Pressure group question

[**pYgmY**]

Hi all,

I have a theory question about calculating the post-dive pressure group for a specific dive profile: max. depth = 28.5m, bottom time = 45min, descending to max. depth at the beginning of the dive and then slowly but steadily ascending to the surface (along a shipwreck), including a 3min safety stop at 5m.

As far as I understand it, the times and pressure groups shown on PADI dive tables always assume a square dive profile. This dive would be multi-leveled, but really is more like a slow ascent from max. depth.. How can I calculate the correct post-dive pressure group for a profile like this? And if there is a way, could someone with more experience help me out by providing the correct result (so I know where to continue in the PADI tables for a repetitive dive calculation)?

Thanks!

 

[tursiops]

How can I calculate the correct post-dive pressure group for a profile like this?

You cannot.
This is why people use dive computers.

 

[arew+4]

Hi all,

I have a theory question about calculating the post-dive pressure group for a specific dive profile: max. depth = 28.5m, bottom time = 45min, descending to max. depth at the beginning of the dive and then slowly but steadily ascending to the surface (along a shipwreck), including a 3min safety stop at 5m.

As far as I understand it, the times and pressure groups shown on PADI dive tables always assume a square dive profile. This dive would be multi-leveled, but really is more like a slow ascent from max. depth.. How can I calculate the correct post-dive pressure group for a profile like this? And if there is a way, could someone with more experience help me out by providing the correct result (so I know where to continue in the PADI tables for a repetitive dive calculation)?

Thanks!

Buy a dive computer. Find a PADI Wheel for multi level dives.

 

[tursiops]

There is a manual way for multi-level. See @arew+4 's post #3:

The PADI Wheel​

​

All dive profile methodologies make assumptions. Stay within the assumptions. For the tables: treat non-square dives as square at max depth.

The Wheel and the eRDPml are not really applicable. He described a approximately linear ascent from the bottom to the SS over the course of the dive. Those devices do not allows that calculation; they are for diving at one level, then another, then another. The levels must be a required distance apart; the Wheel manual says:

If deepest level is:	Next level must be no deeper than:
120-130 ft	80 ft
95-110 ft	70 ft
80-90 ft	60 ft
65-75 ft	50 ft
50-60 ft	40 ft

 

[Wibble]

45mins on air at 28m/95ft would put you well into decompression. Nitrox would reduce the amount of decompression (if you're qualified for Nitrox).

As you said, tables assume a square profile. They are hard to use for multi-level dives.

Other tools such as MultiDeco can be used to enter a rising profile, say 5 mins at each depth.

The reality is you'd use your dive computer and follow a slow ascent making sure that you don't go into decompression.

 

[pgordon]

Easiest way is to buy a dive computer, start deep, and move shallower as you approach your NDL. next easiest would be to get the PADI ERDPML and play around with the different times at different depths until you find a profile that works for the dive you want to do. The wheel is a good manual method for planning out a multilevel dive but it has a bit more of a learning curve.

Edited to remove bad information...sorry

 

[boulderjohn]

Wibble is correct. You cannot do that dive profile with the tables. They assume a square profile, so you would compute the pressure group by simply finding the maximum depth and time. On an imperial version, your maximum bottom time is only 20 minutes.

 

[rick00001967]

simple answer is.....you cant. this is why most people have dive computers now. even an inexpensive one.

or as suggested above, you could use "the wheel". google it. you will see a round odd looking type device you can use to manually plan multilevel dives. i have never owned one but had to plan some dive using it when i did a padi adv diver course.

 

[tursiops]

I have also heard of a round about way to plan a multilevel dive on a normal table but I don't remember the details enough to explain it confidently. Maybe someone can chime in with that method.

The suggested method is not valid. Do not attempt to use a square-profile table for a multi-level dive.

 

[**pYgmY**]

Thank you for your quick responses! I'm aware that in reality most dives are multi-leveled and that dive computers can calculate more complex profiles automatically. I was just wondering if there is a "manual" method that can be used for profiles like this that I'm not aware of. Thanks for clarifying