"Technical." Dive Computers.
- Thread starter Boston Breakwater
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Yeah thankfully most of the world have done away with them.
As I pointed out in another thread Andy has not dived most of these, indeed he probably has never been physically in the same building as some. This is not like a dive magazine taking them diving to see how it goes. This is almost like blokes in the pub discussing the pros and cons of F1 cars. Regurgitated knowledge based on the promo materials of the manufacturers.
His website is there to build a brand to get him students. He has many articles like this on a variety of subjects. Mostly they are better than this one.
Question: Let us assume that a recreational diver performs a NDL dive. If (s)he spends a minute or two more at depth X then does that really change anything deco-wise? Given all the thresholds, safety margins and not-accounted-for-factors, how much would 60 seconds at 60ft or so change in the real life, not just in theory?
I can understand why deep stops were debated in technical diving circles, though. By the way, Subsurface shows a tissue loading heatmap which is quite interesting to study with respect to deep stops... The medium speed tissues...
Now, for a sawtooth-profile we choose a bubble model, right
It's a great question. If I had an answer, I would tell you.
I published an article on deep stops in technical diving last year, and I endeavored to follow it up with a an article on deep stops in recreational diving. When I was done researching, I didn't know what to say. There is really very little research on it, and none of it is conclusive. (That is why I can't understand DAN Europe's recommendation.)
Yes, all dives involve decompression, but there really is a difference between what we call decompression dives and what we call NDL dives. Once a diver has incurred mandatory deco stops, delays in ascent clearly add required decompression time. In contrast, if a diver is within NDL, delays in ascent are still no-stop dives, as long as the diver does not delay enough to incur mandatory deco. Compare these three divers:
- Diver #1 begins to ascend from 100 feet with 3 minutes of NDL time remaining and maintains a 30 FPM ascent rate to 15 feet, where a 3 minute safety stop is completed before ascending to the surface.
- Diver #2 begins to ascend from 100 feet with 3 minutes of NDL time remaining and maintains a 30 FPM ascent rate to 50 feet, where a 2 minute deep stop is completed, followed by a 30 FPM ascent to 15 feet, where a 3 minute safety stop is completed before ascending to the surface.
- Diver #3 begins to ascend from 100 feet with 3 minutes of NDL time remaining and maintains a 30 FPM ascent rate to 50 feet, where the diver notes the increased NDL on the computer and explores the reef for 20 minutes. The diver then does a 30 FPM ascent to 15 feet, where a 3 minute safety stop is completed before ascending to the surface.
As a follow up, I will say that I believe that on most deeper recreational dives, safety stops within 20 feet of the surface a valuable. I have seen people advocate doing a safety stop at half maximum depth and then shortening the shallow safety stop. I do not support that at all.
Without going back and looking it up to make sure, I believe the original research supporting safety stops was quite clear that shortening them was not advantageous, no matter what you did deeper before you got to the safety stop.
All that you say is true, of course.
Half maximum depth has to do with bubble models and the surface tension of a certain class of bubbles. Gas can only enter these bubbles when the pressure drop is greater than ... something ... (it's 3am here) ... and the pressure gradient of nitrogen can overcome the surface tension of these relevant bubbles. I do not recall the detail. It's in Deco for divers. It is better to off-gas to lungs than to bubbles in the bloodstream.
I also recall that practically no on-gassing happens at 15 feet (or 20 feet?). Hence, all time spent at 15 feet or so is golden.
When I do dives requiring mandatory decompression stops (less than 30 minutes of deco), I will first "complete my deco" at 5...6m (or 15...18 feet).
After that I can surface immediately, which I have done, and feel a bit tired, or ascend to 3m/9ft and stay there a long time, which I have also done, and feel better.
Hence, I can attest that extending a shallow stop (or making a more shallow stop) feels good. Subsurface can show the difference in N2 loading resulting from these strategies...
Do not count me as an authority (I would not) but verify my claims.
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