Diving without computers

[KeithG]

Actually I'm a bit confused about that too - the 10% of NDL or greater than 60 feet rules. Are those from some agency?

I mean, if the rule says that with X amount of N2 loading (defined as being within 10% of NDL) you generate an ascent profile that includes a mandatory 3 minute stop at 15 feet, that's a deco stop. The NDL is calculated by the algorithm, so it's model-dependent to say that you are within or past NDLs anyway at any given point.

Agreed - my understanding of the material in this thread has introduced (to me) the existence of the concept of non-mandatory mandatory non deco stops. The stops are claimed to be mandatory. But you can skip them if you like. They also are not deco stops.

I am confused...

 

[boulderjohn]

I know you probably dont like the word rule but that is the way it has always been. its even taught that way in OW. Safety stop is just that. a little blanket of safety not always needed unlike a mandatory stop that is needed. And i think they still talk about 3 min safety stop (still optional) and up to 5 minutes under some conditions, Even the 3&5 in is still not required but recommended.

Do you mean PADI? If so, it is not correct.

In the second paragraph on the back of the PADI tables, it says, "A safety stop for 3 mins at 15 ft is required any time the diver comes within 3 pressure groups of a no-decompression limit, and for any dive of 100 ft or greater."

The course itself was worded slightly differently, saying that it was required any time the diver reached the no-decompression limit, was within 3 pressure groups of the no-decompression limit, or 100 feet or greater. As a specific example, on a dive to 80 feet on the tables, a safety stop would be "required" after the diver reached 26 minutes.

The language in the current computer based version of the course is nothing like it. Safety stops are recommended for most dives, and the word "required" is given some further explanation, both for computers and for tables. Page 199 of the OW manual says (emphases in the original), "...it is a good habit to make a safety stop for three minutes at 5 metres/15 feet before finishing the ascent and surfacing. A safety stop is not required to be within the limits of most dive computers' or tables' decompression models. You make the stop as a prudent, conservative diver to remain well within your computer or table limits. A few computers and tables have a 'required' safety stop. With these, because you are nearing the limits, they call it 'required' to put more emphasis on the importance on being conservative."

 

[Ryszard R]

In my opinion:
mandatory stop=deco stop

 

[doctormike]

In my opinion:
mandatory stop=deco stop

Exactly. I think that we are way overthinking this. All dives involve decompression.

There is only difference between a "rec" diver and a "deco" dive (to use the common phrasing): on a rec dive the algorithm says you can ascend directly to the surface, and on a deco dive it says that you can't do that, you have to stop somewhere for some length of time before offgassing enough to surface with an acceptably low DCS risk.

 

[Bob DBF]

In the early '80's PADI OW manual the topic is labeled Safety Decompression, and discussed as a safety measure when diving on multiple dives per day on tables. These were the old PADI tables, which were probably a clone of the old Navy tables. The tables also included some decompression calculations, in case NDL was violated, and the training included information on dealing with accidental deco.

In the manual, the terms safety decompression stop, and safety stop are used. Effectively the No Decompression Limits and decompression diving are not divided by the the sharp lines of the dive tables, and the new diver needs to consider ways to insure he stays within NDL and having the information to make those choices.

It is easier and faster to make rules and train by catch phrase as is done today.

I use a computer now, and will dive according to it's protocols, except in an emergency, even though it took a long time to get used to 30 fpm ascent and a 3 min safety stop, and it doesn't even care if I violate those conditions.


Bob

 

[NAM001]

Actually I'm a bit confused about that too - the 10% of NDL or greater than 60 feet rules. Are those from some agency?

I mean, if the rule says that with X amount of N2 loading (defined as being within 10% of NDL) you generate an ascent profile that includes a mandatory 3 minute stop at 15 feet, that's a deco stop. The NDL is calculated by the algorithm, so it's model-dependent to say that you are within or past NDLs anyway at any given point.

I don't know where these so called rules are coming from. I consider them advisory's. If I do a dive to 100 ft and ascend at ndl time I should be able to go straight to the surface at the appropriate ascent rate . It is prudent to spend a few minutes at 20 ft in case things like you were working at depth ascent rate got a little high, or you have a health issue you don't know about ect. Computers have the 3 min stop in them that you can turn off if you want to and the books will say the the time increases to 5 minutes if you are with in 10% of ndl or the dive exceeded 60 ft. Some manuals say this is done for accepted precautionary purposes and complies with current doctrine. Doctrine over recommendations or rules, it doesnt matter, the very authoritive word doctrine implies rules to some. The last class I sat in on a couple years ago said that computers have this and you should follow the computer advice. (CYA statement) So it may be the instructor and the way they relay it to the student that makes it sound like a black and white rule rather than prudent advice. When I was there I saw the topic coming when the discussion of safety stops arose. safety stops were not deco stops because they are not mandatory and are not required to be done. (They will be done through out the course) and OW's do not do mandatory stops because those are done for deco that OW's do not do.. At that point the instructor covered the computer and how it handles it.

I still agree with your perspective on this. Its difficult to discuss topics like this when some quibble on the issues of punctuation of use of say rule over strong recommendations when its all moot because it is taken in the light of how the instructor presented it and how the student takes the information.

I know when I am in a 30 ft lake I do not do safety stops for the purpose of safety I do it for buoyancy control once a day and skip it the rest of the time. If I am with a new diver I do the stop with them at 15 for skills profeicency purposes and to display or establish good habits. And habits are not rules either.

 

[NAM001]

Do you mean PADI? If so, it is not correct.

In the second paragraph on the back of the PADI tables, it says, "A safety stop for 3 mins at 15 ft is required any time the diver comes within 3 pressure groups of a no-decompression limit, and for any dive of 100 ft or greater."

The course itself was worded slightly differently, saying that it was required any time the diver reached the no-decompression limit, was within 3 pressure groups of the no-decompression limit, or 100 feet or greater. As a specific example, on a dive to 80 feet on the tables, a safety stop would be "required" after the diver reached 26 minutes.

The language in the current computer based version of the course is nothing like it. Safety stops are recommended for most dives, and the word "required" is given some further explanation, both for computers and for tables. Page 199 of the OW manual says (emphases in the original), "...it is a good habit to make a safety stop for three minutes at 5 metres/15 feet before finishing the ascent and surfacing. A safety stop is not required to be within the limits of most dive computers' or tables' decompression models. You make the stop as a prudent, conservative diver to remain well within your computer or table limits. A few computers and tables have a 'required' safety stop. With these, because you are nearing the limits, they call it 'required' to put more emphasis on the importance on being conservative."

John I believe what you say from a legal/ corporate CYA standpoint. I also know this stuff is not put out to students is so many cases. I know that also they are told OW's have a depth limit of 60'. Other new divers have told me that this issue of mandatory safety stops are covered in AOW because you don't get into the situation of having to do the stop. I'm sure the info you quote in YOUR classes is covered very well. That is not the case in most classes. Its left as a "safety stop is a voluntary stop that if skipped will not put you into the bends because of your 60' diving limit and limited air supply". This item alone is an issue that makes the get your OW and you are good to 130' unreal because they are only taught to make it back to the surface from 60' or less. Many other instructors do not want to deal with the safety stop issue and just say it is a required stop to do to avoid the hassle of covering the reasons for it.

Official doctrine and real world is often far apart.

 

[KenGordon]

Oh dear.... RTFM

There are stops. The purpose is mostly to reduce the probability of a bend. Whether you need these depends on the profile and how much risk you want to take.

If a short shallow dive with a proper ascent has an very low level of risk then we can go directly to the surface.

A longer or deeper dive might make us think twice about the risk, so we add a safety stop to reduce the risk. We didn't expect a bend without a stop but in the population as a whole we will reduce the number of bends if everyone does this. It might also mitigate other factors such as late night partying, working hard on the bottom and so forth which increase the risk above the average for that profile. This is a safety stop.

The same dive, done with a computer, gives us a chance to have extra information about the risk of the profile. A fast ascent is a risk factor. So if the computer spots a fast ascent it can add extra stops. These are more serious than the safety stop as they are just to bring the risk back to 'normal' for this diver on this dive rather than improving outcomes in the general case. However it is not imposed by limits of tissue N2 pressures. They can be skipped and you may be given shorter NDL on subsequent dives. This is a 'mandatory' safety stop.

Longer and deeper again, or with a lower risk threashold, disolved N2 prevents surfacing and you need to wait for it to reduce to an acceptable level. These are 'proper' decompression stops.

These is no such thing as a safe dive profile. Just less risky dive profiles. During the development and testing of tables people get bent AND people do not get bent on the SAME profile, even the same person. The purpose of all the stops is the same, to move the risk off to the right where it will not turn up so often.

 

[doctormike]

The same dive, done with a computer, gives us a chance to have extra information about the risk of the profile. A fast ascent is a risk factor. So if the computer spots a fast ascent it can add extra stops. These are more serious than the safety stop as they are just to bring the risk back to 'normal' for this diver on this dive rather than improving outcomes in the general case. However it is not imposed by limits of tissue N2 pressures. They can be skipped and you may be given shorter NDL on subsequent dives. This is a 'mandatory' safety stop.

I believe that you are making an arbitrary distinction. There is nothing physiologically different between what you are calling a mandatory safety stop and a regular deco stop, it's semantics.

All stops are imposed by the limits of tissue N2 overpressure (as opposed to total N2 loading). On a deep, long dive you are at risk of bubbling if you go directly to the surface at the standard ascent rate because you have so much N2 loaded that you can't offgass fast enough ascending at 30 FPS. On a dive within NDLs you could increase your DCS risk with an ascent rate violation. That's why the computer generates a mandatory stop. The computer can't tell if you are out of shape or dehydrated or have a PFO, but it can tell if you ascend quickly enough to cause a transient overpressure spike.

There are two types of stops - stops that are recommended no matter what the dive profile (safety stops), and stops that "bring the risk back to 'normal' for this diver on this dive", as you put it. You can distinguish between staged decompression stops and mandatory safety stops if you like, but they both do the same thing (bring the risk back to normal), and skipping either increases your DCS risk. This is not the case for an optional safety stop, the reason for which is not dive specific and which we discussed upthread.

 

[boulderjohn]

John I believe what you say from a legal/ corporate CYA standpoint. I also know this stuff is not put out to students is so many cases. I know that also they are told OW's have a depth limit of 60'. Other new divers have told me that this issue of mandatory safety stops are covered in AOW because you don't get into the situation of having to do the stop. I'm sure the info you quote in YOUR classes is covered very well. That is not the case in most classes. Its left as a "safety stop is a voluntary stop that if skipped will not put you into the bends because of your 60' diving limit and limited air supply". This item alone is an issue that makes the get your OW and you are good to 130' unreal because they are only taught to make it back to the surface from 60' or less. Many other instructors do not want to deal with the safety stop issue and just say it is a required stop to do to avoid the hassle of covering the reasons for it.

Official doctrine and real world is often far apart.

I am struggling to understand what you wrote here. The information I quoted is in the official material that ALL students are required to read. It is in the mandatory knowledge reviews. It is on the final exam. I suppose it would be possible for an individual instructor is telling the students that this information is wrong, but I have never heard of it coming from any of the instructors who also taught in either of the shops where I have worked.

You said, "This item alone is an issue that makes the get your OW and you are good to 130' unreal because they are only taught to make it back to the surface from 60' or less." I have no idea what this means. I think students have a pretty good idea on how to get to the surface if they go beyond 60 feet. It will require them to head in the same direction that they used to surface from above 60 feet.

You said, "Other new divers have told me that this issue of mandatory safety stops are covered in AOW because you don't get into the situation of having to do the stop." Why not? I quoted the material in the OW book for computer classes and the tales for those that take that version of the class. This is required learning for the OW class. As I said before, it is on the final exam.

You said, "Many other instructors do not want to deal with the safety stop issue and just say it is a required stop to do to avoid the hassle of covering the reasons for it." That makes no sense. The students are required to learn the material about safety stops during their home study, and they show up in class already having been taught it. It would be more of a hassle for the instructor to tell them to ignore what they were taught in the course materials and explain why.

Seriously, either someone has been feeding you serious misinformation or you are just making stuff up.