Analysis of a 47m deco proifile.

[Clare Gledhill]

Gareth you may want to look at those deep stops again. 45 minutes is at the very edge of 30 second stops to 65 per cent.

Work that has been done by various divers I respect appears to be showing that the intermediate range of stops is more critical to succesful deco than shallow - even on relatively short ocean dives.

That is why if I had to shave deco for any unplanned problem I'd cut it off the 6 metre stop. Having said that, my 'bailout' plan permits me to do normal deco. It would have to be a suit flood or similar to get me out quicker.

[And]

Originally Posted by Mark Chase

OK I can see what your doing but:

75% of 47m is 35.25 I rounded up to 36 by adding 0,75m you rounded down to 33 by taking away 2.25m

The 1min stops from 33 to 21 are not stops as you know they are 3m/min ascent rate with a pause. The run time remains the same I didnt add any run time for the ascent between these stops as they are not real stops.

There is a way of working out the initial deep stops, which are usually between 80% and 65% of the ATA's, and then from 65% to 45% of the ATA's, and so on until you reach your first deco gas. George posted an article about this which is still on the GUE site somewhere. The deep stops will range from 30secs to 5 mins, and then double in the next section. He also said that he worked this out depending on the bottom time, ranging from 0-150 mins and above. How I do this is to say anything below 30 mins bottom time has 30 sec stops, then below 60 mins has 1 min, below 90 mins has 2 mins, below 120 mins has 3 mins, below 150 has 4mins, and then over 150 has 5 mins. Between 65% and 45% you double these.

I also tend to have standard depths at which I choose to start the stops, as only the most anal of people will spend the dive trying to work out the precise depths for 80% and 65% of the ATA's, and I am too busy doing other things, so in this particular dive I choose 36 mtrs, because I know it is 80% of a 45 mtr dive, and I also know there are 5 stops between 36 and 21, so it is easy to work out, in fact, there is no working out, I just ascend to 36 mtrs at 10 mtrs/min, and then pick the stops I want to pick. Mostly this is 1 min, or 3m/min ascent, as I tend to do between 30 mins and 1 hour bottom times.

The 65% stops then double, providing you have not reached a deco gas, and it is not unusual to find yourself extending the 24 mtr stop before the switch, but this is on deeper dives really.

In recent times, there has been some discussion over reducing these stops, as some divers were coming up with niggles, aches and pains, which could not easily be determined as DCI. My own opinion is that many divers just do not ascend fast enough in the first phase. If you can't get to the 80% at 10m/min, then you need to add that into the bottom time. It is an easy way to screw up a deco schedule.

So if i wanted to make my deco faster, would i not redistribute the stop time to the 9m stop to maximize this off gassing effect?

It seems logical but not in line with thinking on ratio deco

That makes the profile more Haldanian, and I might decide to do something like that if my gas is running low, as the choice to do all the internediate stops the same is really just to make it easy, but if for example you had left your bottle on at the bottom and your scooter propwash had been purging the reg throughout, then you might decide to do the stop at 21 and then move a bit quicker up to 9 where you can spend more time there, not that I have ever seen this happen of course . Really this just acknowledges that there is some flexibility here, you shouldn't be on such a knife edge that chopping 2 mins off 15 and adding it to 9 will result in DCI. Thats not the way I think it should work.

So it seems as an outsider looking in that the main areas for reducing the deco on a standard ratio deco profile are:

• Ignore the decent time
  
• err on the shallow side of average depth
  
• Round up the deep stop to the stop above never the stop below.

Hmm, I think this is just a way of fooling yourself really. You should pick the average depth that you did, not try and cheat yourself. The time to cheat is at 6 mtrs, not really before that. I do round the deep stops but usually this is to match a depth that I know, and it might be rounded up for a deeper dive, and rounded down for a shallow one, being aggressive is not the primary reason for doing this.

As I said, in a problem situation where deco gas is in short supply, then I would be tempted to shave some time off the 15 and 12 stops, doing them at 9, and then do the 6 mtr stop for as long as possible before having to ascend. If backgas is in short supply then it might be necessary to shorten the deeper stops by rounding them down, doing 6m/min rather than 3m/min, as the other option is switching deep, which is not something I would readily do. This is the worst example really, and is why we have such a thing about buddy diving and minimum gas requirements.

I am assuming then that after this its just up to the individual diver to shave off deco where they think its appropriate and use this experimentation to form schedules for the future?

This is a perfectly valid approach in my mind but the question is, does the alteration of the profile remain a constant? IE i always reduce the 6m stop by 10% 10mins what ever or does it vary across the depth run time range on an ad hock basis?

Different divers will do different things, and for different reasons. It really is up to the individual how they want to do this, although it necessarily follows that your buddies must also be flexible enough to accept your plan. In my case then I usually end up being the guy who works out the deco, and I veer to the more conservative end of the scale, because even though I might post my theoretical plan here, that might not be what I actually end up doing, its not a precise science, at least, in my world it isn't anyway. Luckily, my buddies so far have been happy with this and I haven't bent anyone yet, although it was close in Pembroke once .

Andy

[Mark Chase]

Thank you for an excelent, informative and extreemly heplfull post Andy.

I have greend you already but it apears i should have waited a bit

ATB

Mark Chase

[Mark Chase]

Originally Posted by Clare Gledhill

Gareth you may want to look at those deep stops again. 45 minutes is at the very edge of 30 second stops to 65 per cent.

Work that has been done by various divers I respect appears to be showing that the intermediate range of stops is more critical to succesful deco than shallow - even on relatively short ocean dives.

That is why if I had to shave deco for any unplanned problem I'd cut it off the 6 metre stop. Having said that, my 'bailout' plan permits me to do normal deco. It would have to be a suit flood or similar to get me out quicker.

But is this just deco on the boat?

What I mean is if i am doing a 12m stop i can get half way through it and start ascending as my ceiling is moving upward all the time.

So if we have a big deco commitment at 3m and we do half or three quarters of it then go up / out are we not just continuing the deco schedule at ambient but now on air as a deco gas and with a very aggressive gradient?

We may chose to stop at 6m for comfort on sea dives but the deco ceiling for dive planning is mostly @ 3m this will be constantly reducing to 2m 1m through the stop so getting out and back on the boat just increases gradient by maybe 0.15 or less.

Being a pendant wouldn't you say then that your not reducing the deco at all your just reducing the in water portion of it and ripping into the off gassing gradient? Lots of deep technical dives end deco on land or on the boat sucking on 02.

If the apparent logic of this concept it correct then it makes sense to me why you can cut short the shallow deco. Because your not actually cutting it short

ATB

Mark Chase

[Clare Gledhill]

Originally Posted by Mark Chase

But is this just deco on the boat?

Or in the car - wherever really

So if we have a big deco commitment at 3m and we do half or three quarters of it then go up / out are we not just continuing the deco schedule at ambient but now on air as a deco gas and with a very aggressive gradient?

Being a pendant wouldn't you say then that your not reducing the deco at all your just reducing the in water portion of it and ripping into the off gassing gradient? Lots of deep technical dives end deco on land or on the boat sucking on 02.

It's not pedantic at all. Every one of us ends our deco on land - regardless of what profile you choose to do.

Essentially shorter deco will always mean that you are increasing the gradient and thus surfacing with a greater liability than planned. I thought your question, with regard to your bailout options, was where would we choose to make it more aggressive - if we had to do so.

Personally I would not choose to do a deco profile which would end with me HAVING to use O2 on the surface. Nothing wrong with staying on it for a bit though.

[And]

Originally Posted by Mark Chase

If the apparent logic of this concept it correct then it makes sense to me why you can cut short the shallow deco. Because your not actually cutting it short

ATB

Mark Chase

This is true. I guess the trick is making sure that you do enough deco in the water so that you can finish it on the boat and not have to do it all again in a chamber, which of course, cannot redo the deep deco for you, but can do the shallow.

If you have done all the deco correctly other than the shallow stuff, it might be possible to help the 'on boat' or 'in car' deco by breathing o2. I would rather be in this situation than the alternative I think.

Andy

[Mark Chase]

Originally Posted by And

This is true. I guess the trick is making sure that you do enough deco in the water so that you can finish it on the boat and not have to do it all again in a chamber, which of course, cannot redo the deep deco for you, but can do the shallow.

If you have done all the deco correctly other than the shallow stuff, it might be possible to help the 'on boat' or 'in car' deco by breathing o2. I would rather be in this situation than the alternative I think.

Andy

Exactly

Running a DOTB(oat) profile id be very concerned about the effort involved in getting back on the boat and on to the bench.


One thing a CCR can show you is when you have finished off gassing. Jack up the PP02 to 1.6 at 6m and your on pure 02. You can see just how quickly it drops off. On the profile i posted you can see from min 87 to min 100 there is a gradual decay in the PP02. Then i did another 02 flush and after that it stays constant at 1.6. So basicly i stoped off gassing significant Helium and Nitrogen at apx Min 100 into this dive.

Thgis is significent when you review the Vplanner profile which ends deco at min 105.

ATB

Mark Chase

[Howard Payne]

Hi Mark
I sorted started all this with a light hearted comment on your Seaford Ferry trip report and it's grown into quite an interesting thread - so I'm not feeling quite as guilty as I would have done otherwise
I really hate "peer pressure deco" mate and you've done vastly more deep mix diving than I have so please don't feel any pressure from anyone, least of all me, to do anything other than what you feel is appropriate for the dive YOU are doing.
If you were feeling a bit crap and having a few PPO2 problems then under similar circumstances - I would probably have erred on the side of caution as well.
You're sitting at home writing a thread about the dive rather than sitting in the pot bored off your tits - so in that sense - you did the right amount of deco for the dive.
It seemed like a lot of deco so me - but I wasn't doing it and I wasn't there.
Besides any dive below 30m with the Dude where you live to tell the tell has got to be "doing it right" hasn't it

[And]

Originally Posted by Mark Chase

Exactly

Running a DOTB(oat) profile id be very concerned about the effort involved in getting back on the boat and on to the bench.


One thing a CCR can show you is when you have finished off gassing. Jack up the PP02 to 1.6 at 6m and your on pure 02. You can see just how quickly it drops off. On the profile i posted you can see from min 87 to min 100 there is a gradual decay in the PP02. Then i did another 02 flush and after that it stays constant at 1.6. So basicly i stoped off gassing significant Helium and Nitrogen at apx Min 100 into this dive.

Thgis is significent when you review the Vplanner profile which ends deco at min 105.

ATB

Mark Chase

Yes, my other complaint about poor deco is the time spent at the surface before getting on the boat. I like to have a full 5 mins before getting on the boat, and in France, was able to get out of the water fully dekitted, reducing physical effort as much as possible whilst that 'bubble rush' is going on.

As for the CCR thing, I am no expert, but just because the po2 stays at 1.5 (not 1.6, if you look at your chart) then does that not just mean that the o2 being metabolised is the same volume as that being injected into the loop? I am unsure what signicance it has in terms of inerts. It could I guess tell you that your blood has been saturated with o2 at a pressure of 1.5 and you could then say that the 'o2 window' is at its optimum, because at the beginning of the stop it will take a few minutes for the inspired po2 to match the rest of your bloodstream (in this case 5 mins perhaps?), but it isn't really an indication of what the state of play is with the inerts is it?

This is the type of question where we need the likes of Rich Walker, as it is too scientific for the likes of me.

Andy

[Mark Chase]

Originally Posted by And

As for the CCR thing, I am no expert, but just because the po2 stays at 1.5 (not 1.6, if you look at your chart) then does that not just mean that the o2 being metabolized is the same volume as that being injected into the loop? I am unsure what significance it has in terms of inerts. It could I guess tell you that your blood has been saturated with o2 at a pressure of 1.5 and you could then say that the 'o2 window' is at its optimum, because at the beginning of the stop it will take a few minutes for the inspired po2 to match the rest of your bloodstream (in this case 5 mins perhaps?), but it isn't really an indication of what the state of play is with the inerts is it?

This is the type of question where we need the likes of Rich Walker, as it is too scientific for the likes of me.

Andy

No trust me it was 1.6 the drop to 1.5ish on the second 02 flush was matched by the rise in my depth (see line above)


Basicly you breath out nitrogen and helium when you off gas. The loop is sealed so it has no where to go but into the loop. The inerts will then drop the PP02. This drop is significant at first.

Once you have finished off gassing and have no nitrogen or helium left to breath out you breath in 02 and breath out 02 and C02. The Co2 is absorbed by the scrubber so all thats left in the loop is 02.

So flush with 02 and wait for the drop in PP02 to slow right down then tripple flush the counterlung to bring you back to pure 02 and if the off gassing is compleet you will not drop your PP02 again.

Instead you loose countalung volume as you metabolise the 02. The KISS automaticly compensates for most of this by the mas flow valve but some manual injection may be needed to maintain minimum loop volume without bottoming out the lung.


Sooo i stoped off gassing at or arround min 100 in this dive. Vplanner level 2 said 98mins for the deco inc 100% for the 6m stop and 105mins for trimix at 1.25 set point.

I thaught that was interesting. Seems Level 2 VPMB was just about spot on for the dive.

ATB

Mark Chase