Riding deco ceiling on ascent

[kaylee_ann]

Well, Suunto is dumb. My only theory is maybe they're putting your stops to where there's just enough difference between the ambient pressure and inert gas pressure in your tissues to begin off-gassing. So if you were below their ceiling, you won't off gas. Instead you'll on-gas and accumulate more deco time.
I don't know, sounds really stupid, but it's Suunto. don't quote me.

 

[inquis]

From past experience a Suunto DC will “punish” you for being deeper than the ceiling on a deco stop.

Makes sense if you don't play ball their way. I suppose the TTS displayed would also be, um, optimistic. Thanks for sharing!

 

[Wibble]

Well, Suunto is dumb. My only theory is maybe they're putting your stops to where there's just enough difference between the ambient pressure and inert gas pressure in your tissues to begin off-gassing. So if you were below their ceiling, you won't off gas. Instead you'll on-gas and accumulate more deco time.
I don't know, sounds really stupid, but it's Suunto. don't quote me.

Which is why very few technical divers use Suunto — those who do a lot of deco.

When I used to use a Suunto as a backup computer to my Shearwater, even with the Suunto in its most "aggressive" mode it would regularly demand many minutes more decompression than the Shearwater. You then decide to either safely ascend knowing the Shearwater is telling the truth or you have the Suunto brick itself for two days.

 

[kaylee_ann]

Which is why very few technical divers use Suunto — those who do a lot of deco.

When I used to use a Suunto as a backup computer to my Shearwater, even with the Suunto in its most "aggressive" mode it would regularly demand many minutes more decompression than the Shearwater. You then decide to either safely ascend knowing the Shearwater is telling the truth or you have the Suunto brick itself for two days.

Before I’d done more research on gradient factors and the Buhlmann algorithm, i had sworn not to get a Suunto because it was too conservative and all that. It’s cool now knowing exactly why it’s not good for decompression dives.
And, yeah, you’d basically need two Suuntos because having a Buhlmann computer will have you run two totally different algorithms

 

[Wibble]

Before I’d done more research on gradient factors and the Buhlmann algorithm, i had sworn not to get a Suunto because it was too conservative and all that. It’s cool now knowing exactly why it’s not good for decompression dives.
And, yeah, you’d basically need two Suuntos because having a Buhlmann computer will have you run two totally different algorithms

Apparently the Suunto eon core can be modified to use Bulhmann. Don’t know if this is extra cost.

However, for a similar price you can get a proper Shearwater dive computer which works exceedingly well and is highly regarded by technical divers and Shearwater computers are used on most rebreathers. Suunto are rarely seen in that community.

Suunto computers are fine for NDL diving or the occasional foray into some light deco. Even this niche is being encroached upon with the Periguine et al.

The requirements for a technical dive computer are simplicity, reliability and technical functionality. A simple and clear user interface is essential — you shouldn’t need to remember obscure button presses to operate the computer. A fragile computer that bricks itself for two days as a punishment is out of the question. Similarly non-rechargeable common sized batteries work extremely well (they don’t go flat overnight).

But most importantly there’s the technical functionality of common Bulhmann algorithms plus all the additional information to help the diver understand or alter their decompression obligations, such as surface GF, ceiling, TTS and changing settings whilst underwater — you’ve set the wrong gas, changing GF-high to get out quicker in an urgency situation. These are the reasons Shearwater dominate technical diving computers.

 

[EFX]

Well, Suunto is dumb. My only theory is maybe they're putting your stops to where there's just enough difference between the ambient pressure and inert gas pressure in your tissues to begin off-gassing. So if you were below their ceiling, you won't off gas. Instead you'll on-gas and accumulate more deco time.
I don't know, sounds really stupid, but it's Suunto. don't quote me.

I don't have access to Suunto's dive computer code. Most, if not all, the commercial personal dive computer algorithms are proprietary and are not available for scrutiny. I can tell you how Erik Baker's (Fortran) and my Excel spreadsheet (VBA) work as these programs are in the public domain. Both programs use Buhlmann's 16 tissue compartments (TC), m-values, half-times, as well as gradient factors (GF's) created by Erik Baker, P.E. GF's are used to modify the TC limits throughout the ascent part of the dive. Gradient factors are expressed as GFLo and GFHi. GFLo is used to calculate the depth of the first stop. GFHi is used to modify the TC limit upon surfacing.

The computer is constantly running the program in a never ending loop usually based on a fixed time interval. It's only inputs are ambient pressure, button presses, and ambient light level if your computer is designed to adjust the screen brightness automatically (if enabled). If a diver is deep enough and stays long enough to require staged decompression stops on the way to the surface the computer will generate a first stop based on the GFLo setting. The GF increases incrementally from GFLo to GFHi as the diver ascends to each stop such that GFHi is used to calculate the final stop at the surface. Typically, most if not all computers will calculate stops in 10 ft (3 m) increments (called the stop interval). The computer is always calculating a ceiling which is a depth the diver cannot ascend shallower until a required stop time is met. The ceiling is then rounded up to the next multiple deeper depth of 10 ft or 3 m. Prior to arriving at each stop the computer calculates a time to remain at that stop. The stop time is incremented in 1 minute (could be less) intervals until the new ceiling becomes equal or shallower than the next shallower multiple of the stop interval. Once the diver has completed the required stop time she may ascend to the next shallower stop. This process continues all the way up to the surface.

As the diver ascends there will always be TC's off gassing. While deco stops are created to limit off gassing to safe levels they do not depend on a certain level of off gassing as you suggest above. A ceiling is calculated for each TC at the current depth and the TC with the deepest ceiling becomes the controlling TC at that point in the dive and its ceiling will be used to generate the next stop depth. Generally, fast TC's clear on ascent while the slowest TC's are on gassing. Usually, for most dive profiles it is the intermediate TC's that control the ascent. Since the computer program is on a loop it is constantly updating TC pressures, and ceilings.

 

[EFX]

One interesting point is that the process I describe above continues on the surface. Once the TC's have saturated at ambient pressure the computer probably stops calculating TC pressures and ceilings until there is a substantial ambient pressure change (ex. ascending to lower pressures or descending in the water on a dive). If memory serves me the Shearwater Perdix uses 1 second or faster updates (the program loop) and switches to 16 seconds after the dive on the surface. This is done to conserve battery power. Data logging on the computer has its own timed loop. The scan (loop) time can be adjusted on the Perdix from 10 seconds, the default slowest time, to 2 (the fastest), or 5 seconds (with the latest software rev.).

 

[kaylee_ann]

I don't have access to Suunto's dive computer code. Most, if not all, the commercial personal dive computer algorithms are proprietary and are not available for scrutiny. I can tell you how Erik Baker's (Fortran) and my Excel spreadsheet (VBA) work as these programs are in the public domain. Both programs use Buhlmann's 16 tissue compartments (TC), m-values, half-times, as well as gradient factors (GF's) created by Erik Baker, P.E. GF's are used to modify the TC limits throughout the ascent part of the dive. Gradient factors are expressed as GFLo and GFHi. GFLo is used to calculate the depth of the first stop. GFHi is used to modify the TC limit upon surfacing.

The computer is constantly running the program in a never ending loop usually based on a fixed time interval. It's only inputs are ambient pressure, button presses, and ambient light level if your computer is designed to adjust the screen brightness automatically (if enabled). If a diver is deep enough and stays long enough to require staged decompression stops on the way to the surface the computer will generate a first stop based on the GFLo setting. The GF increases incrementally from GFLo to GFHi as the diver ascends to each stop such that GFHi is used to calculate the final stop at the surface. Typically, most if not all computers will calculate stops in 10 ft (3 m) increments (called the stop interval). The computer is always calculating a ceiling which is a depth the diver cannot ascend shallower until a required stop time is met. The ceiling is then rounded up to the next multiple deeper depth of 10 ft or 3 m. Prior to arriving at each stop the computer calculates a time to remain at that stop. The stop time is incremented in 1 minute (could be less) intervals until the new ceiling becomes equal or shallower than the next shallower multiple of the stop interval. Once the diver has completed the required stop time she may ascend to the next shallower stop. This process continues all the way up to the surface.

As the diver ascends there will always be TC's off gassing. While deco stops are created to limit off gassing to safe levels they do not depend on a certain level of off gassing as you suggest above. A ceiling is calculated for each TC at the current depth and the TC with the deepest ceiling becomes the controlling TC at that point in the dive and its ceiling will be used to generate the next stop depth. Generally, fast TC's clear on ascent while the slowest TC's are on gassing. Usually, for most dive profiles it is the intermediate TC's that control the ascent. Since the computer program is on a loop it is constantly updating TC pressures, and ceilings.

I thought if you had a deco stop that was too deep, your ‘slow’ tissues would continue to on-gas while your ‘fast’ ones would off-gas, which increases your decompression obligation (?) how does it work?

 

[EFX]

I thought if you had a deco stop that was too deep, your ‘slow’ tissues would continue to on-gas while your ‘fast’ ones would off-gas, which increases your decompression obligation (?) how does it work?

While some slow TC's (tissue compartments) on gas, especially during deep deco stops , they may not on gas to the point of being the CTC (controlling tissue compartment). The amount of on gassing is partly controlled by the TC's half-time. Since TC's 14,15, and 16 have such long ht's they usually don't ongas enough to become CTC's. However, it depends on the profile. Only the CTC will be used to calculate the ceiling and therefore be the determing factor for deco obligations. As the ascent progresses the CTC will gradually shift from faster TC's to slower ones. From TC1 to TC16 here are the half-times in minutes for Buhlmann ZHL-16C:

5, 8, 12.5, 18.5, 27, 38.3, 54.3, 77, 109, 146, 187, 239, 305, 390, 498, 635.

//==============================================================================//
From the help page of the dive Excel spreadsheet:

The rate that each TC can on or off-gas is controlled in part by its half-time. Half-times are the amount of time in minutes where half of the existing inert gas in the tissues will be added to (if on-gassing), or removed from (if off-gassing) if the amount of supplied gas is held constant. For example, TC2 has a HT of 8 minutes. If at time zero the TC contains the equivalent inert gas equal to 80 fsw then after 8 minutes of off-gassing it will contain 40 fsw, another 8 minutes later 20 fsw, and another HT 10 fsw and so on. It takes about six HT's for a TC to on or off-gas to 98% of its final value. In this example it would take roughly 6 x 8 or 48 minutes for TC2 to off-gas from 80 fsw to 1.25 fsw. The TC's are ordered from the lowest (1) to the highest (16). The lower the HT the faster the TC will on-gas and off-gas. One consequence of this is that some slower TC's may still be on-gassing after ascending to a shallower depth (the fastest TC's have off-gassed). Their HT's are so long that the tissues have not been exposed long enough at depth to pick up enough inert gas to exceed the inspired pressure at the shallower depth to allow off-gassing.

//=============================================================================//

My spreadsheet will visually show you the progression in CTC's. You can get my spreadsheet using this link.

 

[inquis]

I thought if you had a deco stop that was too deep, your ‘slow’ tissues would continue to on-gas while your ‘fast’ ones would off-gas, which increases your decompression obligation (?) how does it work?

All (non-saturation) deco dives work that way. It's just a question of where the line is between "fast" and "slow" -- more correctly, between "off-gas" and "on-gas". The "deep stop" debate you seem to be referencing is simply about where to position that line. Bubble-models typically place that line on the faster side of the spectrum (thus more tissue compartments continue to on-gas).

ETA: @kaylee_ann - have you had a chance to grab a copy of Powell's Deco For Divers yet? Highly recommended given your aspirations and the threads in which you seem to participate.