Is it ok to do 2 deeper dives following a shallow dive?

[s.s.seafan]

Thanks for all your responses. I have emailed the dive shop asking an estimate of bottom time for each dive, haven't got reply yet. My air consumption skill is not great. If I remember it correctly (from dives a year ago), for the deeper dives (60'-90') I did 35-40 min, for the shallower dives (40-60') I did 45-50 min. Actually I haven't done any 20' dives (except during training in lake for OW), so I'm not sure what the bottom time would be. The dive shop mentioned the 1pm dive would leave dock at 1:30pm.

---------- Post added December 28th, 2012 at 03:53 PM ----------

I think here is the likely scenario -

for the 10am dive of 20', boat leaves at 10:30am, dive for 60 min
1:30pm dive of 60', for 40 min
SI 45 min
2nd tank dive of 45', for 50 min

I haven't looked at RDP for a while, here is my calculations, please correct me if I'm wrong:

20' dive, round up to 35', for 60 min, makes it group L
SI is 2 hours (11:30am-1:30pm), makes it group B
next dive is 60', RNT=11, ABT=44. dive for 40 min, makes it group Q (do I need to add the 11 min of RNT to the actual bottom time of 40 here?)
SI is 45 min, makes it group H
next dive is 45' (round up to 50), RNT=28, ABT=52. dive for 50 min.

Did I do that right?

Thanks!

 

[sixt3denied]

---------- Post added December 28th, 2012 at 03:53 PM ----------

I think here is the likely scenario -

for the 10am dive of 20', boat leaves at 10:30am, dive for 60 min
1:30pm dive of 60', for 40 min
SI 45 min
2nd tank dive of 45', for 50 min

I haven't looked at RDP for a while, here is my calculations, please correct me if I'm wrong:

20' dive, round up to 35', for 60 min, makes it group L
SI is 2 hours (11:30am-1:30pm), makes it group B
next dive is 60', RNT=11, ABT=44. dive for 40 min, makes it group Q (do I need to add the 11 min of RNT to the actual bottom time of 40 here?)
SI is 45 min, makes it group H
next dive is 45' (round up to 50), RNT=28, ABT=52. dive for 50 min.

Did I do that right?

Thanks!

Well, i'm not getting the same pressure groups as you. I"m getting the following:
1st dive -35' for 60 min makes it group L
SI is 2 hours - makes it group B
2nd dive - 60' for 40min (44min is NDL), makes it group U
Minimum surface interval should be 57 minutes - not 45min. Brings you to group J
3rd dive - 50' for 50min puts you in DECO. NDL is 47minutes.

I'm using Dive Planner Pro... FYI

Your 2nd and 3rd dive seem to be cutting it very close for no deco limits...

But i'd love for someone else to chime in to confirm my numbers...

 

[flots am]

But i'd love for someone else to chime in to confirm my numbers...

That's the point.

The reverse profile dives are probably safe, but all the planning tools I'm aware of have a built-in bias against them, so there's really no way to model them unless you're willing to accept reduced no-deco time, or plan them as deco dives when they probably aren't.

flots

 

[s.s.seafan]

Just got update email from the dive shop, their schedule is the following:

boat leaves dock at 10:30am, 1st dive at 20' for 50 min, return to dock at 11:30am
SI 2 hours, 2nd dive leaves dock at 1:30pm, dive 60' for 40 min
SI 40 min, 3rd dive at 40' for 40 min

so, now the calculation i have is -

1st dive 50 min, makes it group J
SI 2hrs, makes it group A (RNT=6, ABT=49 at 60')
2nd dive at 60' for 40 min, 40+6=46, makes it group S
SI 40 min, makes it group J (RNT=44, ABT=96 at 40')
3rd dive at 40' for 40 min, 40+46=86, makes it group T

now I can't remember what that ABT is used for?

also, a separate question, if a calculation shows the dive time exceeds NDL, does a 3 min safety stop at 15' remove the DCS risk completely (for dives less than 60' or 80' deep)?

 

[ScubaSteve]

NDL = No Decompression Limit

If you exceed that, I guess you missed something in the planning stage.

 

[KD8NPB]

Make sure to budget an air reserve and a deep stop / safety stop.

 

[scubadiver888]

If you aren't trained for deep diving there could be things you won't know to look for. There are no scuba police stopping from diving to 100 feet but diving outside of your training increases the risk. Diving isn't a binary activity. There is no, SAFE/NOT SAFE. The more you dive, the deeper you dive, the more you are increasing risk.

If your training is just getting you by at 60 feet and you dive to 100 feet you might find yourself unable to save yourself. It is safer to skip the 100 foot dive. Some people don't need to be trained to dive around a wreck. Some wrecks are safer than others. Some people need to know what to watch for when swimming around a wreck. I didn't need training to swim around a wreck. You might find you take wreck diving training and realize it is all just common sense. On the other hand, you take the training and think, "I never thought about XXX."

Most the stats and tables are based on doing the deeper dive first. If just don't know as much about doing the shallow dive first. Could be fine. Could be more dangerous. Add to that, if you do it once it could be fine. If you do it 1000 days in a row it could be bad. Without stats, don't know what happens if you do it 500 days in a row. Probably not as dangerous as 1000 days in a row. Could be that it is fine even for 10,000 days in a row. We just don't know.

Add to all of this that what is fine for me might be bad for you. Original tables were created based on Navy divers. I am DEFINITELY not in the same shape as a Navy diver. This could mean the Navy tables are fine for Navy divers but not for me.

My personal experience has been to occasionally do reverse profiles. Diving for 11 years and I'm not dead or even bent.I dive typically twice a day. Some times I'll dive 3 or 4 times. Never 5 times. I always stuck to the tables and use a very conservative dive computer now. Others dive more than me and bent the rules a lot. I don't know anyone who has been bent.

As you learn more and make more friends, you'll be able to judge what you feel 'safe' doing. Ultimately, we can all tell you 'facts' but in the end it is your life.

 

[Mr Carcharodon]

also, a separate question, if a calculation shows the dive time exceeds NDL, does a 3 min safety stop at 15' remove the DCS risk completely (for dives less than 60' or 80' deep)?

The short answer is no. More importantly a safety stop is there to reduce what is already a low risk level. Sure at it's most basic level it is decompression but it is not a plan since it does not consider your depth, times, profile or gas mix.


If dive tables are all you have as a planning tool that is what you should use. But you will find the assumptions used lead to short bottom times compared to dive computers. Using a computer in planning mode is one place to start. There are other computer or phone based tools like v- planner that are worth a look.

 

[dumpsterDiver]

Just got update email from the dive shop, their schedule is the following:

boat leaves dock at 10:30am, 1st dive at 20' for 50 min, return to dock at 11:30am
SI 2 hours, 2nd dive leaves dock at 1:30pm, dive 60' for 40 min
SI 40 min, 3rd dive at 40' for 40 min

so, now the calculation i have is -

1st dive 50 min, makes it group J
SI 2hrs, makes it group A (RNT=6, ABT=49 at 60')
2nd dive at 60' for 40 min, 40+6=46, makes it group S
SI 40 min, makes it group J (RNT=44, ABT=96 at 40')
3rd dive at 40' for 40 min, 40+46=86, makes it group T

now I can't remember what that ABT is used for?

also, a separate question, if a calculation shows the dive time exceeds NDL, does a 3 min safety stop at 15' remove the DCS risk completely (for dives less than 60' or 80' deep)?

why didn't you say so... you will be totally safe. For all dives done from a boat in saltwater.... once you return to shore and remain on land for 15 minutes (or longer) all residual nitrogen instantaneously leaves your body and you are clear to dive again. It is generally thought necessary to stay in the shade during this period and definitely avoid going inside an air conditioned building. Should you do that, the reduced relative humidity will negate the "island effect" of accelerated dissolved nitrogen elimination.

There are alternative means as well, drinking a single Corona beer also has the same effect (just ask any Cozumel divemaster)...

 

[ianr33]

so, now the calculation i have is -

1st dive 50 min, makes it group J
SI 2hrs, makes it group A (RNT=6, ABT=49 at 60')
2nd dive at 60' for 40 min, 40+6=46, makes it group S
SI 40 min, makes it group J (RNT=44, ABT=96 at 40')
3rd dive at 40' for 40 min, 40+46=86, makes it group T

now I can't remember what that ABT is used for?

also, a separate question, if a calculation shows the dive time exceeds NDL, does a 3 min safety stop at 15' remove the DCS risk completely (for dives less than 60' or 80' deep)?

Buy a cheap computer. Go dive. Have fun. If the computer gets anywhere near deco, ascend.