Originally covered Ascents from 30m

I can't answer nor figure out why PADI would recommend a stop at 25m from a recreational dive to 30M max. It really doesn't accomplish much in the way of offgassing, as a matter of fact, I believe that it would only add to the bottom time (IOW, basically just increase inert gas uptake overall vs. ascending at 10mpm to the recommended deep stop depth). I am also a PADI Instructor and have not run across this teaching recommendation (although I admit that perhaps it has come down from PADI HQ and I haven't seen it).

As far as the 5m for 3 minutes, PADI put that into thier curricullum when the DSAT Doppler study years ago showed a tremendous decrease in symptomatic and asymptomatic bubbling when the stop was done and using the standard 60fpm (18mpm)-at the time- recommended ascent rate. They now recommend the 30fpm (10mpm)

I do understand the rationale for the stop at 15M, as that would be indicated as a "recreational deep stop", but was unaware that PADI is recommending this to be done. I know that NAUI has used the RGBM and Bruce Wienke's recommendation of 50% of max depth of any dive past 12m (40fsw) for the recreational deep stop and then :03 at 5m and a 10mpm ascent rate.

GUE's recreational recommendation (per the Triox class) is using a rule of thumb that basically says for dives between 0-24m, deep stop will be 15m off the bottom, for dives between 27-36m, deep stop will be 12m off bottom.

We at GUE then take that :03 at 5m safety stop and stretch it out so that the stops are divided equally at :01 at 10m, 6m, and 3m. This gives us a smoother ascent profile vs. the one stop at 5m for :03, this decreases rapid reduction in ambient pressure and gives cleaner deco vs. ascending to one stop at a shallower depth and waiting the whole time there. This also begins to shape our thinking regarding deco as we progress to more aggressive profiles. IOW, we start to accustom ourselves to this "smoothing" of the deco curve as we ascend and lengthen the shallow stops as we ascend.

The reason GUE (and other agencies) do this comes from decompression research that shows a tremendous reduction in bubbling if we address decompression from a dual phase approach. This dual phase approach says that inert gas exists in two different forms in the body. One phase is called the "free phase" or bubble phase, and one form is the dissolved gas phase. In a nutshell, the bubble phase of offgassing is addressed at the deep stops and the dissolved phase is addressed at the shallow stops. The two different phases use different driving gradients to eliminate inert gas. The Bubble phase gradient is increased w/increased depth (hence stopping deeper) and the dissolved gas driving gradient is increased w/reduced depth (thus the shallow stops).

Deep stops limit sudden reduction in ambient pressure and thus limit the size & formation of more bubbles by keeping ambient pressure high.

This is a very simple explaination of a complex process that is still not fully understood even by the experts. I would recommend doing a bit of research on the subject on the Net...any search for Brian Hill, Yount, WKPP deepstops, Richard Pyle deepstops, Bruce Wienke, RGBM (Reduced Gradient Bubble Model), VPM (Varying Permeability Model), etc. will yield a ton of info.
I hope this helped a bit.

Best Regards,
brandon