Teric & NDL obligation...

[taimen]

I guess I wrote that incorrectly. My Teric begins the safety stop counter once I have ascended shallower than 20’. I usually hang out around 15’ during my SS.

I think the answer to your question can be found in Teric manual pages 24 and 25, and prehaps better explained in Perdix AI "Recreational Nitrox Mode Manual" pages 14 and 15.
There is sometimes controversy related to meanings of "safety stop" and mandatory deco stop. But Shearwater has it's own definition for them.

In Perdix and Teric OC Rec mode (and only this mode) the safety stop timer starts only after mandatory deco stops are cleared.
Thus if you see NDL time replaced by a mandatory deco stop during your dive, you are obliged to hold it and it will be followed by a safety stop countdown.
Usually in typical NDL diving the 1 or 2 minutes of deco oblication at 10ft you see will "melt away" during your ascent. Thus when you reach your safety stop depth you actually do not have any mandatory stops to do and your safety stop timer starts immediately. Otherwise it is just fine to stop at 15-20ft and wait until your deco clears, it will just take a bit longer than at 10'.

Bottom line. In Shearwater OC Rec mode if you see "safety stop" countdown, you have already cleared your mandatory deco stops. Tec modes do not have safety stop countdowns.

 

[taimen]

1) Should you go to your safety stop depth of 20’ and begin your safety stop, assuming the deco obligation will be reduced / satisfied during your safety stop?

Or

2) Go to 10 feet and hold your depth for X minutes until your obligation is fulfilled and then descend to your safety stop depth and hold until fulfilled?

As said above, 1) is better answer than 2). But in Shearwater logic, your deco oblication will be satisfied at 15-20ft and is followed by the saftey stop countdown.
To avoid confusion, you can set "last stop" to 20 ft from deco menu, so the computer will calculate and display your deco stop to minimum 20ft. That way your stop times will be realistic for that dive profile.

 

[broncobowsher]

Deco is mandatory.
Safety stop is optional. Always has been, always will be. Just a good option to take. Some people think it is mandatory. I've been with a guy who was so determined to do his safety stop that he nearly ran out of air (digital pressure gauge said 60 PSI). Had to explain to him that a safety stop is optional, running out of air isn't. When you are that low, just surface.

Makes zero sense to blow the safety stop to get to the deco stop, then go back down for a safety stop. You don't get to pick at what time each gas leaves your body. Pressure (depth) picks when that happens.

Providing you have enough gas to safely do it, do your safety stop first. Usually that will eliminate the deco. A minute of deco obligation at 10' will be gone in 2 minutes at 20' (yes there are errors in that, but the general idea still comes through).

I've done plenty of dives where I'm on the 3rd dive of the day, down at 90' and see a 2 minute deco at 10'. I head up to shallower stuff and even at 30' that time will go away. I've even chased the NDL. On a wall and see 1 minute of NDL left, go up 20 feet and I have 7 minutes left. 5 minutes later, go up a little further. Stay just on top of the deco obligation.

The world doesn't suddenly change when you have a minute of deco obligation. You just have to know it is there and not blow it off.

The default for the Shearwater is pretty conservative. It is likely that it will show deco before most other computers in the recreational market. So you will see a minute of deco and your buddy will show several minutes of NDL left. You can change that if you like. But I recommend you understand it better before you do. I still leave mine on defaults. I like the extra safety margin it provides.

 

[Diver-6873]

I think the answer to your question can be found in Teric manual pages 24 and 25, and prehaps better explained in Perdix AI "Recreational Nitrox Mode Manual" pages 14 and 15.
There is sometimes controversy related to meanings of "safety stop" and mandatory deco stop. But Shearwater has it's own definition for them.

In Perdix and Teric OC Rec mode (and only this mode) the safety stop timer starts only after mandatory deco stops are cleared.
Thus if you see NDL time replaced by a mandatory deco stop during your dive, you are obliged to hold it and it will be followed by a safety stop countdown.
Usually in typical NDL diving the 1 or 2 minutes of deco oblication at 10ft you see will "melt away" during your ascent. Thus when you reach your safety stop depth you actually do not have any mandatory stops to do and your safety stop timer starts immediately. Otherwise it is just fine to stop at 15-20ft and wait until your deco clears, it will just take a bit longer than at 10'.

Bottom line. In Shearwater OC Rec mode if you see "safety stop" countdown, you have already cleared your mandatory deco stops. Tec modes do not have safety stop countdowns.

Exactly how it works for us .... perfect explanation.

 

[taimen]

Providing you have enough gas to safely do it, do your safety stop first. Usually that will eliminate the deco. A minute of deco obligation at 10' will be gone in 2 minutes at 20' (yes there are errors in that, but the general idea still comes through).

Except that in Shearwater OC Rec mode you won't get your safety stop countdown until you have cleared the deco. In real life, you just stop at 15-20 ft and wait until both deco and safety stop are clear. Computer will be happy and the dive profile will be good too.

 

[mcpowell]

To avoid confusion, you can set "last stop" to 20 ft from deco menu, so the computer will calculate and display your deco stop to minimum 20ft. That way your stop times will be realistic for that dive profile.

So why have a 10’ deco stop? It seems so much simpler to have it be the same depth as your safety stop.

 

[RainPilot]

So why have a 10’ deco stop? It seems so much simpler to have it be the same depth as your safety stop.

That will depend on where you have set your last stop depth to. Some like 10', others prefer to do 20' (max benefit when on O2 vs CNS vs surface conditions etc etc)

 

[broncobowsher]

A 10' stop is more efficient at doing deco, providing that the ceiling isn't below that. I vaugly remember playing with a lot of deco software and a 10' stop would take about 5 minutes off a fairly short deco dive over a 20' stop last stop. But when the boat hangs a deco bar at 20' for everyone to use, and the surface swells can make 10' a tough one to hold onto, 20' will do just fine. You burn through your deco gas a bit faster since you are a bit deeper. But if you plan for it all is good.

 

[taimen]

So why have a 10’ deco stop? It seems so much simpler to have it be the same depth as your safety stop.

Exactly. If you do separate safety stops (at 15-20') after deco stops, then I'd definitely set the last deco stop to 20' to for simplicity.

In technical decompression diving people usually do not do additional "safety stops". There if you want additional safety margin to your computer planned ascent profile, you change the conservatism level, for example use a lower GFhigh value. This is why Shearwater tech modes do not have safety stop countdowns.
If deco ceiling is at 10', that is the most efficient depth for decompression. It is often not feasible during ocean dives with swell, but in springs or mines/quarries with calm conditions 10' stop is often doable.

 

[Mike1967]

I'd definitely set the last deco stop to 20' to for simplicity.

In OC Rec mode there's only the 3m/10ft option.