Why do some agencies recommend using a bottom timer instead of a computer

[Tassi Devil Diver]

As a general rule, everyone else I have been diving with has no clue how to adjust their schedule on the fly for deeper/longer/shallower/shorter. I have seen course directors bail on a dive early, then hang on the up-line at 150ft+ until their run time catches up with the shortest stop schedule on their slate. Or go have to go screwing with the +5 function on their Shearwater. Or boast around the shop that their computer died and they have "no idea" how much deco they had left.

This is precisely why I use two decompression computers (1 as a back up) to adjust my dive schedule on the fly, I plan my dive to the absolute maximum depth I could expect, plan the maximum amount of decompression I want to do (time to surface (TTS)) whether that be 30 min, 1hr or 2 hrs and carry the appropriate amount of bailout gas. I then use the TTS display during the dive to determine my dive time, if I go shallower I get more bottom time for my planned TTS, if I go to my maximum planned depth my bottom time is shortened to stay within my maximum TTS for that dive. This where tables fall down for me, lack of flexibility, and I am not interested in RD because the computers perform the job more than adequately for me, and would do for anyone else, so I see absolutely no use for RD.

The one use I can see for RD is when chatting about dive profiles with your team top side you could real off quickly a potential first draft dive plan, then once people were happy with the general plan I would go to my desktop planner to finalise.

 

[Centrals]

There are many ways to skin the cat but just don't tell me your way is the ONLY one.
Exchange of info is great but don't get too personal.
There is no winners or losers here.

 

[Dan_P]

If I were claiming that RD was "optimal" in terms of the physiological decompression process, I could understand objections or calls for substantiation.

If I were saying that using a PDC left you more at risk of DCS, I could understand objections.

But what I'm saying is, it can't be said to be dangerous, I find it highly practical and it is explicitly presented to me in a manner that encourages personal adaptation as one develops.
Which I personally choose to do - adapt it, that is.

These wild objections to that, I find to be clearly motivated by sentiment.

Let me say this:
I hold the Bonus Pater Familias-principle to be entirely pivotal.
But if I were the sort that would stuff my dog in the microwave to dry after bathing it, probably I'd be more prone to litigate, say, microwave manufacturers and the like.

Even so, I stand by this claim:

The snarky litigations against RD seen here, and elsewhere, are likely entirely unscientific in nature, certainly unsubstantiated and probably based on sentiment as well as, in some cases, ignorance.

Now, as I said before we all embarked on this unnecessary deroute (yet again), I don't mind having a conversation on the merits (subject to opinion and a fair discussion) of using a standardized decompression paradigm - but either the litigations go, or I would call for evidence to be presented to support those litigations.

It is with some confidence that I propose this:
What you will find is that no such evidence exists.

 

[Neilwood]

My personal issue with Ratio Deco in all of its guises is that it relies on the human brain to work things out in a clear calculated fashion despite what may be going on in the background.

An analogy would be navigating in a thick forest with limited landmarks or ability to see the sun. In ideal conditions you can pick a direction and stick to it (picking your ratio). You can pick a tree on the right bearing then when you reach that tree, you navigate the same way to the next one(moving between deco stops). Problem is that unless you know the minor deviations between those directions you can veer of course (are you hitting those stops exactly when you should or are slow on ascent) or in the event of an incident such as a fall which takes your focus off the direction you miss the exact tree you wanted but picked one close and are now heading in an unknown direction (sorting an incident at depth causing your calculations to deviate slightly). Even highly trained operators using point to point navigation screw up. The possibilities for human error are too high IMHO.

Compare this with doing the same with GPS (using two GPS devices). No matter how confusing the trail becomes you always have the directions to the end point( adjusted for any minor or major deviations). You might still use natural navigation heading tree to tree but the GPS will keep you in the right direction even if you fall over or forget which tree you wanted.

 

[PfcAJ]

My personal issue with Ratio Deco in all of its guises is that it relies on the human brain to work things out in a clear calculated fashion despite what may be going on in the background.

An analogy would be navigating in a thick forest with limited landmarks or ability to see the sun. In ideal conditions you can pick a direction and stick to it (picking your ratio). You can pick a tree on the right bearing then when you reach that tree, you navigate the same way to the next one(moving between deco stops). Problem is that unless you know the minor deviations between those directions you can veer of course (are you hitting those stops exactly when you should or are slow on ascent) or in the event of an incident such as a fall which takes your focus off the direction you miss the exact tree you wanted but picked one close and are now heading in an unknown direction (sorting an incident at depth causing your calculations to deviate slightly). Even highly trained operators using point to point navigation screw up. The possibilities for human error are too high IMHO.

Compare this with doing the same with GPS (using two GPS devices). No matter how confusing the trail becomes you always have the directions to the end point( adjusted for any minor or major deviations). You might still use natural navigation heading tree to tree but the GPS will keep you in the right direction even if you fall over or forget which tree you wanted.

RD just isn’t that complicated.

Folks are vastly overstating how “complex” it is to come up with a workable decompression schedule.

 

[dmaziuk]

Folks are vastly overstating how “complex” it is to come up with a workable decompression schedule.

Some day when I'm bored enough I might look at Haladne's tables closely and draw a few plots and see if his half-pressure ratios form an easily memorizable pattern...

 

[PfcAJ]

Some day when I'm bored enough I might look at Haladne's tables closely and draw a few plots and see if his half-pressure ratios form an easily memorizable pattern...

Not sure what that really has to do with ratio deco.

Might be a fun excercise though.

 

[drbill]

As a marine biologist I rarely have a pre-defined dive profile in mind. My dive is based on opportunistic sightings of critters. There I could not do pre-dive planning and rely on redundant computers to guide my dive.

Of course even those who use bottom timers are usually depending on tables and formulas calculated by computers!

 

[The Chairman]

@Dan_P

Start by answering these questions.

• Why the f@#$ does Ratio Deco account for such an inordinate number of DCS incidents?

• How can one prevent these kind of DCS incidents?
  • I avoid them by not using Ratio Deco.

• Have any studies shown Ratio Deco to be more reliable than any PDC?

• What benefit(s) would I accrue using Ratio Deco over my PDC? Any?
  • July starts my 50th year of diving without ever having had a DCS incident

• Why the f@#$ does Ratio Deco account for such an inordinate number of DCS incidents?

 

[dmaziuk]

Not sure what that really has to do with ratio deco.

There may be any number of different sets of simple rules for calculating a "good enough" decompression schedule on the fly. One argument seems to be that it's a useful skill to have, the other argument is that the one set of rules is better than a dive computer. I do agree with one of those.