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David The reason there is a significant difference between a last stop at 6 vs. 3 is that you are doing the deco on air which is a poor choice. You are relying upon the pressure gradient to suck out the inert gas in your system. Switch to Oxygen and you'll find very little difference in the deco. David Shimell Project Manager, Sequent Computer Systems Ltd., Sandton, South Africa. Email: shimell@se*.co* <mailto:shimell@se*.co*> -----Original Message----- From: David Reinhard [SMTP:reinhard@oc*.co*.au*] Sent: Monday, June 07, 1999 9:09 PM To: techdiver@aquanaut.com Subject: Last Stop Depth Hi guys I have just discovered something of which I found of interest and wondered if I could get some constructive comments. I have been using Zplan set on a "last stop depth" of 6m, basically because of greater ease in maintaining the set depth if there is a bit of a swell etc. What I have only just realised is just how much difference there can be in deco times when you use different last stop depths. When you run the same schedule using either 3m or 6m last stop depth the deco times can be dramatically different. In fact even a difference of one metre can make a difference (eg 4m instead of 3m). This seems a bit unrealistic given the fact that it would be relatively easy to drift up and down by a depth of a metre during your stop (maybe this underlines the importance of a trapeze or a least an SMB/bag). Here are some examples for a 50m, 20min dive (incorporating deep stops). SET AT 3 METRES Waypoint at 50m for 20:00 (22) on Air, Deep Stop at 30m for 2:00 (26) on Air, Deep Stop at 21m for 2:00 (29) on Air, Deep Stop at 15m for 2:00 (33) on Air, Norm Stop at 9m for 2:00 (36) on Air, Norm Stop at 6m for 10:00 (46) on Air, Norm Stop at 3m for 27:00 (74) on Air, TOTAL DECO TIME: 45 minutes. DIVE RUN TIME: 73 minutes. SET AT 4 METRES Waypoint at 50m for 20:00 (22) on Air, Deep Stop at 31m for 2:00 (26) on Air, Deep Stop at 22m for 2:00 (29) on Air, Deep Stop at 16m for 2:00 (32) on Air, Norm Stop at 7m for 7:06 (42) on Air, Norm Stop at 4m for 35:00 (78) on Air, TOTAL DECO TIME: 48 minutes. DIVE RUN TIME: 77 minutes. SET AT 5 METRES Waypoint at 50m for 20:00 (22) on Air, Deep Stop at 32m for 2:00 (26) on Air, Deep Stop at 23m for 2:00 (29) on Air, Deep Stop at 20m for 2:00 (32) on Air, Norm Stop at 8m for 6:00 (41) on Air, Norm Stop at 5m for 39:00 (80) on Air, TOTAL DECO TIME: 51 minutes. DIVE RUN TIME: 80 minutes. SET AT 6 METRES Waypoint at 50m for 20:00 (22) on Air, Deep Stop at 30m for 2:00 (26) on Air, Deep Stop at 21m for 2:00 (29) on Air, Deep Stop at 15m for 2:00 (33) on Air, Norm Stop at 9m for 2:00 (36) on Air, Norm Stop at 6m for 53:00 (90) on Air, TOTAL DECO TIME: 61 minutes. DIVE RUN TIME: 90 minutes. As you can see from the above examples there is a large difference between the 3m stop (total deco = 45min) and the 6m stop (total deco = 61min). I simply had not realised how much the tables would penalize me for doing my last stop at 6m rather than 3m. Notice the 10 min difference between the 5m setting and the 6m setting (however there is only a 3 min difference between the 4m and 5m settings- how can that be?). My question is how realistic is this? Can a 1m difference in depth make that much difference to the efficiency of your deco that there would be a 10 min difference between the two? Hypothetically what if your depth gauge was poorly calibrated and was showing that you were at 5m when you were actually at 6m - you could leave the water having done 10 min less deco than the tables actually require. Also, if you are doing your deco in a vertical position there could be a significant difference between the deco requirements for your toes as compared with your head - maybe I should be lying in a hammock for my deco stops?!? (or should I do half my deco head up and the other half upside down to even things up...sorry I'm getting a bit silly now...where are my pills...ah, thats better). As you might expect the differences get larger for longer deco times. eg 40m for 40 min (no deep stops) gives 71 min total deco if you do a 3m stop but 98 min if you do your last stop at 6m (nearly 30 min difference!). Given that the Buhlmann tables on which Zplan is based are somewhat old now how would this fit in with more recent thinking about doing deco deeper (or at least starting you deco deeper as in Pyle stops)? Any comments greatly appreciated, Dave. -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Ocean Internet - "The Quality ISP" http://www.ocean.com.au/info.html -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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