Overshooting NDL and mandatory deco stops

[The Chairman]

That "Suuntos are so very conservative" legend is just that. A legend.

I disagree, and I know cave instructors who refuse to teach students who have a Suunto.

I would also suggest that Suunto engineers disagree with you and are proud of their conservatism. Proud. I got to sit with two of them after they heard me make the very statement to a dive buddy at Dive Outpost in Luraville Florida. According to them, these PDCs were developed in the Baltic and North Seas to protect divers who dive in very cold waters. In fact, their HelO2 was developed with negative conservative factors in order to match Pelagic PDCs.

However, conservatism is only the beginning for safe diving with a PDC. I can't emphasize the diver's role in their own safety enough. I can dive more safely with a watch, a red ribbon and the rule of 120 than some divers I have seen with multiple PDCs on their persons. It's only a tool and it's up to each diver to figure out the best practices for them while using that tool.

 

[The Chairman]

Caveat: I mostly dive Shearwater for many, many reasons. While there are many tools out there, I like them the best and I own four for various uses. I gave my HelO2 away. It was perfectly adequate, but I vastly prefer my Shearwaters. For sure, I also wear a Garmin Descent. It has fitness features that my Shearwater does not, like GPS, steps and heart rate.

 

[AJ]

...surface with some 10-ish bar (~150 psi) left ...

I would suggest min 20 bar if using AI, if using SPG don't go below 30 bar because of inaccurate readings. 10-ish is awfully close to the 1st stage causing to stop to functioning.

 

[The Chairman]

10-ish is awfully close to the 1st stage causing to stop to functioning.

I don't understand this statement. Your High Pressure seat in the first stage would simply be fully open once the tank pressure is equal to or less than the set Intermediate Pressure. It would be increasingly hard to breathe, but theoretically, at 20 ft, you could suck that cylinder down to about 10 PSI. IRL, anything below 50psi makes it difficult for me to breathe.

 

[tursiops]

If you don't want to put in the effort required to learn some basic deco science, I'd recommend just following what they taught when I took my PADI OW.

If you overstay your NDL by less than 5 minutes, stay at safety stop depth for 8 minutes and avoid diving for at least 6 hours. If you overstay your NDL by more than 5 minutes, stay at safety stop depth for at least 15 minutes if you have enough gas and avoid diving for at least 24 hours.
(Translated back to English from my OW manual)

Personally, if I overstayed my NDL by more than 5 minutes, I'd spend the rest of my tank at safety stop depth, surface with some 10-ish bar (~150 psi) left and then ask for the O2 bottle when I got back on the boat. And I'd be looking out for any DCI symptoms.

This rule is pretty easy to remember if you think of it as 3's and 5's.
Normal safety stop (always at 15 ft/5m): 3 mins.
Overstay less than 5 mins: add 5 to 3 = 8 min safety stop
Overstay more than 5 mins: multiply 5 by 3 = 15 min safety stop.

The 6h penalty box is because a huge fraction of all DCS symptoms appear in 6h or less, AND because 6h is six half-times for the slowest tissue in the DSAT model.
The 24h penalty box is because almost all DCS symptoms appear in 24h or less, AND because 24h should be a complete washout of all tissues.

 

[Gareth J]

However, conservatism is only the beginning for safe diving with a PDC. I can't emphasize the diver's role in their own safety enough. I can dive more safely with a watch, a red ribbon and the rule of 120 than some divers I have seen with multiple PDCs on their persons. It's only a tool and it's up to each diver to figure out the best practices for them while using that tool.

That's what frightens me when people without any understanding use computers like the Shearwater. They are great computers, but can be set very aggressively, if you wish.

I remember dragging a pair of divers out in the channel who had dived the SS Moldovia, she's a beautiful wreck to dive. We used to use her on ERD and Trimix courses.
Our team where on twinsets and stages, but being April, we kept the bottom times down to 20minutes, and where basically accelerating the deco.
The pair we pulled out hadn't realised how aggressive they where, their computers and punished them badly, they where basically out of gas (air) when we put divers back in (to check on them), and hypothermic.
We had them on 80% for the ride back from our stages.

 

[Storker]

I would suggest min 20 bar if using AI, if using SPG don't go below 30 bar because of inaccurate readings. 10-ish is awfully close to the 1st stage causing to stop to functioning.

This is interesting. IIRC 30 bar is some 500psi, IOW the common safety stop tank pressure if you're diving in imperial units. So I guess that what you're really saying is that the normal safety stop tank pressure in imperial country is so close to zero that you risk going OOG before you surface?

And it isn't consistent. In metric country, recommended safety stop tank pressure is 50 bar (some 700psi). Regardless of whether you're carrying a 10L tank or a 15L tank. That's a 50% difference in remaining gas volume.

 

[The Chairman]

That's what frightens me when people without any understanding use computers like the Shearwater. They are great computers, but can be set very aggressively, if you wish.

That's a decision which indicates a modicum of understanding. Like anything, you get what you put into it. If you want to be lazy and cut corners, then it's easy to fracture the envelope, rather than just push it. Me? I don't like to even get close to that envelope. Since I still don't feel completely comfortable departing from the default they sit right there. A diver has to know their limitations and honor (stick to) them. I'm not going to act like I fully understand something that I don't and don't mind the questioning looks when buddies find I'm still on defaults.

I remember dragging a pair of divers out in the channel

This isn't a PDC issue: it's a training/limits issue. Gear can't train you to dive safely: not even the smartest PDC can do this. Dive within your training and within all of your other limits. Again, it's not the PDC's job to keep them from doing stupid crap. That's all on them.

 

[The Chairman]

This is interesting.

Is it? More problematic is the lack of situational awareness that allows a diver to get into the situation in the first place. All deco should be planned for. There's simply no reason to get into deco without prior thought. Sometimes it's just being sure you have a larger tank, more often it's dragging along additional tanks, including oxygen. Many divers simply rely on their ego to pull these stunt dives off. "It's no big deal" attitude is OK for someone with the training and experience, but not so much for someone doing it as some sort of dare or stunt.

 

[Gareth J]

This isn't a PDC issue: it's a training/limits issue. Gear can't train you to dive safely: not even the smartest PDC can do this. Dive within your training and within all of your other limits. Again, it's not the PDC's job to keep them from doing stupid crap. That's all on them.

Its a mix of both. They didn't understand how the computers would behave, once they had crossed over into decompression. Or the effect of being in very cold water. Or the effect of being deeper than the 3m ceiling the computer was using to calculating their decompression time. So, they where in the water for significantly longer than they allowed for, cold water at that.

Diving is easy, put a regulator in and keep breathing. Its when it goes wrong, or you push the envelop that it gets difficult.

As one of my old diving buddies often says, we where lucky, we survived deep air, single cylinder diving and the advent of Nitrox, many didn't!

There are many who are much better at this diving game than I am. Who have a much better understanding. I have a little experience, often because I survived things, hopefully, a comment from me can reduce the risk of someone repeating the mistakes that I have made.