10 versus 20 foot last stop

[divezonescuba]

On a recent 150 foot deco dive, my backup Perdix was accidentally set for 10 feet while my primary Perdix was set for the intended 20 feet. I prefer to use 20 feet due to surface and bouyancy effects.

The differences between the two computers deco stops was dramatic. After the dive, I ran dive planning scenarios between 20 and 10 foot last stops. The 10 foot last stop plan appears to be way more efficient than I would have guessed as it was much shorter overall and used less gas as well.

So, I believe that one implication would be:

20 foot last stops provide better protection from surface effects and vessels while also giving you additional reserve which you could convert to a 10 foot stop plan in case of emergency. Of course there is also the benefit of easier bouyancy control at 20 versus 10 feet.

Are there any other potential benefits of having a planned last stop of 20 feet?

 

[tbone1004]

what gas were you on for the final stop?

if on 100% O2 you shouldn't have any real difference in deco times. You obviously get a gas savings since you're at 1.3ata vs 1.6ata so that's a big chunk for sac rate 25%.

If you were on something other than 100% O2, then your ppN2's will be a lot lower at the shallower depth which provides a boost in deco efficiency.

No reason not to leave the Petrel set to 10ft final stop. Once it kicks over to 10ft, just stay at 20ft and it will keep calculating. If you are on O2, it won't matter, but if you are on something less than 100% you will have a slightly inaccurate TTS due to the deco efficiency

 

[divezonescuba]

Was actually x36 with the computers set for bottom gas, air, for educational purposes / requirements.

So, this is actually a sort of theoretical question as it would not normally be done.

However, I think the 20 ft versus 10 ft effectiveness and consumption differences would still exist in some capacity regardless of the gas used.

 

[Capt Jim Wyatt]

I set mine to 10 feet. If using 100% O2 as my shallow deco mix I can do the last stop anywhere between 10 & 20 feet.

This keeps the O2 clock from ticking at 2.22% / minute and the deco is equally as efficient.

 

[Boaty McBoatface]

On a recent 150 foot deco dive, my backup Perdix was accidentally set for 10 feet while my primary Perdix was set for the intended 20 feet. I prefer to use 20 feet due to surface and bouyancy effects.

The differences between the two computers deco stops was dramatic. After the dive, I ran dive planning scenarios between 20 and 10 foot last stops. The 10 foot last stop plan appears to be way more efficient than I would have guessed as it was much shorter overall and used less gas as well.

So, I believe that one implication would be:

20 foot last stops provide better protection from surface effects and vessels while also giving you additional reserve which you could convert to a 10 foot stop plan in case of emergency. Of course there is also the benefit of easier bouyancy control at 20 versus 10 feet.

Are there any other potential benefits of having a planned last stop of 20 feet?

My understanding is that the only thing affected by configuring the safety stop setting at 10 vs 20 feet on the Perdix is the Time to Surface calculation (see the manual, page 14). The countdown clock starts/stops using the same depth window, either way, but your TTS number displayed during the dive can be made more or less accurate for you based upon your intention. So, otherwise, the report of both your computers both during and after the dive should have been the same. Actual/measured/modelled off-gassing results, however, are made based on your precise depth, time, and gas mix and so will be different at 10' vs 20'. The shallower you are, the less gas you use, the stronger the partial pressure gradients, and the faster the off-gassing. And that's all great...until it isn't.

 

[Pullnglide]

I keep the last stop at 10'. I usually stop at 20' and stay there but depending on the site I may ascend to 10'. I think some divers doing repetitive dives stop at 10' to reduce the o2 clock.

 

[DevonDiver]

If you weren't on O2, then you should expect some significant variation.

Time to do some reading on pressure differential and gradients

With O2... the pressure differential (ambient - dissolved inert gas) is total.. 100%. There's no off-gassing advantage from moving shallower.

However, there ARE advantages in respect to CNS tracking and gas consumption.

The lower FO2 in the gas used, the more decisive the issue of gradient becomes. But simultaneously, the issue of CNS tracking becomes less influential.

 

[RainPilot]

If ambient pressure is higher (i.e. 6m stop vs 3m stop) would that not affect the rate of bubble growth?

I understand that the inspired PP(inert) vs the PP(dissolved gas) is the primary reason for off gassing, but if ambient is not a factor then surely, once i switch to 100%, i could surface and do the rest of my deco on the boat on O2? If that is not the case, then there must be a physiological difference between 6m and 3m.

If there is no difference, a O2 rebreather hanging on the ascent line would be a game changer.

 

[DevonDiver]

If ambient pressure is higher (i.e. 6m stop vs 3m stop) would that not affect the rate of bubble growth?

I understand that the inspired PP(inert) vs the PP(dissolved gas) is the primary reason for off gassing, but if ambient is not a factor then surely, once i switch to 100%, i could surface and do the rest of my deco on the boat on O2? If that is not the case, then there must be a physiological difference between 6m and 3m.

Yes... gradient/differential on varied FO2 effects off-gassing efficiency. But you're still subject to a ceiling in regards to bubble formation.

When the ceiling remains below 3m... so do you... so that's your 6m stop. Once the ceiling rises, you have the option to stay put, or ascend to the next stop. Once the stop has risen to 3m, you have the option to stay anywhere at, or below, that level.... you could stay at 6m (with CNS concerns) or you could choose to hang at 5m or 4m etc...

The FO2 of your gas doesn't effect your ceiling... it only effects your rate of inert gas diffusion.

P(inert) vs P(dissolved) dictates off-gassing efficiency.
P(ambient) vs (P(dissolved) dictates ceiling (m-value).

*assuming were discussing a dissolved gas model*

 

[ofg-1]

What do you consider as a "dramatic" difference between the times? It is a direct relation to the actual overall dive profile, are you diving 150 feet, or 300 feet? 1 hour bottom time or 3 hours? You do know that your computer will work at a 15' stop, right?

I don't do much open water diving these days, but over the years I found that if you are in 3 foot seas, a 10' stop really isn't 10'. If you hang on the downline, it can pull your arms loose. How much do you want to get bounced around? I would rather be more comfortable at 20' for a few more minutes than jerked all over the place at 10'.

I read all of these threads quibbling and getting nasty and personal about gradient factors, deco gasses, depths....bah. I am 64 years old, and my goal is not to get bent up like a pretzel again, not to be the first one back on the boat. Find out what works for you, and makes you comfortable.