My Journey into UTD Ratio Deco

[Kevrumbo]

When I took the RD class with AG he tried to make the point that small differences in depth or time can be insignificant to the plan. He did not round up as you do when planning with a dive table. Just speculating but he may have waived away a few thousand feet of altitude change as an insignificant pressure change without considering what would happen with five or ten thousand feet of altitude change. That was the approach over ten years ago it may have changed.

(Anticipating @boulderjohn 's opinion on this):

The old school philosophy of GI3 and AG:

It has to change IMO with regard to Ratio Deco Method and its required Deepstops:

If your intermediate/slow tissues have high surfacing supersaturation tensions along with:

• A sequence of consecutive dive days with multiple-dives-per-day further loading and potentially inflaming those intermediate/slow tissues with residual inert gas;

And/or:

• Ascending at altitude where there is a diminished ambient surface pressure.

You will increase your chances of a type I DCS hit.
If you're unlucky enough to have a PFO or other major cardio/pulmonary venous to arterial shunt, your chances of a skin bends, type II DCS or worst case AGE pathology will increase. At the very least, you will have a measurable increase in the level of decompression stress as detected by post-dive Doppler VGE scores. . .

You compensate by either:

• Taking a day-off from diving after three or four consecutive dive days;
• Pad your Oxygen schedule with extra stop time if doing mandatory staged decompression;
• Account for altitude with the appropriate computer setting or table cross correction.

 

[Kevrumbo]

UTD Tech-1 training covers how to blend UTD standard mixes. I am assuming that if you are into DIR standard mix diving then you will be mixing your own brew and that is why UTD tech 1 training includes a chapter on how to do it.

Any shop can blend a standard gas for you just the same as any other mix

True but doesnt hurt to know how to do it if you are that type of person

And you what you fundamentally learn and understand "properly" by rote, trial & error, and practice equally applies to gas blending -whether using a first world convenience of a "smartphone app" working in a Florida local dive store bank fill station; or performing the "art" of partial pressure mixing using a portable gas compressor, manual calculator arithmetic with applied Dalton's Law gas recipe formulas, and an O2 analyzer -in a sweltering east coast Sri Lanka shed on Expedition:

HMS Hermes, Sri Lanka - Technical Wreck Dive - April 2010 - Full Album

 

[boulderjohn]

(Anticipating @boulderjohn 's opinion on this):

The old school philosophy of GI3 and AG:
View attachment 459457

It has to change IMO with regard to Ratio Deco Method and its required Deepstops:

If your intermediate/slow tissues have high surfacing supersaturation tensions along with:

• A sequence of consecutive dive days with multiple-dives-per-day further loading and potentially inflaming those intermediate/slow tissues with residual inert gas;

And/or:

• Ascending at altitude where there is a diminished ambient surface pressure.

You will increase your chances of a type I DCS hit.
If you're unlucky enough to have a PFO or other major cardio/pulmonary venous to arterial shunt, your chances of a skin bends, type II DCS or worst case AGE pathology will increase. At the very least, you will have a measurable increase in the level of decompression stress as detected by post-dive Doppler VGE scores. . .

You compensate by either:

• Taking a day-off from diving after three or four consecutive dive days;
• Pad your Oxygen schedule with extra stop time if doing mandatory staged decompression;
• Account for altitude with the appropriate computer setting or table cross correction.

George's statement is actually sad, especially since it led untold numbers of people down that same uninformed path. There are a number of factors involved when diving at altitude. He apparently only knew that one factor and decided it didn't matter. He was probably right about that one factor, but he ignored the ones that are important, I assume because he did not know about them.

For a more complete discussion about diving at altitude, go to this page and choose the first article on the list.

Deep Adventure Scuba Colorado Technical Diving - For Those Looking to Take Diving to a New Level

 

[leadduck]

George's statement is actually sad, especially since it led untold numbers of people down that same uninformed path.

It seems to me from this quote George Irvine didn't care about altitude diving, hadn't spent much thought about it and this question just caught him by surprise, so he tried to be funny and play it down.

 

[CAPTAIN SINBAD]

So I wanted to address the issue of "mental mathematics" because some degree of criticism is generated by the idea that ratio-decompression requires very difficult mental computations. Before I proceed, I must state that mathematics has never been my forte and if there is anyone on this forum who should be apprehensive about messing up the mental mathematics then it is me. While I was trying ratio decompression during my test dives, I did not come across a situation that required anything more than taking the average of two numbers that would be very close. I am going to explain what mental mathematics go into ratio-deco and if there is anyone here who thinks that what I am describing below is prone to massive human error then please say so.

MIN-DECO TABLE (Nitrox 32):

All UTD divers will know these numbers at the back of their minds. Some are seen carrying a slate but most will have these numbers ingrained in memory as they are easy to remember.

60 ft = 60 mins
70 ft = 45 mins
80 ft = 40 mins
90 ft = 35 mins
100 ft = 30 mins

Unlike a conventional dive table which uses maximum depth, this table uses average depth instead.

Now the mental mathematics: Once the dive begins you are required to do a rolling average every 5 minutes. It would be something like this:

RT 5 mins = Depth 73 (Avg 73)
RT 10 mins = Depth 77 (Avg 75)
RT 15 mins = Depth 85 (Avg 80)
RT 20 mins = Depth 88 (Avg 84)
RT 25 mins = Depth 90 (Avg 87)
RT 30 mins = Depth 93 (Avg 90) = At this point you know that you have 5 more mins at this depth from the above described table.



If you think you can determine the bold number in the bracket without using calculator then the task loading aspect should not be over-whelming. For the particular dives that I was doing with simple straight profiles or gradual depth increases this was not over-whelming.

UTD Ascent Schedule

After finishing the dive, you ascend to 50% of your depth @ 33 ft / minute and then stop for 1 minute every 10 feet until you reach the surface. For a hundred ft average depth you are doing 5 stops of 1 minute each all the way up to the top so that would make it a 5 minute ascent schedule. For 80 feet dive your first stop will be at 40 feet and then 30, 20, 10 so 4 minute schedule.

Does anyone here think that this is too much task-loading as far as mental mathematics go? Thoughts?

 

[huwporter]

Now the mental mathematics: Once the dive begins you are required to do a rolling average every 5 minutes. It would be something like this:

RT 5 mins = Depth 73 (Avg 73)
RT 10 mins = Depth 77 (Avg 75)
RT 15 mins = Depth 85 (Avg 80)
RT 20 mins = Depth 88 (Avg 84)
RT 25 mins = Depth 90 (Avg 87)
RT 30 mins = Depth 93 (Avg 90) = At this point you know that you have 5 more mins at this depth from the above described table.

I don't use or know much about UTD RD (TM), but this isn't how I'd go about working out an average depth.

In practice I'd be doing something like:
RT 5 mins = Depth 73 (avg 80)
RT 10 mins = Depth 77 (avg 80)
RT 15 mins = Depth 85 (avg 80 but starting to think about changing to 90 if I don't move shallower soon)
RT 20 mins = Depth 88 (avg 90)
RT 25 mins = Depth 90 (avg 90)
RT 30 mins = Depth 93 (avg 90)

(Actually I'd be doing it in metres, but same process)

If your example above is how UTD expect you to do it, I can see why people think (UTD) RD (TM) involves too much mental arithmetic.

Edit - and also, the UTD method as you describe it looks like it could seriously underestimate average depth on dives with a bottom profile going deeper to shallower, because the most recent depth has a disproportionate influence on the result. Probably less of an issue for recreational dives, but on an advanced trimix dive, underestimating your average depth by 10 or 20 feet could lead to a significant underestimation of deco required.

 

[boulderjohn]

I am sure you asked for the scientific basis for this approach. Could you provide that for us?

 

[Scuba J7]

I was playing with that dive profile a bit and was curious what the protocol is in the case you forget to calculate average depth as you go?
Just continuing as normal (averaging depths that you do know) produces averages that could have you 10’ shallower. Longer profiles or depth variations could produce even bigger differences using that method, so I assume UTD has a better way to calculate missed depths.

 

[sigxbill]

If you are unsure, you can verify your average depth on your bottom timer. Also, one correction for you Captain Sinbad: ascent to 50% of depth is from average depth (as you mentioned) OR current depth - whichever is deeper ...

 

[Scuba J7]

If you are unsure, you can verify your average depth on your bottom timer. Also, one correction for you Captain Sinbad: ascent to 50% of depth is from average depth (as you mentioned) OR current depth - whichever is deeper ...

I was under the impression electronics were not permitted. So to my next question, if you can use a BT why in tarnation would you try to track it in your head?