SS at 10ft or 20ft -- Why not standard 15ft?

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It sounds like you are setting the initial threshold that the computer will use to start the safety stop. You would not want it set to your target 15' as then it would not start until you are at or above 15' and would then stop if you drifted below 15'. By setting it at 20' you are saying as long as I am shallower than 20' continue countdown of safety stop. All about adjusting for range of diver capabilities and water conditions.
 
I'd guess that PADI et al chose their final decompression safety stop at 5m/16' simply to give some headroom too.
I have no idea what this means. The 5m stop is a safety stop, not a decompression stop. It was started by recreational dive agencies decades ago, The reasons for it have nothing to do with your seriously pejorative description of people doing safety stops. As a diver ascends from a deeper depth, the diver must be sure that tissue pressure does not exceed surface ambient pressure or shallow water by too great a degree. In theory, the diver doing a safe ascent rate can go straight to the surface, but it is nice to be sure. Research indicated that ambient pressure between 3m and 6m allowed the faster tissues to come to equilibrium at a safe pressure, and the slower tissues could not raise pressure beyond that same safe level. Above 3m, the ambient pressure might be too little to be safe. Below 6m, it might be too much. 5m seemed like a good target. A few minutes there to make sure tissues were at safe pressures seems like a good idea.

As for doing the last decompression stop at 3m, there is a good reason for doing that on all dives, especially if you are using oxygen as your final deco gas. If you do the final stop on oxygen at 6m, you have to be pretty darn precise. A foot or two too deep, and you computer gets mad at you for the PPO2. Go a foot too shallow, and your computer gets mad at you for blowing the deco stop. Set it for 3m, and it doesn't matter where you are, because the rate of off-gassing while on oxygen is the same at any depth. If you want to stay near 6m, that's just fine. If you want to go to 3m, that's just fine. Anywhere in between is just fine.
 
Okay, I just took approximately 37 seconds to look things up in the Shearwater manual for recreational mode, which I have never used. Here is what it says about safety stops.

6.1. Safety Stop
A safety stop is an optional stop added to all dives before surfacing. Safety stops can be set to fixed times of 3, 4, or 5 minutes, set to adapt based on dive conditions, or turned off completely. See System SetupDeco Setup for options.

The Perdix does not do “deep safety stops”. That is, there are no extra stops added around 50ft to 60ft (15m to 18m) when ascending from a no-deco dive.

Safety stops behave as follows:

Safety Stop Required Once the depth exceeds 35ft (11m), a safety stop will be added.

Automatic Countdown Countdown begins once the depth becomes shallower than 20ft (6m). Countdown will continue while the depth remains in the range of 7ft to 23ft (2.4m to 7.0m).

Countdown Paused If the depth goes outside of the range 7ft to 23ft (2.4m to 7.0m), then the countdown pauses, and the display instructs to either an ascend or descend.
Here is what it says about decompression stops in recreational mode.
Decompression stops will appear in place of the safety stop when required. After the decompression stops are completed, the safety stop will begin.

Decompression stops occur at fixed 10ft (3m) intervals.
SUMMARY
  1. The 10 and 20 foot stops are decompression stops, not safety stops. You cannot set the depth of a safety stop.
  2. A safety stop begins automatically once the diver ascends past 20 feet.
  3. Decompression stops appear in place of safety stops if you exceed no decompression limits.
  4. After completing decompression stops, you will be told to do a safety stop.
 
It sounds like you are setting the initial threshold that the computer will use to start the safety stop. You would not want it set to your target 15' as then it would not start until you are at or above 15' and would then stop if you drifted below 15'. By setting it at 20' you are saying as long as I am shallower than 20' continue countdown of safety stop. All about adjusting for range of diver capabilities and water conditions.
This is not true. See post #13.
 
Be glad that the computer has a tolerance, if it was exactly 15 feet than a lot of divers would have their computer reseting every time they moved from exactly 15 feet.
 
Be glad that the computer has a tolerance, if it was exactly 15 feet than a lot of divers would have their computer reseting every time they moved from exactly 15 feet.
Are there computers that actually do this? I’ve had 2 lower end computers and both have a good margin, so I’m curious as to which ones don’t.
 
Do your SS at 20 feet. It’s the Maximum Operating Depth (MOD) for 100 % Oxygen. I understand that isn’t how your diving at this point in your scuba training but it is good practice for to start to think and act that way. Doing a SS has more flexibility for depth changes vs a deco stop. Again good practice without the mental pressure of having perfect buoyancy. When you do your SS - do it in a horizontal position- why? - because you are much less streamlined in a horizontal position than a vertical position. Second reason - when you are vertical the micro bubbles in your bloodstream are going to travel to your brain vs horizontal where the bubbles now travel the length of your body.
 
When you do your SS - do it in a horizon)tal position- why? - because you are much less streamlined in a horizontal position than a vertical position.
The explanation is backwards, you are more streamlined in a horizontal position. This matters if you are trying to hold your position in a current.

Second reason - when you are vertical the micro bubbles in your bloodstream are going to travel to your brain vs horizontal where the bubbles now travel the length of your body.
I'm reasonably sure that this is inconsequential. Micro bubbles are constrained within the originating tissue. If they make it into the bloodstream, they will be entrained in the blood flow.
 
Every rec computer that I have paid attention to (which isn't a huge amount) the SS counter will start at 20' if turned on.

As a DM, I choose 15' because if I choose 20, I'd have divers at 25 and 15', if I chose 10', half the people would be at the surface....... It's like splitting two evils, 15' seems right in the middle and a good choice to keep holiday vacation divers incheck - no offense meant. I don't know if it happens, but I don't want someones computer being mad about not completing a SS, I want everyone that has a timer running to reach complete before we surface - that means you have to stay roughly between 20' and the surface to keep it running - I usually gotta pee so I'd rather not yack around......

From a tec point of view, I set my last stop at 20', complete half of it at 20 and move up to 15 to finish it, I never go to 10'. Setting it at 20 and bumping to 15 verses setting at 10 and never getting there theoretically maybe gets you out quicker than your initial TTS, not later - just one of those stupid detail things. I don't do crazy dives, I can watch my surface GF and it seems comfortable to me......
 
The explanation is backwards, you are more streamlined in a horizontal position. This matters if you are trying to hold your position in a current.


I'm reasonably sure that this is inconsequential. Micro bubbles are constrained within the originating tissue. If they make it into the bloodstream, they will be entrained in the blood flow.
You are more streamlined in a horizontal position if your swimming - if you are doing a SS or deco stop - and therefore not swimming- a horizontal position creates far more resistance to rising and descending than a vertical position would.
 
https://www.shearwater.com/products/swift/

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