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