Ceiling/Deco obligation in recreational diving

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

A 3 minute SS at 15 feet are always a good idea.

@Lorenzoid Having recently exceeded the ascent rate (about 63ft/sec) on a 100 foot dive I can confirm a mandatory stop was accessed on my Viper/Air. It was only a 1 minute stop @ 20 feet no problem, and it does not affect the recommended 3 minute SS.

For my clarity, can you please confirm: Your computer was NOT showing you to be in deco when you started your ascent? You had NDL time left on your computer when you began your ascent? Then a rapid ascent resulted in the computer giving you a mandatory deco stop (of 1:00 at 20')?
 
But does the ceiling on a Suunto, based on rapid ascent, occur in association with NDL or is it triggered at any point in the dive when a rapid ascent occurs?
 
A 3 minute SS at 15 feet are always a good idea.

@Lorenzoid Having recently exceeded the ascent rate (about 63ft/sec) on a 100 foot dive I can confirm a mandatory stop was accessed on my Viper/Air. It was only a 1 minute stop @ 20 feet no problem, and it does not affect the recommended 3 minute SS.

Hope you meant 63 ft/min?
 
@Lorenzoid Having recently exceeded the ascent rate (about 63ft/sec) on a 100 foot dive I can confirm a mandatory stop was accessed on my Viper/Air. It was only a 1 minute stop @ 20 feet no problem, and it does not affect the recommended 3 minute SS.

Yep, I'm familiar with this. Suunto refers to it as a "mandatory safety stop" and the computer displays a "ceiling," just as it does if you exceed the NDL, even though Suunto makes it clear that this "mandatory safety stop" is triggered by exceeding the ascent rate and is not related to exceeding the NDL.
 
For my clarity, can you please confirm: Your computer was NOT showing you to be in deco when you started your ascent? You had NDL time left on your computer when you began your ascent? Then a rapid ascent resulted in the computer giving you a mandatory deco stop (of 1:00 at 20')?

Exactly. Here is a brief dive description that shows how stupid I can be. Diving at CCV on the drop off Newton's Wall. Wife/buddy and I drop off I told her I wanted to go to 100 foot as I had not bounced to 100 foot in a while. She said nope and stayed above at 50 foot. I drop to 100' and thought "now what would I do if she or I had a problem" (stupid I know) bad buddy. No where near NDL and I stupidly zipped up to be close to her exceeded 60ft/min. So in effect I violated 2 things fast ascent and bad, bad buddy.

The Viper assigned the mandatory stop during the fast ascent. I knew it was coming at that point, but not a real issue as one minute has easily cleared in ascent. Then we also did a very slow ascent. I can post the Subsurface plot if needed.
 
But does the ceiling on a Suunto, based on rapid ascent, occur in association with NDL or is it triggered at any point in the dive when a rapid ascent occurs?

From my Suunto (D6) manual:

When the ascent rate exceeds 10 m/33 ft per minute continuously for more than 5
seconds, the microbubble build-up is predicted to be more than is allowed for in the
decompression model. The Suunto RGBM calculation model responds to this by
adding a Mandatory Safety Stop to the dive. The time of this Mandatory Safety Stop
depends on the severity of the ascent rate excess.

The STOP sign appears in the display and when you reach the depth zone between
6 m and 3 m/20 ft and 10 ft, the CEILING label, ceiling depth, and the calculated Safety
Stop time also appear in the display. You should wait until the Mandatory Safety Stop
warning disappears. The total length of the Mandatory Safety Stop time depends on
the seriousness of the ascent rate violation.
 
Yeah Charles I edited that .. I did mean ft/min..
 
Stuart, I think you (and DevonDiver) are claiming that you can be at a depth x and have a ceiling of 1mm (meaning you could not reach the surface with the exact tissue loadings you have now, at depth x - ie assuming teleportation to the surface) and not be in deco because if you ascended at 10m/s then by the time you got to the surface you'd have reduced your tissue loadings below the limit and have a ceiling of zero. Further you are saying that computers will call that you are still within the NDL in that case and only set NDL to zero when an ascent at 10m/s would still exceed the maximum saturation on arrival at the surface.

What I claim is that calculating the projected saturation is expensive and pointless and might lead to the issue that the OP is concerned about, so in fact counter productive.

The Baker code to calculate the ceiling doesn't take the time of ascent into account, it does not project forward to see how much you have OFF gassed in the ascent. You see there are no time or speed factors below. ONCE he has a ceiling he does check to see if the ON GASSING will make it too shallow, using the code whose comments you have reproduced, but only to make the stop deeper. So if planning a slow GF ascent you will still get stops planned, even if they are gone when you get to them. In the case above of being at depth x with a ceiling of 1mm by the tissue saturation you will still get a stop at 3m even if the ceiling would be gone by the time you got there.

Code:
C===============================================================================
C CALCULATIONS
C===============================================================================
 DO I = 1,16
 Gas_Loading = Helium_Pressure(I) + Nitrogen_Pressure(I)
 Coefficient_A = (Helium_Pressure(I)*Coefficient_AHE(I) +
Gradient Factor Decompression Program in Fortran
18
 * Nitrogen_Pressure(I)*Coefficient_AN2(I))/
 * Gas_Loading
 Coefficient_B = (Helium_Pressure(I)*Coefficient_BHE(I) +
 * Nitrogen_Pressure(I)*Coefficient_BN2(I))/
 * Gas_Loading
 Tolerated_Ambient_Pressure = (Gas_Loading - Coefficient_A*
 * Gradient_Factor)/(Gradient_Factor/Coefficient_B -
 * Gradient_Factor + 1.0)
C===============================================================================
C The tolerated ambient pressure cannot be less than zero absolute, i.e.,
C the vacuum of outer space!
C===============================================================================
 IF (Tolerated_Ambient_Pressure .LT. 0.0) THEN
 Tolerated_Ambient_Pressure = 0.0
 END IF
C===============================================================================
C The Deco Ceiling Depth is computed in a loop after all of the individual
C compartment deco ceilings have been calculated. It is important that the
C Deco Ceiling Depth (max deco ceiling across all compartments) only be
C extracted from the compartment values and not be compared against some
C initialization value. For example, if MAX(Deco_Ceiling_Depth . .) was
C compared against zero, this could cause a program lockup because sometimes
C the Deco Ceiling Depth needs to be negative (but not less than absolute
C zero) in order to decompress to the last stop at zero depth.
C===============================================================================
 Compartment_Deco_Ceiling(I) =
 * Tolerated_Ambient_Pressure - Barometric_Pressure
 END DO
 Deco_Ceiling_Depth = Compartment_Deco_Ceiling(1)
 DO I = 2,16
 Deco_Ceiling_Depth =
 * MAX(Deco_Ceiling_Depth, Compartment_Deco_Ceiling(I))
 END DO
C===============================================================================
C END OF SUBROUTINE








Without getting too tedious (or maybe it's too late)... First, Baker's code does not do real-time calculations. It is for calculating deco stops based on a well-defined complete dive plan that you feed into it.

Second, here are some excerpts from Baker's code. I am only quoting the comments, so as to avoid expecting anyone to actually read the Fortran code.

Code:
C===============================================================================
C PERFORM A SEPARATE "PROJECTED ASCENT" OUTSIDE OF THE MAIN PROGRAM TO MAKE
C SURE THAT AN INCREASE IN GAS LOADINGS DURING ASCENT TO THE FIRST STOP WILL
C NOT CAUSE A VIOLATION OF THE DECO CEILING. IF SO, ADJUST THE FIRST STOP
C DEEPER BASED ON STEP SIZE UNTIL A SAFE ASCENT CAN BE MADE.
C Note: this situation is a possibility when ascending from extremely deep
C dives or due to an unusual gas mix selection.
C CHECK AGAIN TO MAKE SURE THAT ADJUSTED FIRST STOP WILL NOT BE BELOW THE
C DECO ZONE.
C===============================================================================

The Note in the preceding code comments would seem to suggest that this comment is not relevant for sport diving. But, keep in mind that this code is not used for real-time calculations and it assumes the diver always makes their ascent (or descent) at a perfect, fixed rate. The actual rate used is read in along with the dive profile that is being used as the basis for calculating the ascent profile. The code does allow for different ascent rates to be used at different depths.

Code:
C===============================================================================
C Note: The Schreiner equation is applied when calculating the uptake or
C elimination of compartment gases during linear ascents or descents at a
C constant rate. For ascents, a negative number for rate must be used.
C==============================================================================

Code:
C===============================================================================
C SUBROUTINE GAS_LOADINGS_ASCENT_DESCENT
C Purpose: This subprogram applies the Schreiner equation to update the
C gas loadings (partial pressures of helium and nitrogen) in the half-time
C compartments due to a linear ascent or descent segment at a constant rate.
C===============================================================================

Code:
C===============================================================================
C SUBROUTINE PROJECTED_ASCENT
C Purpose: This subprogram performs a simulated ascent outside of the main
C program to ensure that a deco ceiling will not be violated due to unusual
C gas loading during ascent (on-gassing). If the deco ceiling is violated,
C the stop depth will be adjusted deeper by the step size until a safe
C ascent can be made.
C==============================================================================

Hopefully, this serves to illustrate that, even though Baker's code is not what you would use for realtime on-the-fly deco calculations, even Baker recognized (in his code) that ceilings should be calculated to include the fact that ascents take some time and are, thus, based on a specific ascent rate.

And that an ascent could result in a ceiling being revised downward (i.e. deeper) - even a ceiling that was 0.

Thus, having a deco stop of 0 feet - i.e. an NDL dive - could result in a mandatory stop being imposed because of a too-fast (or too-slow) ascent. Too slow results in more on-gassing than the algorithm was expecting. Too fast results in getting to a shallow depth sooner than the algorithm was expecting. Either one could mean that you have to do a mandatory stop before you get to the surface - even though your computer's NDL was not at zero when you began your ascent. I.e. you could have a ceiling that you never get to, and so your computer doesn't show you as being in deco - unless you ascend too fast and get to the depth of the ceiling before the computer was expecting you to.
 
Stuart, I think you (and DevonDiver) are claiming that you can be at a depth x and have a ceiling of 1mm (meaning you could not reach the surface with the exact tissue loadings you have now, at depth x - ie assuming teleportation to the surface) and not be in deco because if you ascended at 10m/s then by the time you got to the surface you'd have reduced your tissue loadings below the limit and have a ceiling of zero. Further you are saying that computers will call that you are still within the NDL in that case and only set NDL to zero when an ascent at 10m/s would still exceed the maximum saturation on arrival at the surface.

That is correct.

And I did not see anything in the rest of your post that refutes that.
 
https://www.shearwater.com/products/perdix-ai/

Back
Top Bottom