A question about Ascent rate

[Remy B.]

What is so bad about ascending at 6m/min and not 10m/min. ?

I was recently in a training for Trimix and the instructor was adamant about the 10m/min ascent rate, which I fail to maintain or sometimes even reach, my buddy had two Shearwater DC, and he was able to do better job on our practices, I had a Rec computer that was beeping all the way up to the stops.

For the course I had purchased a divecomputer.eu, ( that is what I was able to afford ) which I pickup one day before going to the training and had not time to read the manual, I only entered the gas mixes and the GF, but had not time to read what were the ascent rate bars meaning, I mention to the instructor that I was using it just to watch depth and time, the stops were followed with a slate, beside that I honestly had no clue what else to do with it except change the gases depending on the stops.

Now that I have more time and I'm reading the manual the ascent bars on this tec computer are from 1 to 3m/min ( green bars 1 to 3 ), 4 to 6m/min ( yellow bars 4 and 5 ) and 7m/min ( one red bar ), so there is no way to tell how fast I'm going beyond 7m/min.

 

[RainPilot]

If your dive was planned at the ascent rate you actually use, then generally no issue. A slower ascent rate means more on gassing than the plan accounted for and so you may end up skipping deco even if you follow your plan to the letter.

Your dive computer will take this into account, but it will make your paper backup plans insufficient.

 

[fsardone]

Also you do not need computer bars, just remeber 10 meters per minute is 6 seconds per meter. Count 1001,1002 ... in you mind whenever you have an even meter. if you get to the next before 6 you are fast after 6 you are slow. ...

 

[Remy B.]

Your dive computer will take this into account, but it will make your paper backup plans insufficient.

Correct, the instructor had as well the same point about having two computers that are the same, which I plan and hopefully are able to buy me a second Divecomputer.eu, but it was his persistence about the 10m/min, I understand the on-gassing but as you ascent the on gassing is anyway slower and I think that we are talking no more than 5 min difference at the last stop of 6m, so gas planed is suffice for the dive and that additional time has no great impact, if I'm not mistaken

 

[RainPilot]

Correct, the instructor had as well the same point about having two computers that are the same, which I plan and hopefully are able to buy me a second Divecomputer.eu, but it was his persistence about the 10m/min, I understand the on-gassing but as you ascent the on gassing is anyway slower and I think that we are talking no more than 5 min difference at the last stop of 6m, so gas planed is suffice for the dive and that additional time has no great impact, if I'm not mistaken

True for gas planning but if you have written backups they will be no good. The simplest solution is to have two computers set to the same algorithm and then no issues

 

[RainPilot]

Also you do not need computer bars, just remeber 10 meters per minute is 6 seconds per meter. Count 1001,1002 ... in you mind whenever you have an even meter. if you get to the next before 6 you are fast after 6 you are slow. ...

There speaks a man who remembers timed climbing turns.

 

[Gareth J]

As has been said. Ascent rates are part of the decompression profile (dive profile).

Dive Computers have reduced the issue to some degree, because they calculate decompression requirements dynamically. i.e they take account of your actual depth and time and continuously recalculate your decompression requirement based on what you have done and what you are doing.

The big issue is when you look at hard tables. One of the tables I was taught was the BSAC88 tables.
These give you a dive time which is defined as the time from leaving the surface to reaching the ascent check depth (6m for a no stop dive). The table works on the assumption of a maximum descent rate of 30m/min, and an ascent rate of 15m/min to 6m (1 minute from 6m to the surface - 6m/min).
For a 30m dive your maximum no stop dive time is 20 minutes. So 1 minute to descend, 17 minutes on the bottom (runtime 18 minutes) and 2 minutes to ascend to 6m (runtime 20 minutes). Now, if you ascend at 6m/min, and leave the bottom at 18 minutes runtime, you arrive at the ascent check depth after 4 minutes, i.e. two minutes late (runtime 22 minutes). This would now be a dive requiring a 1 minute compulsory stop - you changed the profile. A computer would now modify your dive, telling you to add the additional 1 minute at 6m, if you where diving under the table, you either need to know that you have this additional 1 minute stop, or you surface outside the dive table calculations.

When I did my mix gas training. Technical dive computers hadn't actually arrived on the market (the VR3 being the first mixed gas dive computer I think). All Trimix dives where done on a hard table, that you generated prior to the dive. So ascent rates where a critical part of the dive. Because they where a critical part of the profile.
Now, with a modern technical dive computer (e.g. Shearwater or OSTC), the computer is dynamically able to recalculate your profile - so you might say what is the issue. The issue is that if you planned the dive, and the gas requirements, you are moving outside that plan, which is arguably may only be a minor correction to your original plan.
The point to remember, is ascending slowly means you are still on gassing some tissues.

In the case you describe, it sounds like you where using a recreational dive computer as a bottom timer (depth and time). Therefore you where diving a hard table, that you had produced before the dive, so to stay within the table you need to maintain the specified ascent rate.

If you have access to the planning software, and it allows you to modify the ascent rates, run two plans, one at 10m/min, and one at 6m/min ascent rates.


To use the BSAC88 table again (using air).

50m stop dive of 19 minutes dive time (surface to ascent check depth of 9m.) After reaching 9m you need to do the following stops, 1min@9m, 3min@6m.
The table assumes an ascending rate of 15m/min so 3 minutes ascent time. Leaving the bottom at 16 minutes of runtime. If you then ascend at 6m/min the ascent time is approximately 7 minutes. So an overrun of 4 minutes. The dive table now gives you a 24 minute dive (there isn't a 23 minute option). This pushes the stops out to 2min@9m, 12@6m.
If you had followed the original plan you would have blown off 8 minutes of stops!


Gareth

[1] I picked the BSAC88 because of the way the dive time is defined. NOTE they are air tables.
Also Note, these are viewed as quite aggressive tables by some, when we look at changes in modern dive practice.

 

[Remy B.]

In the case you describe, it sounds like you where using a recreational dive computer as a bottom timer (depth and time). Therefore you where diving a hard table, that you had produced before the dive, so to stay within the table you need to maintain the specified ascent rate.

If you have access to the planning software, and it allows you to modify the ascent rates, run two plans, one at 10m/min, and one at 6m/min ascent rates.



Gareth

Yes I have Multi-Deco and I can adjust my ascent rate and I set it too 6m/min as my tec computer have a maximum of 7m/min on alarms

If I do a run on the program, with 60m/20min and ascent rates of 6m/min and 10m/min the only difference is 5min overall time between the two ascent rates.

 

[Remy B.]

What ascent rate do the experts use from bottom to 21m/70ft stop ?

 

[rjack321]

10m/min on average. I try to start out a little faster like 12-15m/min at first to get off the bottom. But slow down to 6 or even 3m/min in the 26-21m range if I have come up from significant depth. This is more about not overshooting 21m than it is about agonizing over ascent rates. For short bottom times in the 20-40min and 35-65m depth range the ascent rate isn't super critical. Longer and/or deeper it starts to be a real issue. You absolutely have to efficiently move from 90m up to your first stop (10m/min usually) or you are just a packing on extra dissolved gases.

Most divers, especially new deco divers, ascend far too slowly. 6m/min might not be a big deal on a 45m dive but its not correct and is going to be a real issue on 65m+ dive.