Nitrox Question

[Rolobingo]

Hi everyone...

My questions concerns doing a dive on air, then a second dive on nitrox.

Can one's pressure group, on the air and nitrox tables, be interchanged?

If one does an air dive surfacing in group R, interval to group F, say. Can that person then plan a nitrox dive starting in group F on the back of the nitrox table?

Does it all work out the same when you use EAD's and air tables?

Im going to break out my tables and have a look for myself. I just wanted to get everyone's opinions...

Are there any brawbacks or risks with this practice?

Tanks for having a look...

RH

 

[Dsix36]

It has been a long time since I used tables and i do not remember them well enough to give you an answer to using the same grouping from one table to another.

We often did nitrox dives on one mix and used a different mix for the second dive. You logic of the tables seems to make sense since air is actually just 21% nitrox. If I am wrong then someone with more current knowledge of the tables will certainly correct me.

 

[kaerius]

I think going from air to nitrox is ok, just not the other way around, if I remember right. IE: one-way only. Similarly some(most/all?) dive computers will let you swap mixture from air to nitrox at any time, but will not let you switch from nitrox to air as easily.

You can dive air after diving nitrox, you just have to treat it as 21% nitrox, you can't treat it as just air. IIRC this is due to oxygen toxicity buildup. The DSAT Oxygen exposure table also lists air for this reason.

 

[tomfcrist]

I think the op's question is if the pressure groups are interchangeable (same n2 load) when moving from an air table to a nitrox table. Honestly, I have never given it any thought. I may do some reading and find out though.

 

[ekremer]

Use your next mixes Equivalent Air Depth and then continue with the standard tables.

 

[SeaHawk12]

I believe that they are, but a really easy way to check would be to make a hypothetical dive using 1) only the air table with EAD to account for nitrox and 2) the same profile but using the nitrox table. If the end groups are the same, all is well. If not, then you know you can't. I'll give it a shot and post back in a sec...

 

[kaerius]

Yeah the pressure groups are interchangeable, as long as you're talking PADI RDP and PADI nitrox tables at least. Don't mix systems, IE: don't for example mix navy tables with padi nitrox tables.

 

[wve]

The PADI (DSAT) nitrox and air tables are interchangeable with each other, although they are not interchangeable with other tables (eg, USN, Naui, etc).

The practice of switching between air and Nitrox is fine, and makes sense in the right circumstances. Although it is often said that Nitrox is used to prolong one's bottom time, Nitrox can alternatively be used to lower your risk of DCS.

I do not recommend bending rules, but we all know that it happens sometimes. So if your situation starts to push the limits -- such as diving at altitude, diving in reverse order (ie, deeper dives follow shallow ones), or making multiple dives with minimal surface intervals -- then anything that helps to offset the risk can be helpful. Nitrox might help in such situations, although it is not a panacea or guaranteed to keep one from getting bent.

 

[tomfcrist]

I would still use EAD, but yes as long as its from the same source, the pressure groups should be interchangeable. My advise, buy a dive computer, or calculate EAD and use the air tables accordingly.

 

[Rolobingo]

From the OP

I'll be shore diving at a spot with both air and nitrox fills. I dive a computer, rarely use tables. Both my back tanks and bailout are full of air. To avoid the process of emptying them and getting nitrox fills i was hoping to do the first dive on air, then filling nitrox. Want to have the data/ tables ready in case the switch throws my computer into some kind of lock out mode. (never tried it before).

I think ill find the EAD and continue planning from air table.
Thanks for your advice!

RH