Modifying rule of thirds

[aainslie]

I still don't get why it's reckless. Care to explain?

I'm suggesting keeping the SAFETY MARGIN constant, not the RULE.

Thirds is like setting a speed rule. We all know that increased speed increases the probability of an accident. but we also know it's a function of the road conditions. So we a) alter speed based on e.g. weather (slowing down in fog, etc) and b) alter speed limits based on road conditions (e.g. 65 on a highway, 35 in a town).

My suggestion is to implement something like the speed variation based on road conditions, which (like cave flow) are reasonably invariant. Thirds is like setting a speed limit of 50... whether you're on the highway or in the middle of High Springs. And flow is a very objectively measurable aspect of a cave.

You should add more margin if the cave is narrow... silty... unknown... deep... etc. etc..

Attached is a sheet with the calcs I put in the earlier discussion. Note I put a piece in there to add a psi "safety net" for conditions other than flow. And notice how easy the arithmetic is. Measure psi on entry, on turn and on exit. Input next dive's starting psi. Based on just those 4 numbers, plus the addition of a safety margin, you can keep a safety margin that stays constant as cave flow changes. I am ONLY controlling for flow. You need to control for other sources of gnarliness.... which btw you'd better be doing with thirds also.

It's another reason I hate thirds. it's such a simplistic rule that people think it's good enough. It isn't. It's frequently too liberal.