how long can i stay down on dive 3

[kubefish]

dive 1 65 feet for 25 minutes
one hour rest

dive 2 65 feet for 25 minutes
one hour rest

if i make a third dive at the same depth how long do i
need to wait before dive 3?

 

[Warhammer]

I don't understand your question. You said that you planned to have a 1 hour surface interval after dive 2, then you asked how how long that same surface interval should be? If you were planning on dive 3 being the same as 1 & 2 then, using the PADI RDP, you'd need to wait at least 1hr 11min before dive 3 instead of the 1 hour you indicated. Here's why:

Dive 1; 65 ft for 25 minutes:
Your pressure group would be L after this dive. If you had a surface interval of 1hr, then your pressure group would be C.

Dive 2; 65 ft for 25 minutes:
Your starting pressure group is C, so your residual nitrogen time from dive one would be 12. Add that to your actual bottom time of dive 2(25 minutes) and you get a total bottom time of 37 minutes, putting you in pressure group S. Also note that a 3 minute safety stop would be recommended for this dive.

Dive 3; 65 ft for 25 minutes:
The minimum surface interval for a diver in the pressure group S would be 1 hr 11 minutes, putting you in the pressure group E. As a E diver the max NDL on dive 3 is 25 minutes, which when added to the residual nitrogen time from dive 2(15 minutes), would put you in pressure group T at the end of this dive. Also note that a 3 minute safety stop would be required on this dive.

On the other hand if you weren't planning on making dive 3 for the same time and depth as 1 & 2, then it depends on how long and how deep you want to go.

 

[jenmichigan]

Warhammer,

Excellent well thought out response. This is why I like this board. Everyone takes the time to gives great responses.

Jennifer,
http://www.MichiganShipwrecks.com

 

[kubefish]

thank you for your response. you answered my question. although, i had my question worded wrong. thanks.

 

[Rick Murchison]

An interesting question... and just to show you that nothing is quite as straightforward as it seems, the answers vary from "you can't do dive #2 without a decompression stop" (using the Navy tables and the Navy definition of bottom time and assuming your 25 minutes is viewed as "bottom time") all the way down to about 32 minutes using the algorithm used in many dive computers. As has been previously mentioned, using the PADI RDP and PADI's way of figuring times, you need about an hour and eleven minutes surface interval before the last dive...
Sorta makes you think, doesn't it.
Rick

 

[Warhammer]

Your welcome and as Rick indicated, NDLs vary somewhat depending on the table or computer used. If this is an actual dive plan your pondering; if I were you, I'd take a little extra precaution on dives 2 & 3, espeacially 3. On dive 3, you'd be at the max NDL for that dive, going by the PADI RDP. And on dive 2, your just 3 minutes away from the NDL. I'd either make the last 2 dives a little shallower, increase surface intervals, increase safety stop times, or a combination of some or all of those, depending on the exact conditions. I try to avoid pushing the limits of charts/tables or computers, just for added safety. Even though we all know that you rarely, if ever, stay at your max depth for the entire dive and when diving charts like the PADI RDP you just have to assume that you did just that. Which limits your max bottom time and NDLs somewhat. If you want to get the most out of your dives then consider investing in a dive computer or at least a wheel type table/chart that gives you the advantage of diving multi-level profiles. But even then, you'd be wise to not push the limits.

 

[Walter]

.......I won't use the Bend-a-Friend tables. Even dropping the 3rd dive, this profile is, IMHO, dangerous, yet the Bend-a-Friend Tables allow it. Using the YMCA tables the second dive requires an 18 minute decompression stop at 10 ft. NAUI tables require an 8 minute decompression stop at 15 ft. MDEA tables requires a 10 minute stop at 10 ft. The US Navy tables require an 8 minute stop at 10 ft. It's time to think long and hard about which tables you are using.

Walter

 

[Warhammer]

Walter,

Thanks for the introducing other tables into this senario. I always thought the RPD was more conservative than the Navy tables. I'll have to give them another look.

 

[Warhammer]

Just ran the profile through the Navy tables and all I can say is damn. Like you said, the Navy table would require an 8 minute stop at 10ft for the second dive. It also requires an 18 minute stop at 10ft for the third dive, if you had an 1hr 11min surface interval after dive 2. And to think this very profile will just squeeze into the PADI RDP, without any decompression other than the required 3 minute stop at 15ft. That's a sure enough eye-opener if I ever saw one. Never relized there was that much difference in the tables. I always assumed that since the RPD is more conservative at every depth, on the first dive, than is the Navy tables, that the same would be true for repetitive dives. I'm both amazed and shocked to think that I've crossed the line several times in the past. Thanks.

 

[Warhammer]

Any ideas as to where I might get my hands on the Navy tables in card(wet-proof) form?