[emmbee]

Hi all,

Apologies in advance if this is all well-known useless cr*p...

During my AN&DP course my instructor mentionned in passing the factoid that in the recreationnal nitrox range the sum of the O2% and max depth for a 1.6bar ppO2 was 70-ish. Said it might be useful to sanity-check max depth on the boat for low O2% deco mixes (obviously everyone knows them for 50%, 80% and 100%).

Did the math on train this morning. It works because the sum, 100*fO2-10+10*ppO2/fO2 reaches a minima of Smin=20*sqrt(10*ppO2)-10 at fO2=sqrt(ppO2/10), and doesn't grow up too much around it.

So basically, for a ppO2 of 1.6bar, the minima is 70, at 40%, and the sum remains between 70 and 72 for a fO2 between 32% and 50%.

For a ppO2 of 1.4bar, it's 65 (64.8), at 37.4%, and the sum remains between 64.8 and 66.8 for a fO2 between 30% and 47%.

To sum up, in the specified range, you can quickly guess the MOD by just doing 70-O2% (1.6) or 65-O2% (1.4), and it will be a bit on the conservative side of things, by at most 2m (well, modulo 20cm at 1.4).

For instance:

1.6:
mix quick real
32%: 38m 40.0m
34%: 36m 37.1m
40%: 30m 30.0m
44%: 26m 26.4m
48%: 22m 23.3m
50%: 20m 22.0m

1.4:
mix quick real
30%: 35m 36.7m
32%: 33m 33.8m
35%: 30m 30.0m
36%: 29m 28.9m
37%: 28m 27.8m
38%: 27m 26.8m
39%: 26m 25.9m
40%: 25m 25.0m
43%: 22m 22.6m
44%: 21m 21.8m
47%: 18m 19.8m

Now obviously everyone has properly marked cylinders, knows his or her MOD and dives with a Nitrox/Trimix computer that they know how to use and have actually set up properly, but maybe in the real world...

Thoughts?

Cheers,

Matthieu

 

[Starwatcher]

Not come across this before, but looks like a nice quick "rule of thumb". Thanks.

In practice I can't see the sightly conservative estimate making any difference. Who dives a site to such precision?

 

[petlowe]

Thoughts?

Cheers,

Matthieu

I think you have a pending meeting with Nigel H and a shining career on YD, where gas geeks come to vent
Welcome, you will make many friends here

Cheers,
P

 

[12480]

A couple of things concern me here, first is the use of the term ish by an instructor.

At 1.6 air will give a MOD of 66.19 ... at that depth, the working part of the dive then 1.4 is the accepted limit giving a depth on air of 56.6

Why use rule of thumb when it can be precisely worked out?

For those not good with maths things like this are available

UKDivers.net - MOD Calculator

If people wish to ish then good luck to them, if I cant do the maths on the boat in my head I could have a laminated table or look at my computer pre dive, there are many more options to get an exact reading...

 

[The Duck]

ish works for sanity checking - I can work out quite a few things precisely, unfortunately on the odd occasion they can be precisely wrong :redface: so a ball park figure can give a good check.

 

[12480]

ish works for sanity checking - I can work out quite a few things precisely, unfortunately on the odd occasion they can be precisely wrong :redface: so a ball park figure can give a good check.

Fair enough but isn't it written on your tanks when you analyse like a good little diver?

 

[Mal Bridgeman]

1.4 and 1.6 are considered by some (me included) to be "aggressive" in CNS terms for a bottom/working phase of a dive. I would tend to plan on 1.2 ppO2 for the bottom phase so that I can more safely increase the ppO2 for the decompression phase.

An alternative to the rule of thumb you have proposed is to just limit the number of gases mixtures you work with and then learn the appropriate values .... e.g. 32% is 30m (as a bottom phase gas), 50% is 21m as a deco gas, O2 is 6m as a deco gas.

HTH
Mal

 

[11431]

It's not something I've seen before, but neither can I see any practical use for it;

- I wouldn't use it to check my pre-calculated MOD is "in the right area", as the difference between it and the accurate figure varies, by up to 3 metres for a 1.4 PO2 and over 5 for a 1.6 PO2 (taking the range of 28 - 40% mixes). Taking 40% as the gas in the cylinder, if I gave myself a 3 metre variance around the fudged 1.4 MOD, I could end up diving the gas to 28 metres, resulting in a PO2 of 1.52. Give myself 5 metres around the 1.6 fudged MOD, I could end up at 35 metres, a PO2 of 1.8!!! To ensure that I didn't push myself over the odds, I'd have to assume the fudged method was accurate with no leniency... but then I'd just end up checking every single MOD again (with the exception of 40%)

- I wouldn't use it as a guide to guess an MOD for a Plan B, if the main dive was cancelled. The mix would have been prepped for the dive, so the planned depth of the dive gives me my limiting factor if I ended up facing another option (e.g if I was blown out).

And at the end of the day, working out the MOD of a gas is hardly difficult.... you can almost do it in your head (you just need to get rid of all those horrid +10, -10 nonsense bits in the equation)!!!

1.4 / Fraction of O2 = Max Depth expressed in Atmospheres for a 1.4 PO2

1.6 / Fraction of O2 = Max Depth expressed in Atmospheres for a 1.6 PO2

 

[Digger]

In reality, you will do a few dives and then realise that you know exactly the MOD off hand.

Everyone regularly diving adv nitrox or trimix dives will know the MOD of 21%, 32%, 50%, 80% and 100%. You therefore know roughly where any of the other numbers should be - if someone told me that the MOD of 40% was 40m I'd know they were out because that's the MOD of 32%.

It's a very rough rule, and if you were brand new to nitrox I can understand it, but then why not just have the MODs written down on a table that you can refer to for sense checking? The entire MOD table and % could be written on a credit card and easily readable, that way you're not doing jungle maths on a boat rocking about thinking about MODs.

Digs.

 

[Captain Calamity]

Thanks. Clever.

The small difference between your "rule of thumb" and the calculated MOD does not matter as the O2 analyser is only accurate to 1% anyway.

Also going to a PPO2 of 1.41 at depth will most likely not lead to your immediate demise. IIRC correctly the figures were reached by exposing lots of test subjects to a range of high PPO2 and recording when they convulsed. Some got all twitchy earlier than others, and the 1.4 and 1.6 represent levels at which most divers are not likely to convulse.

I'm not suggesting everyone go and ignore the PPO2 limits, just that there is some leeway in both analysing and in the level at which toxing occurs, so your rule of thumb is good enough for me.

CC

 

[tufty]

i'm with po2/absolute pressure in atm easy

 

[chrisch]

It's not something I've seen before, but neither can I see any practical use for it;
...

Me neither. Looks complex to solve a non existent problem.

 

[matthewoutram]

1.4 and 1.6 are considered by some (me included) to be "aggressive" in CNS terms for a bottom/working phase of a dive. I would tend to plan on 1.2 ppO2 for the bottom phase so that I can more safely increase the ppO2 for the decompression phase.

Mal

I'd be interested to know why 1.4 is considered aggressive?

Thanks
Matt.

 

[matthewoutram]

And at the end of the day, working out the MOD of a gas is hardly difficult.... you can almost do it in your head (you just need to get rid of all those horrid +10, -10 nonsense bits in the equation)!!!

1.4 / Fraction of O2 = Max Depth expressed in Atmospheres for a 1.4 PO2

1.6 / Fraction of O2 = Max Depth expressed in Atmospheres for a 1.6 PO2

And it's even easier in your head without the decimals...

140 / %O2 = Max Depth expressed in Atmospheres for a 1.4 PO2
160 / %O2 = Max Depth expressed in Atmospheres for a 1.6 PO2

Cheers
Matt.

 

[chrisch]

I'd be interested to know why 1.4 is considered aggressive?
...

I don't think there is any hard and fast rule as such but the 1.4 figure was generated by the recreational agencies in the early era of Nitrox on OC. With the advent of CCR divers are exposed to much longer periods of elevated PPO2 than was the case. Furthermore the use of even higher PPO" for deco adds to the amount of O2 free radicles you are subject to in a dive.

Many agencies now are moving away from long periods of high O2 as a precaution, since the long term effect on the body is not know. Ordinary everyday humans have not had this kind of exposure before. Maybe a few US Navy Seals but they tended to retire at 30, not carry on a career of diving.

The dangers of O2 in the body are well documented and anti-oxidants are one of the biggest selling drugs in the world (they are much over-used and becoming a [dangerous] wellbeing "fad" in some countries, notably the US).

There is masses of information out there to read if you want to know more. Sadly a lot of it is bunk (on both sides of the debate) and as with global warming there are some pretty big commercial interests at work (drug companies in this case) which blur the picture.

Mal is erring on the safe side, which seems like a wise thing to do to me. I dive to 1.4 on OC and would go to 1.5 for a short while without much thought for the matter. I don't get to dive all that much these days and its mostly max 20m and air - just for cost and convenience - so its not an issue for me personally.

 

[Dyson Diver]

handy as a guesstimate ,, computer does it for me though , but after marking loads of tanks you kinda know whats going on the label ,,, thats an insanity cheque

 

[Captain Calamity]

I'd be interested to know why 1.4 is considered aggressive?

Thanks
Matt.

From an ox-tox point of view 1.4 for the working portion of the dive is fine.

From a CNS loading point of view the CNS for a given period of time 1.4 adds a lot more to the clock than 1.3 or 1.2. Doing your deco on 1.6 might be very efficient but it hammers the oxygen clock.

For a long deep deco dive with accelerated deco it is very easy to hit 80% CNS or higher, adding the need for air breaks. I would describe this as an aggressive dive. If you run a lower PO2 on the bottom and during deco it is often possible to keep the oxygen clock much lower without adding much to the run time. Try playing around with some examples on V-planner etc, but when I looked at this on a course some time ago I was amazed at the difference 0.1 PO2 can make to total oxygen exposure.

CC

 

[NotDeadYet]

I'd be interested to know why 1.4 is considered aggressive?

Thanks
Matt.

Because it sounds good and makes you think your weekend of 45min nitrox dives is an expedition.

For the average diver not doing any/much deco and/or doing deco on backgas 1.4 is about as aggressive as a smurf on dope. A lot of deco or long dives then it's a different story.

If I'm doing deco then I dive on a setpoint of 1 (which realistically is 0.9 on the Hammerhead) so that I can crank the PO2 on deco up at 6m and not care (much). If I'm doing no-deco mooches then I'll happily dive 1.5 on open circuit if I have to (not much point on CCR when I can get whatever PO2 I want).

On long dives (2hrs+) I've had throat/chest irritation on 1.2 on CCR where it's been a constant PO2. Never noticed it on OC, I guess the variation in depth gives a bit of relief. Plus I don't do dives like that OC any more so my memory may be getting hazy.

 

[chrisch]

.... about as aggressive as a smurf on dope. ....

You can't trust the little blue bastards. Always picking a fight they are. They might be docile with a bit of good weed but once they get their hands on a few pints of Stella its a different matter.

 

[matthewoutram]

Because it sounds good and makes you think your weekend of 45min nitrox dives is an expedition.

For the average diver not doing any/much deco and/or doing deco on backgas 1.4 is about as aggressive as a smurf on dope. A lot of deco or long dives then it's a different story.

If I'm doing deco then I dive on a setpoint of 1 (which realistically is 0.9 on the Hammerhead) so that I can crank the PO2 on deco up at 6m and not care (much). If I'm doing no-deco mooches then I'll happily dive 1.5 on open circuit if I have to (not much point on CCR when I can get whatever PO2 I want).

On long dives (2hrs+) I've had throat/chest irritation on 1.2 on CCR where it's been a constant PO2. Never noticed it on OC, I guess the variation in depth gives a bit of relief. Plus I don't do dives like that OC any more so my memory may be getting hazy.

I'm wondering if it is no longer fashionable to dive with PPO2 as high as 1.4?

Personally I'm diving 1.3 for the dive and either running 1.3 for all the deco or running all the deco at 1.5. I don't see the point in using 1.5 only for the last stop as it makes such a small difference.

I've tried gas-switching on ascent (from gas to nitrox/air, various depths) but although this saves time it also causes incomplete decompression (for me at least), so I don't do it any more.

I've also stopped air-breaks for CNS < 200%, seems to work ok.

Cheers
Matt.

PS Please don't dive like I say as it's probably not safe for you.