Nitrox for the PS diver

[Lucy's Diver]

Good question
I'll be echoing allot of what has already been said but bear in mind the vast majority (over 90%?) of teams will never go below 60ft - allot won't exceed 30ft.
We don't want bottom times longer than 20min even if the tissues allowed it - the concentration level of the brain and environmental exposure maxes out about that point anyway when doing multiple dives - ie up to 6-8 a day. If we keep the dives short we CAN do that many dives safely in a day. Nitrox doesn't give you anymore MULTIPLE dives which is a big consideration for rescue or recovery. Once we max out a diver he's done for the day and we need to find a new one

Unfortunately cost is always a factor for PSD but so is time to train. It would take allot of effort to train a team (and maintain competency!) in handling nitrox. Time and money that could be spent in other areas.

There is always a multitude of team members that handle one's gear. Both for checks and set-up. many are non-divers. Too much can go wrong without COMPLETE understanding of nitrox ie - bad mixes.

Most of the time it is the tenders that handle the dive profiles, monitor SAC, bottom time, depth and especailly the search pattern etc (because the diver can't see). Tracking O2 exposures would be a nightmare.
Also, we're supposed to analyze gas before the dive which won't happen in PSD

Basically, when dealing with these shallow depths and times (less than 60ft and 20min) there are many more complications to using nitrox than the benefit it provides

Why multiple short dives?
We, a surface supply air recovery team, strive for the longest bottom times we can, but our SOP is one dive per diver per day. In less than 30 feet 2-4 hour bottom times are typical. Part of this is that swapping divers on a SSA rig is quite a time investment.
We do use nitrox on dives over 30 feet, but we also have a nitroc compressor so its essentially free.

 

[bridgediver]

Why multiple short dives?

A few reasons. Mainly diver attention span and logistics. Heres what I mean.

1) attention span. For you commercial guys you're probably focused (or at the very least engaged directly) with what you're supposed to be doing so your mind is "occupied" by your task. With us, we haven't found it yet and with blackwater or even zero vis the mind will begin to wander and we lose concentration. Without full concentration the effectiveness of the search is reduced - may go too fast, not realize what we're touching, get paranoid, depressed that "I'll never find it!" etc.
A good span of attention is of course variable for each person on any given day but a reasonable amount of time can be 15-20min.
As an example, there was a team looking for a drowning victim utilizing long dives. After a time they eventually found the victim in a spot they had already serched once before and it dawned on the tender that handled the diver in that location on the first pass that the diver said he remembered finding a "big ball of mud" (which turned out to be the head of the victim) -- he had found the guy the first time but wasn't focused enough to realize it and kept on plodding along

2) Unknown search plan. We don't really know how long we may be searching. It could be the first dive and it could last the week. If we look at the tables and we want a diver to have say 4hrs of BT he will have much less N2 loading if he does short dives vs. 1-2 long ones. We want to limit our N2 loading because we may need the diver for more dives/days

3) Environmental exposure. We may not be as prepared for long exposures as you guys are. It would be rare to see PSDs with heated suits for example. We may not be in a rested, fed and hydrated state when the call comes in (which would contribute to more heat loss). We could be (and often do) find most calls at the end of our shift or early evening
Also, don't forget that the back-up and 90% ready divers are also going to be subjected to the surface environment before they dive for the duration of the dive (or twice that if its the 90%)

4) Search urgency. Some searches may be urgent (rescue) and others recovery but either way it looks bad if the team needs to "take a break" while little johnny is still in the lake. We should be able to have a diver turnaround of only a couple of minutes and keep the op going while guys rehab as they need it. Unless you're fortunate to have allot of divers we have to cycle through what we have - 4 is optimum.

5) fatigue. With long dives we pretty much have to switch out all 3 divers and tenders after each dive. Time to undress, grab a drink , a pee and redress could be 15-20min depending on the fatigue level - thats allot of time spent not searching at all. Let's say the dive is 1hr, the back-up and 90% are sitting in the wind/snow/rain/sun/bugs (whatever) for that hour then the back-up should be ready to do his 1hr dive - now the 90% becomes the back-up and must sit another hour and will essentially get wet/cold/hot/hungry/dehydrated (need BATHROOM!!) for 2 hrs before he can look forward to a 1hr long dive. Up to 3hrs in his gear he will not be an effective searcher in this state.

6) Rehab. Its a fact that it takes allot of time to rewarm and rehydrate team members if they begin to get chilled. If they get mild hypothermia they are pretty much finished for the day and maybe for a couple of days. Most teams don't have that many members to call upon. The best course of action is to prevent the team from getting cold, dehydrated and tired in the first place. Short dives allow frequent rehab and we don't burn anyone out
*Also, tenders can get cold and tired too


So here's what we do. If we have 4 divers we can cycle each guy through a 20min postion of primary, back-up, 90% ready and rehabbing diver. The longest a guy will be dressed is 1hr before he can take a break and refresh.
If we have 5-6 divers we can rehab each guy longer each hour. We could run an op for 8-10hrs straight without stopping the search - thats allot of searching!
If we have 6-8 divers we can also run 2 seperate searches in this manner doubling our efforts.
It works pretty slick

hope that makes sense

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