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SurfLung
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Nitrox Checkout in Cozumel

Mon Feb 22, 2021 3:59 pm

Nitrox Checkout in Cozumel

Rich (50,001 Questions) and his wife Marie joined Me (SurfLung) and my wife Jill on a wonderful trip to Cozumel last week. We stayed at the fabulous Secrets Aura Cozumel resort all inclusive with airfare, hotel, food, and drink for one bargain price. Rich and I booked three 2-tank boat diving trips and got plenty of experience with my new Nitrox certification. Rich and I spent the mornings diving and the afternoons romancing our wives by the pool.

The diving was run by Dive House and they had several boat/crew options. We told them we wanted small groups and swim throughs. Raul obliged us by putting us on Dive House IV with Captain Rudolpho and Dive Master Victor. Wow did we have fun! Victor kept us no deeper than 80 feet with our Nitrox 32 tanks and led us through lots of Swim Thrus. We saw huge crabs, Angelfish the size of turkey platters, huge nurse sharks, eagle rays, sting rays, skates, HUGE Moray eels, spotted eels, lobsters, I could go on and on if I could remember them all. Visibility was too far to estimate. The water was simply beautiful... And warm. I dove with just a swimsuit, no wetsuit. Now THAT is the way to go once you have your weights figured right. Happily, we figured our salt water weights almost perfectly from having practiced in the freshwater pool here at home.

I dove my Argonaut Kraken double hose regulator with matching Kraken SPG and Safe Second. I used an Oxycheq 18 lb. BCD and the VDH DSV mouthpiece. Everything work just perfectly. Rich dove conventional with his ScubaPro stuff. Victor showed me how to use the oxygen checker and our oxygen percentages varied from 31.5-33.8... We rounded up to 32-34%... Noted it in the log and set the dive computers that way. Otherwise there was nothing different about Nitrox diving except that our bottom times were longer. Us old farts were actually the last to come up as we were so comfortable and effortless we just weren't breathing hard at all.

Nitrox Maximums vs. Minimums: We worked with two different max partial pressures. My SDA certification was for PP of 1.2 and the course gave a max depth of 100 feet on NItrox 32. Rich's PADI course used a PP of 1.4 and max depth of 110 ft with Nitrox 32. THEN, I set my ZOOP for 1.2 and 32% and it gave me a max depth of 87 feet. Then we had the day when our tanks measured 34% O2. The ZOOP set for PP 1.2 calculated the max depth for Nitrox 34 at 80 feet. It is of some concern (which is correct?) when we get different maximums from different sources. But we simply used the shallowest recommended maximum depth as our limit.

I was a long time coming around to the benefits of Nitrox in Scuba diving. But now I see how useful it can be and I think I'll try and incorporate it in more of my local Midwest diving. It might depend on convenience as we don't have a diveshop locally that fills Nitrox. We'll see...
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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luis
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Re: Nitrox Checkout in Cozumel

Wed Feb 24, 2021 9:59 pm

Below are some actual calculations based directly on O2% to calculate partial pressures at the respective depth.

1st column actual depth in feet
2nd column Equivalent Air Depth
3rd column Partial pressure of O2

First page is for Nitrox 32 and second page for Nitrox 34

I think your computer must have some extra safety margin added to it.


- Nitrox calcs-32 copy.jpg
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luis
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Re: Nitrox Checkout in Cozumel

Wed Feb 24, 2021 10:07 pm

That wasn't very clear.

Let see if this is better.

Image



Image



Those are calculation sheets that I setup a while back using a software called MathCAD. It is just a mathematical software like Excel, but with the advantage that you can actually see the equations in a readable format.

I like MathCAD a lot because I can just setup the calculation sheet and just change any values to see how it affects the results.
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SurfLung
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Re: Nitrox Checkout in Cozumel

Thu Feb 25, 2021 11:23 am

Yes that explains alot... I DO have my ZOOP set for 2 levels of personal protection. I call them "Old Guy" and "Fat Guy" but the ZOOP just has P1 and P2. When I get home tonight I'll try setting it for P0 (no special protection) and see what it gives for max depths.

I notice that you are using the 1.4 value for the toxicity partial pressure. My SDA certifying agency uses a lower 1.2 value. I had my ZOOP set for the 1.2 value... Which I figure must also add the equivalent of yet another personal protection?

I looked up the Recreational Diving Standards that my SDA certrification complies with and was surprised to see they list a maximum O2 partial Pressure of 1.6. You're using 1.4. And my SDA course uses 1.2. What's your opinion on this?

https://wrstc.com/downloads/04%20-%20En ... cation.pdf
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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luis
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Re: Nitrox Checkout in Cozumel

Thu Feb 25, 2021 3:45 pm

Hi Eben,

I would like to first say that just like anything involving humans (or any living creature), nothing is exact. In reality there is never a sharp threshold when it comes to O2 toxicity or for that matter decompression obligation. (IMHO) I always roll my eyes :roll: when I hear the term “undeserved decompression hit”, but that is a subject for another discussion. :)

How susceptible we are to O2 varies per individual and it even varies by the time exposure and some other factors. But it is never an exact threshold limit.

From most of my reading, the 1.4 atm seems to be a fairly acceptable limit (both for the technical community and recreational divers), but you may notice the three calculations on the left of my page.

I normally use 1.4, but for short periods I have momentarily even exceeded the 1.6 atm threshold. At the same time for most of my actual diving we rarely get even close to O2 exposure of 1.2 atm.

Notice that 1.6 is often referred to as the contingency threshold. It is the often the allowable limit in shallower water during decompression when a diver may be using higher content O2 (even pure O2 at 20 feet max) to accelerate decompression.

The first calculation is for D-max1.2, the second is for a D-max using 1.4, and the third equation is for what we call D-cont using 1.6 (that is for maximum contingency depth).


So with the most common Nitrox 32, I like to stay shallower than 100 feet, but I am not too concern about 110 ft (for a short excursion). Most of the dives are probably shallower than 90 feet, which keep my PPO2 exposure to 1.19 atm or less.


Both of my computers (the Perdix and my older one) are both set to a max PPO2 to 1.4, but I also have the alarms on silence. I do keep an eye on it.



BTW, I do remember when I started diving it was considered acceptable a PP of O2 up to 2.0 atm. That is why air at the time was considered OK down to 298 feet of salt water. Never mind the narcosis level, but this was based on the believed acceptable limits for CNS (central nervous system) O2 toxicity.

A pure O2 rebreather was considers OK down to 33 feet of salt water.

And the Navy used to test their divers for how O2 susceptibility.

In a chamber or with a full-face mask the Navy and commercial divers still pushes O2 exposure during decompression. With a full-face mask you are not as concerned about drowning if you have a seizure from CNS high O2 exposure. You are not going to drop the regulator out of your mouth.
Luis

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SurfLung
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Re: Nitrox Checkout in Cozumel

Fri Feb 26, 2021 11:26 am

Thanks again for clearing some things up. I think there are some misconceptions even among divers who have taken the course and got certified for Nitrox. And I think these misconceptions may have kept me from realizing the value of nitrox in my recreational diving.

Misconception #1 - Dive Nitrox but use air tables and air computer settings. In other words, just use Nitrox as an added level of safety. This, I think, is a legitimate use but I think it defeats the purpose. Since you're not using the extended bottom time and shorter surface intervals, you could get the same extra safety with air by extending safety stops and surface intervals.

Misconception #2 - The shallower depth limits of Nitrox limit the safety of a buddy because if he goes too deep, you can't go after him. Your discussion on previously believed limits and modern contingency threshold, shows that an emergency deeper decent is most likely not going to be lethal.

Misconception #3 - Dive the first dive with air because that will be the deepest dive. Dive the second dive with Nitrox. Rich and I were disagreeing on this in Cozumel but I think he was talking about our Fortune Pond diving... We usually do one bounce dive down to the 130' depth for about a minute and then come up to finish the dive in the 60-80 feet depths. In Cozumel, we were only doing a 2-tank dive trip each day. And I argued that doing the first dive on air and 2nd on nitrox was defeating the purpose of nitrox. First dives were always in the 70-80 ft depth so nitrox is proper and it extends bottom time and shortens the surface interval as well as helps off gassing between the 2 dives.
- I was pleasantly surprised that our computers gave us about 45 minute 1st dives and 55 minute 2nd dives. AND we were set for 2 personal safeties and PP O2 of 1.2.

Diving to 130 ft. on Nitrox
- If I am reading you correctly, we could do our brief 1-2 minute dive to 130 ft with Nitrox 32 and still be within the 1.6 PPO2 contingency limit... Then ascend to 80 ft and shallower for the entire rest of the dive... staying well within the 1.2 and even 1.4 PPO2 limits.
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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luis
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Re: Nitrox Checkout in Cozumel

Fri Feb 26, 2021 6:10 pm

When I dive Nitrox, I set both of my computers to the Nitrox mix that I analyzed. I don’t get too hung up with high precision. I am OK with in 1 or 2%. Even some of the O2 sensor/ gauges may not be as precise as we would like. With some O2 gauges, you can even induce a slight error by opening the valve a bit high. But it is not a big deal.

Setting the computer to air is like the people that set their alarm clock fast so they get up earlier. If it works for you, go for it, but to me that is like lying to myself. I prefer to have the best information on hand and make my decisions based on the information I have.

For extra safety, I am a strong proponent on just doing extra long safety stops (deco stop). I have had this discussion with several hyperbaric medicine doctors/ researchers and they all like my approach. There seems to be a big emphasis on shortening the deco/ safety stop. If I am cold, yeah I want to get out of the water, but if I paid to go to warm water to dive… I don’t want to spend the time on the boat. I rather be in the water.


Yes, you are reading the data correctly. The calculations are numerically accurate, but just keep in mind that nothing else is that precise. It is entirely up to the diver to decide the risk one is willing to take. There is always a balance between risks and rewards.


The column with the EAD (equivalent air depth) is very telling of the advantages of Nitrox. With modern computers you don’t need to know the EAD (since you just set your computer and it does all the calculations). But, just like learning to use the dive tables give you a good idea of what is going on, looking at the EAD is very telling of the advantage of Nitrox.


Divers don’t call it Voodoo gas anymore… :?

Now you hear the term geezer gas… :shock: I don’t like that term. :roll: :)
Luis

Buceador con escafandra autónoma clásica.

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