Reverse dive profile advice

[epolice]

I plan on doing two shore dives in Bonaire on regular air. No Nitrox training for me yet, unfortunately.

First dive should be max depth of 45 feet for around 45 minutes. I am hoping to follow that up with a 40 minute dive to 75 feet. For both dives, I can't imagine I am at max depth for more than half of the time mentioned.

I planned on staying out of the water for 75 minutes between dives. Is this long enough and/or should I plan for a safety stop of longer than 3 minutes on my second dive?

 

[Eksilbergenseren]

Taking a longer safety stop is OK, but planning so that you need a longer stop is deco-planning. That is a game changer, and opens up a lot of new things to consider. When you can no longer go straight to surface you need detailed contingency planning.

 

[Eksilbergenseren]

I would not be very worried about going to 75 on the second dive, but you need to calculate your avilable time to NDL

 

[Jonn]

You’re asking us?! Just looking at it it seems like a reasonable day of diving, but I certainly wouldn’t trust that with my health. What does your computer (or old school tables) have to say? You do have one and know how to use it... right?

The safety stop is a safety stop, not a deco ceiling.

 

[oly5050user]

I plan on doing two shore dives in Bonaire on regular air. No Nitrox training for me yet, unfortunately.

First dive should be max depth of 45 feet for around 45 minutes. I am hoping to follow that up with a 40 minute dive to 75 feet. For both dives, I can't imagine I am at max depth for more than half of the time mentioned.

I planned on staying out of the water for 75 minutes between dives. Is this long enough and/or should I plan for a safety stop of longer than 3 minutes on my second dive?

Buy a computer. A watch and depth gauge can cost more than a computer

 

[tursiops]

OP, just look at tables and/or your Planner on your dive computer.
(There is no fundamental issue with reverse profiles, so long as you manage your nitrogen.)
Your tables will say you cannot do the second dive, even if it is your ONLY dive....too deep and/or too long....the square profile makes it not possible. Your computer planner will agree.
On air, your only safe option for that second dive is to come up when the NDL on your computer says you have only a few minutes left. You will NOT get a 45 min dive if you spend any time at 75 ft. Sorry.

 

[BoltSnap]

First dive should be max depth of 45 feet for around 45 minutes. I am hoping to follow that up with a 40 minute dive to 75 feet. For both dives, I can't imagine I am at max depth for more than half of the time mentioned.

I planned on staying out of the water for 75 minutes between dives. Is this long enough and/or should I plan for a safety stop of longer than 3 minutes on my second dive?

Can't you figure it out on your own using the dive tables from your certification agency? What is the purpose of your asking here?

I am very concerned that you are unable to do it yourself and for you to ask if you should "plan for a safety stop of longer than 3 minutes" on your second dive. This question and the way it was phrased is very concerning indeed and shows a fundamental issue with your understanding and knowledge of dive tables, dive planning and decompression theory even at the entry level -certified- diver especially when considering that it is very easy and uncomplicated to figure out the answers using simple flat dive tables that should have been part of every basic diver certification course.

BTW, if you are going to be using dive tables for planning your dives' NDL's, you should start with the deeper dive first for it would give you better allowed dive times for the deeper dive (but not the dive times you are specifying) without overstaying your NDL's. Dive Tables do not give you credit, any credit, for not staying the entire dive time at the max depth of your dive, they are a "square profile" tools and don't work and can't be used assuming not spending the entire time at the max depth of the dive. If you are stating that you won't be using dive tables and going to be using a dive computer, then it will be impossible to say for certain if these dives are doable for few reasons but primarily since it is difficult to predict what NDL's the dive computer you will be using will give with the profile you are describing. The dive planning feature of dive computers will give you less NDL's than what paper/plastic tables would give you for "square profile." For the "multi-level" computations of a dive computer, you won't know what you can do and can't do until you are underwater making the multi-level dive and following your dive computer's NDL's (riding your computer's NDL's).

When using NAUI's dive tables, it is very clear that you can NOT do the 75' dive for 40 minutes even if it were the first dive without any RNT let alone doing it as a second dive, repetitive dive, with RNT from a prior dive. You will have almost no dive time allowed at 75' if it were the second dive after a 75 minute SIT and it should NOT be made at all (all assuming using dive tables) for all practical reasons.

I suggest that you review deco theory, dive table and dive computer use with your local instructor if you are actually a certified diver and if you are not a certified diver, then you should sign up for an accredited diver training course before proceeding to do any diving.

 

[AfterDark]

Staying longer at a SS has no downside that I'm aware of, I've been doing it way before the term SS was coined. Sometimes it can be the most relaxing part of the dive.

That being said if you don't know, the answer to your dive planning questions you should see your instructor and ask him/her why you can't determine this on your own.

 

[Steve_C]

You have not given us your profiles. Just a start, an end, and a depth somewhere in between. Unless you are starting at the top of a wall, these will be far from square profiles.

 

[tursiops]

You have not given us your profiles. Just a start, an end, and a depth somewhere in between. Unless you are starting at the top of a wall, these will be far from square profiles.

But the point is his only planning tools appear to be tables that he apparently doesn't have or perhaps even know how to use....so the only safe assumption for him is a square profile, so the only safe answer for him is to shorten the second dive. If he uses the PADI RDP, the first dive (50 ft) puts him in PG O, which after a 75 min SI is PG E. That's 13 mins of RNT at 75 (80) ft, so his remaining BT is just 13 mins. He could do a 13 min dive, but not a 45 min dive.
If he has a computer, the game changes....but only a little. Even if his first dive (being multi-level plus a 75 min SI) gives him zero residual nitrogen, he still can't go to 75 ft for 45 mins on the second dive. His second dive must also be multilevel, and most likely also be considerably less than 45 mins.

Added: If the eRDPML is used, instead of the RDP, you can work with 5 ft intervals instead of 10 ft. On that basis, the first dive ends in PG M, reduces to C after 75 mins SI, and allows 24 mins BT on the second dive to 75 ft.

If the dives are treated as multi-level, using the eRDPML, and the first dive is 45 mins to 45 ft followed by a 75 min SI, the second dive is possible as a multi-level dive, for example: 15 mins at 75 ft followed by 30 mins at 45 ft, plus a normal safety stop.

This kind of dive planning is not possible with the usual tables, nor with most computers, but the eRDPML makes it easy.

Advice to the OP: do NOT use my dive planning for YOUR dive. do your own dive planning.

ADDED LATER:
The OP should NOT do these dives, even with the multi-level aspects outlined above. With the usual AL80 tank, he MAY not have enough gas to do the first dive, and he WILL NOT have enough gas to do the second dive, not with a single AL80. He asked questions about his nitrogen management. His bigger concern is his air supply.