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.

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.

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.

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.

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.

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

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.

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 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.

	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 .
	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.

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..

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.