So, for the last two weeks we’ve been looking at Oxygen Toxicity. Week one of the three part series was a reminder of what oxygen toxicity is, and how we do some of the fundamental calculations behind it. In week two, we looked at the two types of oxygen toxicity in a bit more detail, and some of the science behind tracking them.
This week, we’ll look at how to put that into practice with a planned dive – within the realms of a PADI / DSAT Tec 40 Diver – in which we’ll pick our gases, track our oxygen exposure and make sure that what we’re doing in the water is safe.
Step one: Pick your dive
For this example let’s work on the assumption that we’re going to dive a wreck sitting in 40m of water, and that she stands 6m proud of the bottom. We only plan to dive to the halfway line on the hull, however, so our target depth is 37m.
Step two: Pick your gas
So.. what gas should we take?
Given that there’s a bottom at 40m, we’d be hard-pressed to get any deeper than that. Personally, I’d sacrifice a little bit of dive time for the reassurance of knowing I can’t get too deep for my gas to be too rich – so I’d plan a gas for a 40m dive rather than a 37m one.
Referring back to our Pressure T from post number one:
We can see that the FO2 – the fraction of oxygen in the mixture – can be found by dividing our PO2 by the pressure at depth. In our case, the depth will be 5ata (40m) and our PO2 is going to be limited at 1.4ata… so we end up with:
FO2 = 1.4 / 5 = 0.28, which translates to 28% oxygen, or EAN28.
So, that’s our bottom gas sorted.
We’re also going to take a cylinder of EAN50 on which we’ll do our decompression for added conservatism. In that case, we know the PO2 we’re going to switch at (1.6ata) and we know the fraction of oxygen (50% = 0.5)… so we can find the pressure at which we’ll make our gas switch by covering up the P:
P = PO2 / FO2 = 1.6 / 0.5 = 3.2ata
Back to open water maths and we know that each 10m of water accounts for 1ata, and there’s one for the surface. Subtracting that one first leaves us 2.2ata of “water pressure” – or 22m… and there’s our switch depth.
Step three: Plan your bottom time & stops
As a Tec 40 diver, we’re limited to 10 minutes of decompression – so a bit of trial-and-error using your favourite brand of dive planning software eventually gives us a dive time that fits that limit:
Note that I’ve set my bottom gas to EAN28, and although I’ve got EAN50 as a decompression gas it isn’t turned on (no tick in the box) – so I’m using it for conservatism not accelerated decompression.
But… that means my oxygen calculations in the dive planner are off. It thinks I’ll be using EAN28 when actually I’ll be using EAN50 from 22m and upwards… so it’ll be under-estimating and we need to calculate it by hand.
Step 4: Plan your Oxygen Exposure
To calculate our oxygen exposure, we’ll need to know our PO2 at each stage of the dive… which means working it out using the same formula as above
(PO2 = FO2 x P)
Our first double-check here is simple, and relates purely to CNS Oxygen Toxicity. Ask yourself the easy question:
Is my PO2, at any point above 1.4ata at any point on the bottom, or 1.6ata during decompression?
If the answer to either of those bits of that question is yes, do not do the dive.
For Pulmonary Oxygen Toxicity tracking, we need to track both the “CNS Clock” and Oxygen Tolerance Units.
Let’s start with the CNS Clock
We need to look at the NOAA table (in last week’s post) and find the single exposure limit for each PO2 in the table above. We can then work out the percentage of that single exposure time we’ve used up based on the stops list above:
|Depth||PO2||Single exposure limit||Our time at that PO2||% of limit|
|40m||1.4||150 mins||18 mins||18 / 150 = 12%|
|12m||1.1||240 mins||1 min||1 / 240 = 1%|
|9m||0.95||300 mins||1 min||1 / 300 = 1%|
|6m||0.8||450 mins||3 mins||3 / 450 = 1%|
|3m||0.65||570 mins||5 mins||5 / 570 = 1%|
Adding up those percentages (note that I rounded up to 1% for anything less than that, for conservatism), we can see that we’ve used just 16% of our total limit – well within them.
The easiest way to do OTU tracking is with a set of tables which give you an “OTU per minute” figure for each PO2 – but you can do it with a formula:(tx is the time of the exposure)
For reference, here’s the result of that formula for the PO2’s between 0.6 and 1.6ata in 0.05ata increments:
|PO2||OTUs per minute|
From there, tracking our OTUs for the dive is a simple matter of working multiplying the “per minute” figure for each stage of the dive by the number of minutes we spend there:
|Depth||PO2||OTU per min||Our time at that PO2||% of limit|
|40m||1.4||1.63||18 mins||18 x 1.63 = 29.34|
|12m||1.1||1.16||1 min||1 x 1.16 = 1.16|
|9m||0.95||0.92||1 min||1 x 0.92 = 0.92|
|6m||0.8||0.65||3 mins||3 x 0.65 = 1.95|
|3m||0.65||0.37||5 mins||5 x 0.37 = 1.85|
Again, add those together and we achieve a total of 35.22. But… 35.22 out of what?
There’s a (yes, another!) table that tells you how many OTUs you’re allowed on a given day based on how many days in a row you plan to dive. The good news is, you almost certainly don’t need it. Once you exceed 10 days in a row your limit drops to 300 OTUs per day and stays there, whether you do 11 days or 1,100 days. So… remember 300 as a daily limit and you’re good regardless of how many days.
So… what’s the conclusion?
We’ve just worked out a dive plan for a 40m, 18 minute dive – with 9 minutes of decompression, using EAN28 for back-gas (even if you’re on sidemount!) and EAN50 for conservatism on the decompression.
Despite the gas switch to a higher mix we’re still well within our safe limits – the highest PO2 we should experience on the bottom is 1.4ata even if we reach the sea bed (remember we actually plan to stay at 37m, so even more conservatism built in there) and we’ll be switching to our EAN50 after we pass 22m… so less than 1.6ata there. Our first decompression stop will be at a very safe 1.1ata. That’s CNS toxicity neatly covered.
Pulmonary toxicity was a bit more convoluted, but we know that by the end of the dive we’ll have used 35.22 OTUs (out of a limit of 300) and 16% of the CNS clock. Again, plenty of safety built in there.
It might seem like a lot of planning, but consider for a moment that same dive compared to a standard recreational dive planned on the RDP. At 40m your no-stop time is just 9 minutes; assume it takes you 2 or 3 to get down there and you’re left with just 6 or 7 minutes on the wreck. Because it’s a dive deeper than 30m (and to a limit of the RDP) you’ll have a required 3 minute safety stop to do on the way up, and you’ll surface right on the edge of the safe decompression threshold.
As a Tec 40 diver you’ll have double the total bottom time, and 15 or 16 minutes down there on the wreck itself. Your ascent time includes 9 minutes of stops (6 minutes more stops for 10 minutes more on the bottom!) and you’ll surface with tons of conservatism thanks to a combination of planning the deco for a dive deeper than the one you actually made and planning the stops on EAN28 and doing them on EAN50.
Doesn’t that sound better to you? It does to me!
About the author: Kevin is a PADI Master Instructor, DSAT Tec Trimix Instructor, TDI Advanced Trimix Instructor, IANTD Instructor Trainer and IART CCR Instructor. He’s been diving professionally for more than a decade and has been fortunate enough to dive in many of the world’s most famous technical dive destinations.