Deep Dive into Reading

First stop, which one?

Peter Bennett Ph.D, DAN

Before you continue reading, review this article: Gradient Factors in a Post-Deep Stops World, because this post was inspired by it. In addition, I recommend you read: Gradient Factors and multi-model approach to decompression.

What first stop should I use? How should I modify GF to get a better and cleaner decompression?

 by Ross Hemingway

Deep stops are always in discussion, some swear by it. But recently deep stops are starting to fallout in the diving community because of a few studies published. But keep in mind that “no clear answer is yet to be found”.

Much of the bouncing, aka, recreational diving will NOT be affected by it, just follow the model and the computer you chose and, always ascent slowly.

I used the following deco procedure, only 3 methods are mentioned here, because they are related to “deep stop” (first stop is a better description).

The “first stop” should be done after the tissue in control starts to off-gassing, this depth is part of the calculation in bubble models, like VPM.

It is practical to control the model by varying the ascent rate instead of using a fixed rate during decompression, this is called slow ascent during off-gassing and switch rate after the first stop. Remember that many of the initial deep stops will fall between “30 seconds and 1 minutes”, in practical terms, this is a way of controlling bubble growth through ascent rate (AR). Changing your AR will control bubbles as well.

The third method is extending the first gas switch at 1.6 for 2 to 5 minutes. This will eliminate some stops due to the O2 windows effect. For example, NITROX 50 at 21 meters for 3 to 5 minutes.

By using the procedure or methodology described above, you will find that Bühlmann and VPM will have comparable results.

What GF do I use to get less deeper stops? 50/85 with slow ascent rate or VPM-B with change on ascent rate and accelerate deco with O2 windows effect.

To illustrate this, I’m going to follow the same example used in the post “Evolving Thought on Deep Decompression Stops” from PADI’s TecRec blog written by John Adsit, PADI Master Scuba Diver Trainer and Tec Trimix Instructor, which is: 20-minute dive to 60 meters/200 feet using TMx 18/45.

ModelFirst Stop Depth
Bühlmann 18 m
Pyle, RGBM, VPM, Bühlmann w 20/80 GF36-40 m
Ratio Deco approach 1:2 ratio deco profile30 m

What I do first is find the point when the tissue starts off gassing, this is done via Varying-Permeability Model (VPM), in this example, the off-gassing depth start relative at 45 meters, if you add a couple of minutes to the bottom time it will not make any differences in this regard.

If you want to use Pyle stops, the standard depth is 36 meters and 24 meters for 1 minute each, also you may use 30 meters which is calculated based in 45 meters and 18 meters. 30 meters also coincides with Ratio Deco (50% of the depth).

So, the “first stop” should be done after reaching this depth. The question remains if I should use the first stop described by VPM table, the answer is embedded in my deco-gas. The standard gases used in this type of bouncing dive are NITROX 50 and Oxygen.

It’s perfectly possible to force VPM to use a shallow depth, like 27 meters. Using slow ascent rate up to the first switch gas, in this case Nitrox 50 at 21 meters for 2 to 5 minutes, raise PO2, opening the windows effect.

Doing a deep stop at 27 and 24 meters for 2 minutes each with the back-gas/ bottom-gas will give you time to prepare for the gas switch and it is physiological good. All other stops will remain the same, 18,15,12,9,6,3 meters.

In any case, if you use straight ZHL16(b) the 18 meters stop will be replaced by 21 meters with NITROX 50 and a 12,9,6,3 meters stop remains, this fact shows the importance of using the O2 windows effect.

Two crucial factors are in play, first the ascent rate and second accelerate deco. The relationship of these two factors will give you the best possible decompression relief.

Below you can compare both tables:

Depth metersVPM minutes (Vplanner)ZHL16C No GF minutes (Zplanner)ZHL16C GF 47/85 minutes (Freedom)Mix
213 (5)3350
153 (2)50
124 (5)350
97 (6)7550
total 444340
Comparison table
 by Ross Hemingway

Continue to follow your dive computer and use print tables or Ratio Deco as a backup. Learn to follow different ascent rates throughout the dive, identify what’s better for you and always use slow ascent rate velocity. Near the surface use super slow ascent. If you are performing a Non-Stop Dive, always do a safety-stop at 4.5 meters for 3 to 5 minutes and super slow ascent to the surfaces.

If you settle for Pyle stop (between 1 and 2 minutes), it’s better to use straight ZHL16 without GF or set it in 90/90. The algorithm will modify the compression curve by itself.

2 responses to “First stop, which one?”

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