Deco Setpoint

[beldridg]

The discussion was about increasing the setpoint (at deco) and whether there's any issues arising from this, aside from the benefits of a shorter TTS.

Answers to include references to conservatism and risk assessments. Would be good to get references to current thinking.

Yes, but he also expressly mentioned "or to improve conservancy by doing the same stop durations as indicated whilst on 1.3 but with a higher setpoint (circa 1.4-1.5)."

Either I don't understand what he means by that statement or I don't know a practical, easy way to do that (or perhaps both).

- brett

 

[Tracy]

Personally, I tend to leave my setpoint on deco the same as my bottom phase. I do manual o2 flushes periodically at my 6m stop, but still leave the controller set the same.

 

[JonG1]

Yes, but he also expressly mentioned "or to improve conservancy by doing the same stop durations as indicated whilst on 1.3 but with a higher setpoint (circa 1.4-1.5)."

Either I don't understand what he means by that statement or I don't know a practical, easy way to do that (or perhaps both).

- brett

Apologies I was vague, basically I was thinking along the lines of the 6m and 3m stop (if used) as being the point where conservancy is added, so as you arrive at 6m if you have a TTS off 30 mins for example on an SP of 1.3, but 20 mins when you bump up to 1.4 or 1.5, you still do the 30 mins even tho deco would have cleared.

 

[beldridg]

Apologies I was vague, basically I was thinking along the lines of the 6m and 3m stop (if used) as being the point where conservancy is added, so as you arrive at 6m if you have a TTS off 30 mins for example on an SP of 1.3, but 20 mins when you bump up to 1.4 or 1.5, you still do the 30 mins even tho deco would have cleared.

No problem, now I understand.

I generally do pad my last stop but I don't generally increase my set point. As @rjack321 mentions, changing from 1.3 to 1.4 on deco a "normal" 50m-60m dive won't change your TTS that much.

I have my GFHi set to 80 on my Petrel controller but I almost always surface with 70-75 because I pad my last stop after I clear deco.

Having done a fair number of wreck dives at those depths, IIRC, a general rule of thumb for a "normal" dive duration at that depth, every 1 extra minute of deco is about 1% reduction in SurfGF.

I like doing my last stop at 6m/20' because of conditions in SoCal waters and it is easier to maintain a set point. The reason I mention that is that I tend to slowly make my way to the surface when I'm doing that extra deco. So, if I have cleared deco at 20', I might move to 18' and do a minute or two and then move to 15' and do a minute or two, etc.

Regards,

- brett

 

[sea_ledford]

Apologies I was vague, basically I was thinking along the lines of the 6m and 3m stop (if used) as being the point where conservancy is added, so as you arrive at 6m if you have a TTS off 30 mins for example on an SP of 1.3, but 20 mins when you bump up to 1.4 or 1.5, you still do the 30 mins even tho deco would have cleared.

You could change your shallow GF to be more conservative or just do a 5 minute safety stop after your deco has cleared. What would probably make more difference would be a surface deco stop where you stay in the water, on O2, at the surface for a few minutes. Works great if you are cave diving, in calm water, or waiting for a boat to come pick you up. A bit of a nightmare if you are bobbing around in rough seas holding on to a surface line from a tied off boat. Or if you are on an SF2.

If you want to be really conservative, add an OC buddy to the team! You'll be a good 10-15 min out of deco before they clear.

 

[broncobowsher]

And then there is the CNS. How high are you willing to go with it?
I'll drop a little O2 and take a little more time when I start seeing the CNS really climbing.
I'll do a little sweep of PPO2 while hanging on deco. Look at TTS at various points of PPO2. It's not linear. I might choose a lower PPO2 where it might only add a minute or two. Or add a little O2 if it takes a lot of time off and have the CNS to absorb it.

As for planning, 1.2 is a general number, 1.1 is optional for repetitive, deep, or multiple days of diving. 1.4 deco. But they are not absolute, just a typical starting point. If my CNS is pretty high from the morning dive, the PPO2 will be dropping for the second dive. Even if it means cutting the dive a little short.

 

[cathal]

What's the rationale on the flush Cathal and is the 1.5 to improve the efficiency of the deco or reduce the time to surface.

Re, the high ppO2, I think its just to get the ppN2 down as much as possible. There are no refences as to its efficacy, just individual empirical feedback from a number of divers for the last couple of years. While it does reduce the overall TTS, there does be no rush to get out. I have never felt better after a deco dive than having done an additional 30 - 40mins at 6M on O2. Re the loop flushing, people just felt better after each dive when they flushed the loop wth pure O2 a couple of times, We reckon that it gets rid of any small percentages of Co2 in the loop that may have been retained, again no references, just a number of peoples experience.
These dives range from 2-3hrs, between 60 - 80M, the Malin Classics stuff.

 

[rjack321]

at 6m/20ft your 1.4 setpoint is an fO2 of 87.5% setpoint of 1.5 is an fO2 of 93.7%. 6% more fO2 is not really relevant, or alternatively, 7 vs 13% inerts

 

[beldridg]

at 6m/20ft your 1.4 setpoint is an fO2 of 87.5% setpoint of 1.5 is an fO2 of 93.7%. 6% more fO2 is not really relevant, or alternatively, 7 vs 13% inerts

And trying to maintain a 1.5 at 6m/20' is, at least in my experience, a real PITA and definitely not worth the effort.

- brett

 

[rjack321]

And trying to maintain a 1.5 at 6m/20' is, at least in my experience, a real PITA and definitely not worth the effort.

- brett

I bump mine up and let it decay but it's more to give me something to do and keep me amused than any substantive conservatism