Current best practice for deco gas: 75% vs 100%

[tbone1004]

Hi I’m confused by this as I’ve yet to a technical course. I have a few questions if you don’t mind answering:

What is the difference in efficiency between O2 and EAN 50 for deco?

Why do not many people like 80%?

Is there a massive difference between 80% and 75%, why have people used 80% but 75%?

Thanks

1. significant. One is half full of the gas you are trying to get rid of, the other doesn't have any. Your body on and off gases based on pressure gradients between the blood and the tissues.
This is super basic and is just to try to have you understand the concept, this does not really represent how it actually works, but the concept is there.
Think of this as a pipe with two pumps on each end attached to a bucket where one represents the blood, the other your tissues. After a dive on air to 2ata with an equalized system, both pumps are pushing nitrogen at a pressure of 1.6 in both directions so there is no movement, and the buckets are unit 1.6 and full.
The total pressure of the system is brought to 1ata when you surface so the blood bucket becomes .8 unit and is pumping at .8 units. Tissue bucket stays at 1.6 and the pump stays at 1.6. There is only a .8 pressure differential so it will take a long time for the tissue to drain its bucket to .8 units by overflowing the blood bucket.
If the blood bucket is reduced to .5 by breathing 50% and the pump is reduced to .5, then the pressure differential is 1.1 units and the tissue bucket will drain faster.
If the blood bucket is reduced to 0 by breathing 100% and the pump is turned off, then the pressure differential is 1.6 units and the tissue bucket will drain even faster.

We always strive to maximize the gradient between the two during decompression but are limited in the gases that you can breathe at depth. The sooner you can get on pure O2, the more efficient your deco will be.

2. No point in 80%. With 50%, you can get on the gas at 70ft so you gain a huge benefit with that pressure gradient early. This advantage is why it is the preferred gas for normoxic trimix dives especially with a low GF-lo. With 80%, you can't get on it until 30ft so you minimize the benefit of getting on that deco gas early, and because it still contains inert gas it isn't nearly as good as pure O2. About the only time you can justify using it is if you have mandatory 30ft stops and can't safely decompress at 20ft or shallower for the bulk of your decompression.

3. no real difference between 80 and 75, I have only ever heard of 80% being discussed because it conveniently works with our O2 limits at 30ft/2ata. 75% would start around 40ft

 

[Gareth J]

Interesting read. Reminds me of the old days.

When I first started using Nitrox (in the days when it was supposed to kill us), we used to use 50% a lot. Getting hold of that was hard enough.
We would dive a twinset on air, then run the decompression on 50% (using an air computer), from a 3 litre mounted on the side of the twinset. Generally that would do dive 1 (am), skip using it for dive 2 (shallow dive), then the following day use it for dive 3, and if there was any left drain it down on dive 4.
We could change sites because we didn't have MOD issues on the back gas, and then flush our systems on the 50%. Worked very well, especially when the sites could change from 30m dives to 50m dives dependent on what the conditions where like on the day.

Later, when I did ERD. We often used anything form 60-80%. This was mainly due to the lack of O2 and there being very few booster pumps. It was also advantagous in very lumpy seas, when holding a stop at 6m was problematical.
At 80%, the stop time difference compared to 100% was marginal, with the advantage that your O2 dose was lower.

Now, boosters are more common, so full cylinders of 100% are easier to get.
To a greater or lesser degree its not an issue I worry about, diving CCR. Even now, most of my O2 is decanted, currently I'm only getting 100bar of O because my bank is getting low.... I don't have a booster.

Being old school, I work with what I've got, or am likely to have over an expend'. Then i'll plan the decompression schedules and dives around that.

Gareth

 

[rjack321]

3. no real difference between 80 and 75, I have only ever heard of 80% being discussed because it conveniently works with our O2 limits at 30ft/2ata. 75% would start around 40ft

40ft on 75% would be 1.66 bar O2 so its in between normal 10ft stop depths

A ppO2 of 1.6 on 75% would be about 37ft which is awkward to say the least. One reason among many why 75% has never even been debated (until now).

 

[EireDiver606]

Thanks @rjack321 and @tbone1004 for your informative posts.

If you use O2 at 6m will it be twice as fast to complete deco than 50%? I know this is kind of a stupid question but I have no idea.

 

[rjack321]

If you use O2 at 6m will it be twice as fast to complete deco than 50%? I know this is kind of a stupid question but I have no idea.

No not at all.
For a given 25min dive at say 150ft, the time to surface roughly the same (an hour) regardless of which gas you bring. You can switch to the 50% sooner though. The time to get up to 20ft counts a little bit to burn off your deco obligation, but also blows up your rock bottom since you need to carry enough gas for 2 divers to ascend all the way from 150 to 20.

 

[JohnnyC]

If you use O2 at 6m will it be twice as fast to complete deco than 50%? I know this is kind of a stupid question but I have no idea.

Nope. Sometimes it's close, but there are plenty of other variables at play, especially once you start getting to longer depths and run times.

As an example, GF 50/80, 60m dive for 45 minutes, 18/35 bottom gas, all stops are the same until 6m.

50% calls for a 72 minute 6m stop for a 166 minute total run time.
100% calls for a 45 minute 6m stop for a 139 total run time

If O2 were twice as fast. the 6m stop would be 36 minutes.

That's just the deco, no gas, CNS, or other considerations.

 

[elmo]

For a given 25min dive at say 150ft, the time to surface roughly the same (an hour)

I agree with the point of your post, but are you sure about the one hour tts after 25min at 150ft?

 

[rjack321]

I agree with the point of your post, but are you sure about the one hour tts after 25min at 150ft?

Depends on your GF settings and bottom gas but 25mins on 21/35 with GF of 50/70 has a TTS of exactly 60 mins with 50% deco. Its 65 TTS mins if you have 100% mostly because the backgas deco to get through 50, 40 and 30ft takes 17mins.

 

[Ryan Mcshane]

Depends on your GF settings and bottom gas but 25mins on 21/35 with GF of 50/70 has a TTS of exactly 60 mins with 50% deco. Its 65 TTS mins if you have 100% mostly because the backgas deco to get through 50, 40 and 30ft takes 17mins.

At 50/70 it’s still only 30 mins?

 

[rjack321]

At 50/70 it’s still only 30 mins?

TTS = time to surface
So 35mins of deco after the 25 min bottom time. I used TTS because it was too confusing to talk about time on backgas to get to 20ft as deco time.

Its only 150ft for 25mins how much deco did you think you'd have?