on the bar / in the water.

http://img.photobucket.com/albums/v465/MarkChase/Trip%20Reports/new-4.gif



ATB

Mark

Major Clanger
22-05-2013, 01:37 PM
80% for reason stated. Plus, if I've really fooked up I don't want to have to think about air breaks as well.

steve6690
22-05-2013, 09:19 PM
80%. For reasons of fill pressure and the fact that the extra deco over 02 is marginal.

Diving Dude
22-05-2013, 10:12 PM
I say 80% simply because that's what Chasey says is in the tin.

SilentDiver
24-05-2013, 08:20 AM
Doesnt this sort of question normally demand a Poll..........:devil:

Personally my soon to be set up drop tank will have 80% in it.

matt
25-05-2013, 04:49 PM
Personally wouldn't mess with 80% at 6m and would go with O2 - it if is tied to the line how will the diver get deeper?

No use of course if the diver has hefty stops from 21m up so we'd normally have a deep string with air at 60m and a 50% at 21m.

Matt.

Kev
26-05-2013, 12:28 AM
If I need it then you send down whatever you like!

:)

bletso
26-05-2013, 11:17 AM
6: The 50% tin is the one your most likley to empty if you look at bailout plans with abreaviated deep stops. So it may be necessary to get on the 80% at 9m

7: The 6m stop is where your most likley to be with other divers who can provide gas so if you run out of 80 because you used it on the 9m stop there should be plenty of gas about.
Mark

Very true. 50 is the only one we have ever run low on. 80 is nice except in the field you will have more trouble topping off. O2 is easy, so I use 100. Just boost and go. It is more a logistics problem unless you decant / boost from a large premix 80.

an example of 50 is we had a cut short dive and calculations showed we would be short 1-2 min on 50 on a subsequent dive. We were short 1 minute.

Point 7, spot on.

Safe Diving

matt
26-05-2013, 01:39 PM
Very true. 50 is the only one we have ever run low on. 80 is nice except in the field you will have more trouble topping off. O2 is easy, so I use 100. Just boost and go. It is more a logistics problem unless you decant / boost from a large premix 80.

an example of 50 is we had a cut short dive and calculations showed we would be short 1-2 min on 50 on a subsequent dive. We were short 1 minute.

Point 7, spot on.

Safe Diving

When you say point 7 is spot on, it made me think a bit more. For a staged cylinder, like the one in the photo, it is spot on. Although I still contend a cylinder fastened to a bar represents no hyperoxic risk.

However for a _drop_ tank, where are all these other divers coming from? Why were they not present at the time the drop was requested? Or am I confused and we're heading up the line (which IMHO is the very best solution) and the gas may be either _staged_ or _dropped_ or _borrowed_?

Matt.

bletso
26-05-2013, 03:18 PM
I would think that a drop cylinder should be left at its MOD. 80 at 9 msw and 100 at 6 msw. Certainly a diver should accept some responsibility. The dive briefing should outline where and what will be available.

Now in cave, we drop our deco gases 3 msw below where we will need them to allow us to pick it up, and proceed to 1st stop. That is planned so being deeper than MOD is no problem. In an OW out of deco gas scenario it should probably be left where I mentioned, at MOD. It should not be an emergency situation as the diver can always switch to back gas.

Dale

Mark Chase
26-05-2013, 08:08 PM
I would think that a drop cylinder should be left at its MOD. 80 at 9 msw and 100 at 6 msw. Certainly a diver should accept some responsibility. The dive briefing should outline where and what will be available.

Now in cave, we drop our deco gases 3 msw below where we will need them to allow us to pick it up, and proceed to 1st stop. That is planned so being deeper than MOD is no problem. In an OW out of deco gas scenario it should probably be left where I mentioned, at MOD. It should not be an emergency situation as the diver can always switch to back gas.

Dale



You cant go back on to back gas with a CCR failure.

ATB

Mark

bletso
27-05-2013, 11:38 AM
You cant go back on to back gas with a CCR failure.

ATB

Mark
That is correct but you should certainly have diluent bailout capability. I have both on-board and off-board bailout diluent available on all dives deeper than 10 msw and just on-board for less than 10 msw. (30 cf 850 l)

Dale

Mark Chase
27-05-2013, 01:55 PM
That is correct but you should certainly have diluent bailout capability. I have both on-board and off-board bailout diluent available on all dives deeper than 10 msw and just on-board for less than 10 msw. (30 cf 850 l)

Dale



Not all units have the posabuility of accessing on board diluient without a functioning loop.

On my JJ i can get to Diluient via the ADV but only if the loop isnt flooded or I just get a mouthfull of water.

Same on my KISS and on my rEvo.

My inspo classic with large lungs would work (so i am asuming a front lung meg would as well) but only in the upright position and even then only up untill the lungs filled up which wouldent take long with the optional small lungs on the Inspo or the standard lungs on a Meg.


The main issue is the dropping of the tank. If your on loyal watcher they drop the tank "near " you. It took three attempts to get the drop tank to Howard in such a position as he spotted it. End result it was a good 20mins from sending up the request to him getting on the gas.


For this reasion the 9m stop which will usualy be a long stop, is the best period of time to receive the gas in preporation for it being required.


ATB

Mark

Gary
28-05-2013, 07:30 PM
I think I'll just stick with the 80% as theres little compelling arguement to change to 100%

Scuba steve
29-05-2013, 08:40 AM
We use 80% bigger margin for error in a stressed diver and the marginal differance in deco is a no brainer . We also have a safety diver with deeper dives .

But to be honest if i was low on gas with a massive deco ahead of me just get me somthing breathable that will last the duration . Anything else i can work out after that . ( the only exception to this whould be really lean nitrox or air if id been doing deeper dives )