Quote: (Originally Posted by jradomski)
3 hours isn't really necessary (its just the ideal time), latter dives are just longer by a bit (not really a bad thing), 1.5 hours is really the minimum SI I would use, but I always shoot for at least 2hrs.. (this is true for custom tables and real time calcs, for the generic published RGBM table stay within the rules as these are set up for ranges)
I have never had any issues with the explorer on repetitive dives with the second being the same depth or deeper (again the profiles are a just bit longer)
The 3-hr sit time goes back to the DCAP tables that Hamilton created for the Key West Consortium tables that were ranged. It's basis was two-fold. First because oxygen is belived to have a 90 minute half time a 3 hour sit time will reset the CNS clock nicely. Furthermore if the final deco gas is oxygen or something with a PO2 of 1.4 or greater a repet dive can be done with the exact same deco times as the first dive but by shortening the bottom time by 5 minutes.
This seemed to work very well as a "rule of thumb". For more than 10 years I never did a repet on gas with less than 3 hours of sit time until 2004 when we were on the USS Monitor flying the new Cochran EMC-20s and I took the risk on a 90 min sit time. The pucker factor was high but the deco was clean and efficient. Even so today I prefer a 2.5 hour sit time unless I have to make the dive for operational issues.
For a 200 fsw dive for 30 min a 90 min sit time on a set point of 1.3 with 17/50 diluent should yield a decompression for a repet of the same profile about 30 minutes longer. Whereas a 3 hour SI will only increase the deco on the repet by about 15 minutes so the longer SI will cut the deco time. The only question is do you want to sit on the boat or sit on the deco line ... Time is Time ......
cheers