Some common practices, myths and mistakes on decompression.

Presented below is a collection of concepts, common practices and deco model behaviors that are often overlooked or miss understood. The topics shown were selected based on comments in current discussion groups. 
Please note that these topics apply to mixed gas decompression ascents. NDL dives have other considerations not shown.

First stop = 80% ATA of depth ?

Some tech divers like to use simple rule based systems to create their deco schedules. These are often called Ratio Deco, or Deco on the fly. They are comprised of a simple formula that can be used in-water but also require other specific settings such as mix and depths. One of these rules is used to calculate the first stop depth as: "First stop = 80 % of ATA depth".

80atadepth (2K) Does this look right? The first stop distance gets smaller on shallow dives?

The 100 ft dive has the smallest gas load and requires the least amount of decompression. However, the x% first stop rule causes this 100ft dive to commence decompression stops almost at the bottom. The distance permitted from bottom to first stop is not sufficient. The same can be said for the 200ft example, and all depths in between. The problem is the deco schedule commences too early in the ascent: 

The 80% value is valid at 300ft only. At other depths, the 80% value goes out of scale, and gives an invalid first stop dimension.

The x% rule was originally designed for use with 270 to 300ft dives at WKPP but needs a correction to remain in context with any other depth. The efforts to extrapolate the rule base from the 300 ft dives to all other depths, have failed to address the change in scale required.

A better solution to the "simple rule" first stop:

2atadepth (3K) There is an easier "simple rule" that approximates this first stop depth quite accurately.

The first stop depth is related directly to the pressure change from the maximum dive depth (distance from bottom). The first stop location is primarily to limit and prevent bubbling quantities and sizes from growing large early on in the ascent.

For most dive parameters, the fastest tissue cell gas loads begin to stabilize after 10 or 15 minutes. The dimension of the first stop (distance from bottom) will be very similar across many decompression dives.

This approach uses the same first stop distance for all decompression dives. It corresponds nicely with many planning tools and with validated table data in use today.

Note: All these simple "one variable" based rule systems will fail at some point. Decompression stop placement is more complex than a single variable or constant can account for.

Warning: All these methods are approximations, offered for quick check purposes only, and require additional rules (bottom mix controls, depth ranges, deco gases req'd, etc) . They are not to be used as substitutions for actual computed plan dimensions. Simple rules like these cannot account for all dive variables.

A simple comparison of first stop depths:

1ststopcompare (3K) The first stop depth in older, original and dissolved models, is typically 3, 4 or 5 ATA's from the bottom. These models have performed millions of successful dives.

In the 90's, deeper stops became more wide spread. Pyle stops, WKPP, Gradient Factor models, and then Bubble models all produce plans with stops that typically started at around 2 ATA from bottom. All these methods reported better post dive feelings, and the ability to reduce overall decompression times.

The first stop distance for the Ratio Deco 80% method, when used on dives of 200ft or less, will produce the deepest of all first stop depths. This method does not have any benefit over those at 2 ATA.

Slow ascent off the bottom ?

Q. What is the right ascent rate when leaving the bottom?
A. Any rate will work, however....

The diver must use the rate as planned. This area of the ascent is critical in the timing and gas load. Any extra time spent in this deep section, will incur additional gas loading. If a diver was to ascend slower than planned, then the diver will incur a bigger gas load than originally calculated and require additional deco time to match it.

Bubble models will account for a fast ascent and the potential bubbling. They will stop your ascent before bubble quantity / size becomes a problem (this is the primary function of bubble models).

1ststopint (3K) If the ascent rate is A, then the First Stop is encountered at a deep location.

If the ascent rate is just right for the conditions, it will be B. In this case the diver has carried out just enough deco during the ascent. The ascent was rate was perfect match to the conditions. Hence the dive program will display the first required stop higher up the curve.

If the diver is slow off the bottom (slower than planned) as in C, then the diver incurs more on-gassing, and requires a new ascent schedule (2).

Note: the ascent C is perfectly OK, but the dive must be planned using the slow ascent rate.

Limitations of each ascent above are set by:
  A: The fast ascent rate and bubbling limitations.
  B: The modest ascent rate and bubbling limitations.
  C: Increased bottom time leads to increased deco requirements.

Adding deep stops ?

The risk of adding a deep stop onto a bubble model plan, is over staying in the deeper sections of the dive. This will require additional decompression to compensate. A Bubble model will provide all the necessary deep stops.

adddeepstop (3K) When an extra deep stop is added to a bubble model plan, it becomes an actual multi level dive. The extra stop is really an extension to the bottom time and the diver incurs increased gas load. There is no avoiding this situation. A diver will on-gas from the highest ambient pressure - precisely what happens with these extra deeper add on stops.

This type of extra deep stop has no benefit. The diver needs to rise in the ascent sufficiently to create a pressure gradient to commence off gassing (usually 1.5 to 2 ATA off the bottom). Adding deeper stops will either make additional gas load and more required deco, or no gas exchange will occur (fast cells are equalized at ambient pressure) and the stop is just wasted time. 

Please note that these topics apply to mixed gas decompression ascents. NDL dives have other considerations not shown.

Super slow last ascent ?

A common practice is the very slow ascent from last stop up to the surface. It is often said that this super slow ascent greatly improves the divers post dive feelings. This practice is perfectly OK, because...

superslow (2K) The decompression plan shown here is the green line and the super slow ascent by the diver is the brown line. The deco program actually ignores the selected last ascent rate setting. When the program clears the last ascent to surface - it considers all deco requirements as complete.

The super slow ascent has in effect extended the last stop time by a significant amount. The passage from last stop to surface is extra time. i.e. 5 minutes extra deco on O2.

Note: It is perfectly OK to perform this extended time & super slow ascent.

Q. Is it the ascent, or the 5 minutes extra deco that helps a diver to feel better?
A. Both are beneficial and they compliment each other. Using O2 deco for an extra 5 minutes probably contributes the most, just as it does on the surface and post dive.

Q. Why do deco programs ignore the last ascent rate set?
A. They need to prevent cases of inadvertent skipped deco. If the program was to consider and include the slow last ascent, the time to surface shown would be a lot earlier. The diver would then be obliged to carry out the slow ascent on all dives. In real life the diver might do some ascents at the regular ascent rate, and therefore the diver will inadvertently skip over a large amount of required deco.

20ft / 6m last stop ?

The last deco stop at 20ft / 6m. This works well for 100% oxygen deco only.

20laststop (2K) The 100% oxygen gas has the unique ability to deliver a 0% ppInert at any depth.

For all other mixtures, the ppInert reduces as the diver ascends. The reducing inspired ppInert gas is the driving force in the off-gas process.

The 20ft / 6m last stop is a benefit only to divers using 100% oxygen for decompression gas. The use of any other deco mixture requires the continued reduction of inspired ppInert (the diver ascends) to drive the off gas procedure.

Divers using Air, 32%, 50%, etc. should continue the stepped ascent to the surface and include the normal 10/15ft or 3/4.5m last steps.

Extended & Accelerated stops

This shows the Extended stop and Accelerated stop procedures applied to the basic decompression ascent and how they modify the plan. The accelerated stops are also known as "Pushing the Gradient", and "Fast helium".

stopshape (2K) The ascent curve in A shows an ideal ascent for a single gas decompression curve.

The ascent shown in B is the normal decompression of stops and steps in normal use. We have forced decompression into blocks of time and distance that suit our needs.
Ascent C shows a multi gas decompression curve. The diagram shows the junction (s) between deco mix 1 and 2. With each mix, the ascent rate slows in response to the reducing ambient pressure, and the reducing gradient between tissue ppInert and inspired ppInert. The off-gas rate is refreshed with the new mix, and increased ppInert gradient.

extend (2K) Extended Stop: The diver elects to stay longer at the switch point (s) to carry out the Extended stop (e). Off-gassing continues and will reduce the required stop times at adjacent levels. The advantage of continued use of the Extended Stop is limited by the high ambient pressure. The required ascent after this extended stop will steepen initially (3), but will return to the base line as the ascent continues.
The extended stop works in combination with the gas switch and reduced ppInert. It cannot be applied in the deepest sections, due the risk of increased gas loading: see Adding Deep Stops

accel (2K) Accelerated Stop ("Pushing the Gradient"): This procedure permits the diver to violate the base line ascent ceiling (a). During this period, the off-gas gradients have increased and fast inert gas (helium) responds well ("pushed the gradient"). The diver then reaches the mix switch point (s) and applies the Extended stop procedure (e). The brief period of higher off gas rates (a), work in conjunction with the Extended stop (e). At the completion of the Extended stop, the diver is now ahead (3) of the base line curve (2) and will complete the decompression faster.

The additional risk to the diver is the period during (a), in which the Pss is increased briefly. The fast transfer rates of helium can respond quickly to the increased off-gas gradient.

The Accelerated stop procedure shows that existing models and calibrations can indeed calculate the "fast helium concept". In the past, models would not permit the diver to violate the stop ceilings to take advantage of this condition.

Deco on helium mix

Some trimix divers will use a decompression gas made with a trimix or heliox blend. They often report improved results in post dive conditions over traditional EAN based deco gas. The diagrams shows how that improvement is achieved and shows the gas loads during decompression for both the EAN and mix based deco gas.

heliumdeco (9K) Tissue Pressures

Using EAN 50...At the switch onto 50%: No inspired He, so the helium gradient has been maximized, leads to rapid off gas of He for the rest of the dive. The N2 inspired component is virtually unchanged during the switch, and the off gas rate of N2 is controlled by ambient pressure reduction alone. N2 off gas is slow.
Using EAN 50...At the switch onto 100%: The He off gas gradients are maximized during the entire time from the switch to 50%, and this will continue through the 100% O2. The He tissue pressure levels are very low. N2 tissue pressures are high. N2 off gas rates increase during the 100% O2 period, though it will take some time to achieve.

Using 50/50...At the switch onto 50%: The helium gradient has not changed - still inspiring up to 50% He, and the off gas rate of He is controlled by ambient pressure reduction alone. No inspired N2, so the nitrogen gradient has been maximized, leads to increased off gas of dissolved N2 from tissues.
Using 50/50...At the switch onto 100%: The N2 off gas gradients are maximized during the entire time from the switch to 50%, and this will continue through the 100% O2. The N2 tissue pressure levels are at modest levels. He tissue pressures are high. He off gas rates increase during the 100% O2 period. The benefit of "fast helium" can now be utilized to off gas a lot of He in a short time..

pp50compare (10K) Partial Pressure values
These two graphs show the pp values for a sample dive across the various stops. The top graph has 50 EAN, the lower with graph with 50/50.

Using EAN 50
Note how the N2 (red) ppInert pressure remains high through the dive. At the switch onto 50% mix (70ft), the N2 pp has increased to the level experienced several stops deeper (earlier). The effectively brings N2 off gas rates to a halt. N2 off gas later resumes several stops further up the ascent.
The Helium has a high gradient driving its off gas from mid way through the decompression.
This gas choice places the emphasis on getting the Helium out.

Using 50/50
Note how the N2 (red) ppInert pressure drops to 0 at the 50% mix switch point. This give the slower Nitrogen the highest gradient possible to off gas for all of the remaining decompression and ascent.
The Helium gradient is relatively consistent reduction through the ascent and deco mix changes. The faster rate of helium allows the off gas process to keep pace with the ascent, and is driven on ambient pressure reduction alone. At the final stops, when 100% is used, the helium off gas rate recieve a boost from maximized inspired inert gas partial pressures.
This gas choice places the emphasis on getting the Nitrogen out.

The difference is the amount of nitrogen remaining after a dive. Both approaches use 50% O2, but with a different inert gas. Remember that the purpose of deco is to remove bottom mix from our bodies. The rates for off gas is controlled by the inspired mix and pressure gradients between each inspired and dissolved inert gas. Any inert gas we breath during deco will slow this process. i.e. if we breath He in deco, it slows the off gas of He.

Q. What does helium deco achieve?
A. The 50/50 (or 50/25) is used to remove more N2 earlier! It does not remove He faster! In fact a diver retains more He up to the switch onto 100% O2. Only then does the "Fast helium" off gas rates used to complete the deco.

Q. Why do I feel better using a 50/50 (or 50/25) mix for deco?
A. The diver has reduced N2 levels upon surfacing.

Q. I can shorten the required deco times on a 50/50 plan. Why?
A. This is "Pushing the Gradient" See the Accelerated Stops procedure above.

Q. Helium is fast, so breathing helium in deco should speed deco up?
A. The deco process is to remove bottom gas from our tissues. Deco gas is normally not absorbed. Breathing He during the deco inhibits the off gas of He.

Q. The plan gets longer when using a trimix deco. Why?
A. Breathing helium slows the off gas of helium, so more time is required to achieve the same reduction in tissue pressure levels. In a trimix dive, the Helium component is the dominant factor during the bulk of the deco. The diver has set a margin of safety or conservatism value into the program, and the program is carrying this out.

Q. Still not sure?
A. Breathing back gas is the slowest deco (no pp inert gradient to assist). Breathing O2 gives the fastest deco (maximum pp inert off gas gradient).

Note: Some of these diagrams are not to scale. Exaggeration has been applied in the areas of interest.