Decompression question?

[thedecimator]

If you dive to 25 feet underwater what is the maximum bottom time you can stay under if you are an A diver?

What is the maximum time you can stay underwater at 25 feet and not get the benz?????

 

[scubadiverjunkie]

Well I would like to know what depth I have to so I cant get The Benz.....now is it the CL or SL series......


j/k

 

[HammerNoMore]

If you dive to 25 feet underwater what is the maximum bottom time you can stay under if you are an A diver?

Your instructor will cover how to read your tables during your OW class.

 

[rab]

Well, it depends on how you want to use your tables (or which tables you use)

PADI's RDP says this gets treated as a dive to 35ft, so if you start in pressure group A, your NDL is 195 minutes. That doesn't guarantee you wont have a decompression problem, but that shallow and it's quite unlikely.

In some tables that I found online, the answers seemed to vary. One had 225 min, another 575 min, and a third had no limit at 25ft. All three of those tables had an unlimited NDL for 20ft or less.

That said, I hope you're not already certified because it would be unfortunate if you hadn't learned this in your basic Open Water course.

-Rob

 

[cyklon_300]

Backgas - air (21% O2)
Deco - EAN50

Descend to 25' for 500 min.
Deco at 20' for 1 min.

Not a bad profile, but the gas requirements are a little stiff...527 cu ft. Guess I'd have to dust off the double-350s...

 

[pipedope]

I will bet that you will reach one or more of your own limits long before any significant risk of DCI at 25'.

My longest dive at 25' was something like 4 hours.
I was very tired after the dive but that was because I was dredging the whole time.

 

[DepartureDiver]

Why do you want to know how long it takes you to become an "A" diver? If you have a specific application, then there may be another way to do this. Any shallow dive such as this to a maximum time will not track well with any repetitive group designation – which is one reason the tables force you to use a deeper depth. This is because a “much slower compartment” than what the surface intervals are based upon will limit such a dive. So to answer the other part, the times at the "shallow" depths will vary greatly upon the table or "theory" being used. Many of the bubble models do a good job of calculation deco for dives in the 100 to 300' range, but their predicted no-stop times appear to get a little off, especially in the shallow depths. The minimum bends depth appears to be in the area of 20 feet. So deeper than 20’, there needs to be a maximum time. Since this has to be a theoretical question, I would recommend a maximum of 470 minutes. The question is actually a good theoretical question since divers need to look at the no-stop limits so the table/software can be evaluated as to its level of conservatism or aggressiveness. Most tables will have times very similar in the middle to deeper depths such as 70’ to 120’. But they can vary greatly in the 20 to 35’ range. At 30’, the maximum no-stop times should be in the area of 300 minutes in my opinion. The shallow times need to be looked at since it is still an indication of the model. A model allowing very long no-stop times in the 25 to 30 foot range needs to be considered very aggressive.

 

[DA Aquamaster]

The US Navy used to consider a direct ascent from a saturation dive at 33 feet as being ok, so in effect there was no limit down to 33 feet. There is some validity in this I think as when the USS Squalus sank in 1939, the crew was on the bottom for something like 36 hours (2160 minutes) at a pressure equal to 27 feet and ascended directly to the surface with no decompression.

However my 1980's era PADI tables give a limit of 310 minutes, but then they also give a limit of 300 minutes at 10', so I suspect that 300 odd minutes is considered more than enough for any practical purpose and the tables just don't go any farther. My current SSI nitrox tables list a limit of 205 minutes at 30 feet on air but again I suspect the reason for this may again be the space available on the table.

 

[DepartureDiver]

The US Navy tables that everyone is familiar with never used a 2:1 ratio or considered that there was no time limit at 33 feet. Prior to this table, the 2:1 rule was used and believed for a little while. Many "new" tables today have adopted the no-stop times from the RDP table and research by DSAT (205 minutes at 35') with some rounding down to a number that is already on the Navy table. Others since then, such as SSI, have slightly reduced these times even more. It is also interesting to note that the no-stop times on the RDP were reduced slightly from those tested in the deeper depths. For example, the times tested at 130' was 12 minutes, but on the printed table that was released, it was kept at the U.S. NAvy 10 minute limite and was not increased to 12 minutes.

 

[Diver0001]

The US Navy used to consider a direct ascent from a saturation dive at 33 feet as being ok, so in effect there was no limit down to 33 feet. <snip>

I don't recall anyone saying this. IIRC even Haldane himself saw that 2:1 didn't work all the time. I also recall that this was reset to 1.6:1 at some point but I don't recall who did that.

I also recall reading (from DSAT) that the PADI tables don't work reliably for very long repetative shallow dives in the 33 ft range, which would lead me to believe that the ndl they quote is too long.

R..