Difference between gas mixes

[seekerGH]

A brief article, with some good information on some of the mixes..
leaves out much about Trimix or Helox, (79% helium and 21% oxygen) but oh well!

Many new divers incorrectly call their diving cylinder an oxygen tank. For some, it’s just a turn of phrase; they know full well that the standard diving gas is good, old-fashioned air. But divers can breathe other gases, including the commonly used nitrox, as well as technical mixtures. What’s the difference between all these scuba diving gas mixes anyway?



The Difference Between Scuba Diving Gas Mixes ? Scuba Diver Life

 

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[ropesfish]

I love me some Nitrox, the very best thing that has happened to old divers in a while.

 

[Boatlode]

I'm definitely taking the nitrox class this spring. I figure I'll need it to dive the U-352 this summer (115 feet).

 

[seekerGH]

You should look at Trimix, would extend your bottom time significantly and comfortably.



Interesting note from the article: Minimum time to fly or ascend to altitude is 12 hours after one dive, 18 hours after two dives and 24 hours after three dives.

http://www.advanceddivermagazine.com/articles/rgbmphase/rgbmphase.html

 

[Jason in Enid]

You should look at Trimix, would extend your bottom time significantly and comfortably.

View attachment 1410804 View attachment 1410805

Interesting note from the article: Minimum time to fly or ascend to altitude is 12 hours after one dive, 18 hours after two dives and 24 hours after three dives.

RGBM Full Up Phase Model Decompression Tables • ADVANCED DIVER MAGAZINE • By Bruce R. Wienke, PhD and Timothy R. O'Leary

Trimix has its own set of concerns.

 

[Boatlode]

So according to those tables, at 115 ft with EAN32 I can stay down 20 minutes, but with air my NDL is 13 minutes (making a 3 minute safety stop at 15 feet in both cases). Am I right?

 

[SADS 669]

No Mike, the table above is for 110 ft and you only get15 mins no stop, if you wanna do 20 minutes your virtual overhead ( deco stops) are1 min @ 10 to 15 ft depending on weather....

US Navy tables You only get 2 minutes deco at 10 to 15 ft for 120 ft after 20 mins so that's worth doing in my opinion, but it's not a safety stop...it is a deco obligation.....

I haven't looked at nitrox for 120 ft but I think a 30% mix Works better at that depth off the top of my head

 

[seekerGH]

SADS,

That is a great comment, as all Nitrox is not created equal, the differing percentages make a difference, as 35% I think, is the most (with 20% being normal air)

Using this formula, a diver breathing enriched air nitrogen, (EAN 35) will have a maximum operating depth of 99 feet, or 4 atm. The recreational limit of 1.4 PPO2: 1.4 / 0.35 = 4 atm, 33 feet x (4 atm - 1 atm--the one surrounding the earth) = 99 feet.

"For example, based on the U.S. Navy dive tables, a diver on air at 100 feet reaches his or her no-decompression limit and must come up after 25 minutes. At 60 feet, the diver's maximum time would be one hour. Nitrox changes the numbers. When you replace some of the nitrogen with extra oxygen, there's less nitrogen available, which means it won't dissolve as quickly, allowing a longer no-decompression limit. This concept is known as the equivalent air depth (EAD). For example, divers at 105 feet with 36 percent nitrox will dissolve nitrogen into their blood and tissues at the same rate as when breathing air at 80 feet. Therefore, the diver's ordinary no-decompression limit of 20 minutes extends to 40 minutes--that's double the bottom time."

 

[SADS 669]

Seeker, I managed to do 68 dives in 2 weeks at Truk lagoon simply because dives 2,3,4 and,5 each day were mostly 32% after a 160 to 210 ft first dive each day, on air.

Long safety stops on top of the deco....because I am both old and fat...ha ha

I felt really safe but looked a thousand years old at the end of the week and spent 2.5 days off gassing before flying home.



Japanese sub I 169 at 210 ft....look up the story, very sad...

 

[ropesfish]

Seeker, I managed to do 68 dives in 2 weeks at Truk lagoon simply because dives 2,3,4 and,5 each day were mostly 32% after a 160 to 210 ft first dive each day, on air.

Long safety stops on top of the deco....because I am both old and fat...ha ha

I felt really safe but looked a thousand years old at the end of the week and spent 2.5 days off gassing before flying home.

Does 2.5 days of offgassing go more smoothly with a single malt and some ice (and a DAN bottle of pure O2 for the aftermath)?

 

[SADS 669]

Depends on the single malt? Why do you want me to do some R & D?

Ps watch your language.....ice? Ha

 

[Boatlode]

No Mike, the table above is for 110 ft and you only get15 mins no stop, if you wanna do 20 minutes your virtual overhead ( deco stops) are1 min @ 10 to 15 ft depending on weather....

US Navy tables You only get 2 minutes deco at 10 to 15 ft for 120 ft after 20 mins so that's worth doing in my opinion, but it's not a safety stop...it is a deco obligation.....

I haven't looked at nitrox for 120 ft but I think a 30% mix Works better at that depth off the top of my head

So really, its better to just keep your bottom time at 15 minutes and use the EAN32 as an additional safety factor, right Kev?

 

[SADS 669]

Sort of........I am of the school that if you are happy to stay for an extended time at 20 ft ( and the diveboat guys don't complain too much) it doesn't really matter to me if its air or nitrox once you get over 110. At those depths you really should think about going into deco diving procedures as the norm ( certainly Long safety stops if your dive is a NDL) by that I mean 30 ft per minute ascent and a minute stop at 60 ft for sure on the way up

I think nitrox is best in the 60 to 110 ft range otherwise you need to drop the percentages of O2 and dive shops find that irritating on the whole, they just wanna bang out 32's all the time in my experience.

No doubt nitrox is safer but don't get too focused on it being the " no bend snake oil" there is no such thing as a bend free dive....it's like every landing in a plane is a crash landing except most of the time the undercarriage can stand the hit.

 

[ropesfish]

Here on the Treasure Coast we don't do much deep diving, in fact I spend most of my time in 10 feet of water. It's a long trip out to anything more than 110 fsw. My affection for Nitrox has to do with feeding my 61 year old lungs some extra oxygen. If you are doing physical work underwater, it's a wonderful thing.

As some already know, I sometimes post things I think might be of use - not because I want to look like I know something, but because I already looked it up, thus saving someone else the time and effort.
Just a couple of resources for air vs EAN.
A quick Maximum Operating Depth table: DiveNerd - Nitrox Maximum Operating Depth (MOD) Table

Other useful calculators from Divenerd: DiveNerd

The NOAA No-Deco Nitrox tables: http://www.omao.noaa.gov/sites/default/files/documents/1.4 Nitrox Tables New ver 7 Oct 2016.pdf

NOAA No-Deco Air tables: http://www.omao.noaa.gov/sites/default/files/documents/NOAA No-Decompression Table_032316.pdf

Tech divers might like this one- It includes all the math and reasoning behind the no-deco and deco tables as well as treatment protocols for deco chamber rides - http://www.omao.noaa.gov/sites/default/files/documents/DMT Formula book 111816.pdf

 

[Boatlode]

I'll probably use nitrox and stick to the air NDL (15 min), and make a 3 minute safety stop at 15 feet. I'm not trained in technical diving. But after reading Shadow Divers, I want no part of DCS. Its a horrible way to die.

BTW - if any of you haven't read Shadow Divers, I highly recommend it. Its the story of the discovery and identification of U-869.

 

[TreasureCDave]

Nitrox gives me a headache and I don't like to use it on shallow dives. It seems to be only on shallow (or shallower) dives as I found out when I used one of Ropes on a 10-20 last summer.

When diving on air or mixed .... I prefer a safety stop at 20 ft. when diving past 100 ft. just the same.

 

[ropesfish]

Nitrox gives me a headache and I don't like to use it on shallow dives. It seems to be only on shallow (or shallower) dives as I found out when I used one of Ropes on a 10-20 last summer.

When diving on air or mixed .... I prefer a safety stop at 20 ft. when diving past 100 ft. just the same.

You're just not old enough....yet. We'll be using a hookah this summer, so Nitrox will only be an option for the reef roaming portion of the day.
I'm in favor of the 20 foot 2 minute minimum stop for anything over 60 fsw. It gives all your parts a little time to adjust.

 

[agflit]

I'll bring up a "subtle" reminder re: eanx ...or air (21%).

NDL... "No decompression limit"..is just that...a "no decompression requirement" dive. Thats why the recreational dive tables are designed to a depth of 130'. Theoretically...you are able to ascend DIRECTLY...to the surface non stop provided you maintain the correct ascent rate and maintain your depth/time ratio... IE no Safety stop requirerment.

Safety stops are just that...a safety stop. It's designed to build in a conservation factor. That being said...I ALWAYS...do a safety stop. and 90% of the time I'm the last one on the boat...I'll hang at 15' til the last diver surfaces...it can only help to extend a SS...no down side to it.

I try to always keep in mind that the dive tables are designed by the US Navy and revolve around 20 year old kids in the best shape of their lives...and I'm not 20 anymore!!

Way I look at it...once above 60' I slow my ascent way down..by 40' I'm at 30' per minute ascent rate...and hang out at 15' for at least 4-5 minutes..longer most of the time if I'm not holding things up.

Just my .2 ...just the way I dive.

ag

 

[SADS 669]

Ag, I don't agree with anything you said above, not because it's untrue......just because!!

 

[TreasureCDave]

Safety first applies to most endeavors.

When shore diving to 100 ft. + in G.C., I used to go up to 15 or 20 ft., equalize and slowing swim back to shore.

Safety stops are mandatory for most recreational dive operations were I have been.