ascending too slowly (long-table divers please read)

[Diver0001]

This topic hasn't been mentioned in a while so I think it's time for a new discussion about ascending too slowly, especially when using tables.

The good news is that people are generally aware that ascending too quickly can be harmful. When I look around me I'm now seeing divers who ascend slowly, or at least much slower than the rates we were seeing 10 or 15 years ago. The advent of computers has helped a lot because divers can now look at their ascent indicators and at least get an idea if they're going too fast.

When you're using a computer, pretty much any reasonable ascent rate will do as long as the computer stays out of deco. Some people call this "riding the computer" and don't agree with the practice but the reality is that if the computer stays out of deco, the dive is probably safe.

We've also seen ascent rates being brought down by some of the major agencies. Some from 25m to 18m to 14m to 10m per min in the last couple of decades. PADI recommends 18m/60ft per min when using their tables and we'll get back to why this is relevant in a minute.

The drop in ascent rates has lead a great many divers to make the blanket assumption that slower is better. Maybe this is true to a point but if you're using tables then "slower is better" can have negative side effects. I'll explain using an example.

Assume you're making a table (square) dive to 40 metres for 9 min (the limit on the PADI tables). Your bottom time is calculated from the beginning of the descent to the beginning of the ascent. On a 40 meter dive you don't have any significant off gassing until you are back to somewhere between 30 and 20 meters (80% or 2ATA - depending on your religion) so any time you spend deeper than something like 25 meters (to pick an average) is spent on-gassing during your ascent.

In other words, on this dive, a slow ascent from 40m-25m is doing nothing more than extending your bottom time. More bottom time means more nitrogen in your system, which can become a problem if you ignore it when planning subsequent dives. Your tables are calculated using a particular ascent rate and the assumptions of residual nitrogen made on your tables are based on this ascent rate. If you ascend too slowly then you'll have more residual nitrogen in your system than your table is designed for. Needless to say this isn't good.

The point being that the ascent rates used to calculate your tables are not only "maximums" but "target" ascent rates. The going thinking is that an ascent of 10m/30ft per min is fast enough and when leaving the bottom the *first* part of the ascent should be made at your target ascent rate. Once you're shallow then slow down but don't start slow, start at your target.

R..

 

[Rick Inman]

This topic hasn't been mentioned in a while so I think it's time for a new discussion about ascending too slowly, especially when using tables...

Would you also add that, from 5 meters up, there is no such thing as too slow?

 

[gkndivebum]

As a general set of guidelines when we're doing deco diving, we ascend to our first deep stop
at 1 atmosphere/min (33 feet per min). Generally we slow it down to 10 feet per minute on the
subsequent stops , though if the dive is deep enough we'll ascend to a second deep stop at
1 atm/min before slowing down.

Our last stop is at 20FSW; we take10 minutes to ascend from 20 FSW to the surface.
Yep, that's 2 feet per minute.

For recreational diving (i.e., within your favorite set of no-stop limits) I still do a 5 minute hang
at 20 feet, and take at least 3-4 minutes to surface from there.

In a nutshell, the shallower you are, the slower you go.

 

[MikeFerrara]

Good subject Rob but I don't agree with your conclusiuons. For any profile and ascent rate there is a depth where "offgassing" begins. That depth will change depending on the model used, I suppose, but in theory you don't want to dally too much below that depth because it essentially adds to your bottom time. Spending too much time at deep stops which are meant to address gas in the free state and reduce bubble formation will just add to the desolved gas and lengthen the shallower decompression.

The problem with tables is they don't give you credit for the time you spend at shallower depths. You deal with bottom time measured to the time you start a direct ascent to the first stop (whether a real stop or the surface/safety stop) at the prescribed ascent rate. This is dealt with by treating the dive as a multilevel dive whether you use a computer or another method.

you are correct that adding too much time too deep can get you into trouble. discussing specific way of dealing with it probably requires deciding what kind of dives we are going to discuss. Sometimes, for the sake of simplicity, we count bottom time from the time we switch to back/bottom gas to the time we get to the first stop with everything inbetween treated as a multilevel dive. for us that means using a "profile depth". That works well for cave dives since you can't ascend any faster than the cave does. Wreck dives, which are often more of a square profile are treated more conventionally and we pretty much hit the schedule (realizing that we're not dealing with an exact science).

Unfortunatley the way divers learn decompression can cause them to do a couple of different things. One is to ascend too quickly because they think they are out of time...ie there is really no reason to rush to leave 30 ft to get to 15 (legal safety stop) thinking it will put you off the table. The other is to ascend too slowly adding ongassing time. Either way the goal is to spend enough time above the transition from ongassing to offgassing on a curve that minimized bubble formation and yet illiminates as much loading as possible prior to surfacing. Just to pick a number, I haven't ever run a profile where spending time at say 30 ft would make things worse. It might be possible to come up with one in theory but in practice?

 

[Detonate]

Studies from DAN show that most people that get bent are well within table/computer limits. Why is this? Acending to fast. So while you article is interesting, I still ingrain "slower is better" into my students brains. Especially when ascending from 33ft to the surface.

 

[pt40fathoms]

To date the practice in most tech diving has followed 60fpm (18 metres per minute) until you reach 2 ata (33 feet or 10 metres) then it is reduced to 30fpm (9 metres per minute) to the surface. The slower is better is most benifical when going from 2 to 1 ata. In the last 15 feet (5 metres) is where many divers tend to get impatient and come up too fast. This may explain to some extent the onset of "undeserved DCS". Too slow from your planned bottom means just as you have suggested, further ongassing of Nitrogen or Helium that is accounted for if you have a tri-mix dive computer, but not if you are diving tables.

Good subject, one that needs discussing.

 

[pt40fathoms]

Sometimes, for the sake of simplicity, we count bottom time from the time we switch to back/bottom gas to the time we get to the first stop with everything inbetween treated as a multilevel dive.

Same for us up here, bottom time ends at the first planned deep stop.

 

[Diver0001]

Studies from DAN show that most people that get bent are well within table/computer limits. Why is this? Acending to fast. So while you article is interesting, I still ingrain "slower is better" into my students brains. Especially when ascending from 33ft to the surface.

Well.....I'm digging at this very point but from the point of view of new divers making their first deep dives.......

I'll agree about one thing. All the research I've seen shows that slower is better from 1.6ATA to the surface. (what is that? about 20 ft). The way I understand it is that no significant ongassing can occur from 6metres to the surface so no matter how long you extend these stops there is no damage. Chances are if you've been deeper than 9 metres that the 9metre stop also results in off gassing. (and so on but this is beyond recreational limits)...

However......and get your pen and paper ready because this is important...

......."slower is better" breaks down when we go deep.

Do you teach deep specialties?

I'll give you the point that a lot of new divers ascend too fast but I've been seeing experienced divers (including pro's) making their first deco dives on tables and their reaction is invariably the same. They ascend too slowly and put themselves in danger and then say "slower is better, isn't it?".

Slower is better........shallow.

If you're deep.....achieve your ascent rate

.... at least until you're getting into the shallow zone.

We have a long way to go before we've made this discussion understandable for the intended audience, namely new divers. Help me out here. Some of you understand what I'm on about. Help me translate it for new divers making thier first deep dives.

R..

 

[MikeFerrara]

We have a long way to go before we've made this discussion understandable for the intended audience, namely new divers. Help me out here. Some of you understand what I'm on about. Help me translate it for new divers making thier first deep dives.

R..

I agree. Though, I'm not sure how to simplify it betond saying that when your still ongassing you can't dally unless you take the time into account and above some depth, slower is better. Knowing when to do what takes a little study. Granted some rules of thumb could be used within the limits they were developed.

Of course it's all for nothing if a diver doesn't know how to control their ascent rate in the first place and I think that's what's behing much of the "too slow" and "too fast" ascents.

 

[boomx5]

My advise to the new divers out there is to get some proper training and not learn this stuff on the internet. Without going into details, there are some good things and some bad things being taught in this thread, and without the ability to differentiate between right and wrong you can get yourself hurt.