Ceiling/Deco obligation in recreational diving

[KenGordon]

So, which one are you saying? If you are within your NDL, you don't have a ceiling, so you can go straight to the surface, as fast as you want? I mean, you have tried to make the point several times that your NDL is not at all calculated based on ascent rate.

According to the dissolved gas limits of the model you can go directly to the surface. Of course that risks being bent all the same.

Or are you saying that, even if you're within your NDL, you could have a ceiling, which you COULD break if you ascend too fast (thus having a significant increase in chance of DCS)?

No, if you have a ceiling you are no longer within the NDL.

Say you are one of those nutcases that thinks that a free swim to the surface is a plan for running out of gas. The claim is that during a, so called, recreational dive this is possible because there is no ceiling. It would be a bit embarrassing to discover at 3m just as you have got to the end of a nice long 'ooommmmmmm' that you needed to hold a stop for a minute.

 

[KenGordon]

That's fine, but I have been talking about what current computers actually do.

There are lots of computers, we can't actually talk about what they ALL do. But we can postulate reasonable behaviour.

 

[KenGordon]

Suunto's behavior is based on a different model which yields the same conclusion: Ascend too fast from an NDL dive and you can get a mandatory deco stop.

No, you get a thing which they call a mandatory SAFETY stop. If you skip it you get shorter NDLs later, but not locked out.

The Suunto looks at critical bubble radius (I think).

No, an ascent rate warning is based on exceeding the max ascent rate and the time that was done for. You will get one if you bounce to 15m in 2 minutes. You do not need to have a high saturation for it to happen. BTW, if uses M-values too. (At least the pre 'Fused' ones do)

neither is based on NDL. Both are based on violating the constraints that are programmed into the computer.

The NDL is based of the constraints programmed into the computer. They are all linked. How they are linked is what we are arguing about.

 

[nick73]

Say you are one of those nutcases that thinks that a free swim to the surface is a plan for running out of gas. The claim is that during a, so called, recreational dive this is possible because there is no ceiling. It would be a bit embarrassing to discover at 3m just as you have got to the end of a nice long 'ooommmmmmm' that you needed to hold a stop for a minute.

Precisely my thinking and the reason I asked at the beginning of the thread. To be honest, in a similar dive I am very much tempted, provided I convince my instructor to finish the dive without one, to shoot my computer to the surface with an smb/reel approximating a 9-10m/min ascent and see if it will log a ceiling violation at any depth (implying that a deco stop was required after all) or it will log "only" the expected mandatory stop violation (due to the high rate of ascent) that always activates when ascending at any rate faster than what the computer likes.

 

[EFX]

We don't need to speculate. Most, if not all, computers using symmetrical dissolved gas algorithms employ the Schreiner equation, or a variation of it, to calculate tissue compartment (TC) gas loadings. The equation shown here is the general form used for all phases of the dive (descent, constant depth, and ascent):

P = Pio + R(t - 1/k) - [Pio - Po - (R/k)]e^-kt
where:
Pio = initial ambient pressure minus water vapor pressure
Po = initial compartment inert gas pressure
R = rate of ascent/descent times the fraction of inert gas
t = time interval
k = half-time constant = ln2/half-time
e = base of natural logarithms

Prior to the dive the computer will set Po for all the TC's to the surface pressure. As the dive progresses the computer will calculate P for each TC by first setting Po to the previous calculation's P. The computer will calculate P every x seconds or a fraction of a second depending on the model. For the descent phase R will be the actual descent rate. For NDL and ceiling calculations the computer assumes a constant ascent rate of 30 fpm. For NDL, the computer calculates a value of t where P equals the m value for each TC. The NDL is a projection based on the current depths gas loading plus additional loading at the assumed ascent rate that will not exceed the m value. Increasing R above the assumed rate (you actually ascend at a rate higher than 30 fpm) will push P to higher values. If P gets high enough an ascent ceiling is imposed on the dive. This ceiling is usually rounded down to the nearest 10 ft (3 m). There are different ways of calculating NDL and ascent ceilings. For more information see Stuart Morrison's paper "DIY Decompression". (I tried to upload the file here but SB wouldn't let me do it).

The computer calculates both NDL and an ascent ceiling for each iteration of the program. Some computers will display an NDL until it gets to zero and then display the deco stop depth and time at that depth. The ceiling is positive below the surface, zero at the surface, and negative above the surface. If the calculated ceiling is negative or zero the NDL is calculated and displayed. Once the ceiling goes positive the stop depth and time at the stop are displayed.

The m value (a constant) stands for maximum value and is the pressure difference between the tissue's pressure and the surface pressure to prevent DCS. It is expressed in equivalent absolute values of fsw (feet of sea water) or msw meters of sea water). Each TC has it's own m value and half time constant.

 

[Miyaru]

Last week we did a cave dive, using:
diver 1: ShearWater Perdix set to GF40/75
diver 1: Suunto D4i Novo
diver 2: Suunto Vyper Novo
diver 2: Suunto Gecko
The dive profile is from 2 dives with a very short surface interval of less than 5 minutes. Dive 1 goes into a cave to 28m, turns after 26min at 6m, same way out and is repeated again.
The profiles from the SW and Vyper Novo are shown below. Dives were done on air.

GF 40/75 seems to be more conservative than what the Vyper Novo is using. The Gecko and D4i showed much longer stops (over 15min longer).

Looks like Suunto has deviated from their proprietary RGBM model, and since the OP was using a Vyper Novo as well, this might even add to the confusion.....
Anyone with more insight into what Suunto has changed in this divecomputer?


Edit: diver 3 had a Heinrich Weihkamp set to GF40/80. This computer never showed a ceiling.