Nitrox

[Alban]

Hello all

I am planning a few nitrox dives but only have one clean cylinder. My manual states if possible use your weakest mix first.I am unsure as to the reason for this ?

I have tried a search but came up with nothing.

Many thanks Alban

 

[cornfed]

I am planning a few nitrox dives but only have one clean cylinder. My manual states if possible use your weakest mix first.I am unsure as to the reason for this ?

Work the dives out both ways on your tables and see what happens...

 

[H2Andy]

1. "weakest" means more nitrogen, thus a "deeper" dive nitrogen-wise (same as "do your deepest dive first").

2. a "weaker" mix will allow for a physically deeper dive without O2 risks, thus you are
likely to dive deeper on your "weakest" mix (again, same rationale as "do your deepest dive first)"

i find that with a nice surface interval and cautious dive planning, it doesn't really
matter which mixture i use first or whether i do the deepest dive first or not.

 

[Alban]

Thanks guys

 

[wedivebc]

1. "weakest" means more nitrogen, thus a "deeper" dive nitrogen-wise (same as "do your deepest dive first").

2. a "weaker" mix will allow for a physically deeper dive without O2 risks, thus you are
likely to dive deeper on your "weakest" mix (again, same rationale as "do your deepest dive first)"

i find that with a nice surface interval and cautious dive planning, it doesn't really
matter which mixture i use first or whether i do the deepest dive first or not.

Not trying to hijack this thread but does anyone still teach deepest dive first?

cheers,

 

[Atticus]

To echo what has been said before, but perhaps with more explanation --

The idea is to do your deepest dive first. With various nitrox (or air) mixtures what you do is compute your EAD's for each dive, and dive the deepest EAD first.

Many people do reverse profile dives (shallow dive first), however the majority of the agencies request that students be taught that their risk is lowest if they do their deepest dive first.

There have been various discussions on this subject both here, in print, and even in DAN articles. Here is a link to a DAN article that suggests that divers do their deepest dive first:
http://www.diversalertnetwork.org/medical/articles/article.asp?articleid=14

 

[MikeFerrara]

Not trying to hijack this thread but does anyone still teach deepest dive first?

cheers,

Of course.

Check your tables, you get more bottom time/less deco that way.

Also do a search on this board there's some interestsing statement by Bruce W.

What came out of the reverse profile workshop was a statement that for no-stop dives with a difference in depths no greater than like 30 ft it didn't seem to be a problem.

The magazines hosed the whole thing up in several of the articles claiming that reverse profiles were ok because people were using computers. LOL At least that's what many readers came away with.

Also check out "Technical Diving in Depth" by Bruce W. for a thorough examination of reverse profiles using multiple decompression models.

If reverse profile were such a good idea you could do your safety stop at the beginning of the dive. LOL just kidding...sort of.

 

[wedivebc]

I used to occasionally skipper a dive charter boat out of Nanaimo BC. I was amazed at the number of divers who would pass up, (IMO) the best dive the area had to offer, Dodd Narrows because they were saving their deep dive for the wrecks. Due to tide considerations we were sometimes forced to do Dodds first (85ft) then the Saskatchewan (120ft) later but many divers did not want to do that for fear of reverse profile.
Some would even dive Dodd on air then use a nitrox EAD to go shallower on the wreck.
My feeling is especially for infrequent divers, a shallower dive can put an out of practice diver at ease making the second (deeper) dive less stressful.

I will check out Bruce Ws book although much of that mathmatical stuff is way over my head.

 

[cornfed]

I will check out Bruce Ws book although much of that mathmatical stuff is way over my head.

None of his stuff would qualify as a light read, but the section on reverse profiles isn't that bad. It would also be a good idea to ask google about the reverse profile study MikeF mentioned.