Setting computer to 21% when using 33%. Issues?

I guess my main question is am I putting myself in danger with O2 Toxicity.

How I dive and what am I checking.

Buddy and I are both PADI rescue diver. He is at 200+ dives I am at 51 dives.

Computer is a Uwatec Smart Z plus I also have and analogue SPG as backup.

I have never seen the O2 indicator go over 10%.

I also check occasionally the RBT indicator just to make sure I don't get too close to the limit.

90% of my diving is at the same site where the maximum depth is around the 20m/60ft mark and I am always within a 5 minute hard swim or 10 minute cruisey swim back to the safety stop area.

I always make sure I am back to the safety stop area with 50 - 70 bar remaining and hang around this area until 30 - 40 bar then slowly surface. Its a 10.5 litre tank which I think is an 85 cu ft tank. I get 45 minutes out of it with 70 bar remaining then hang around the 10 to 5 metre area slowly going up for the next 15 minutes doing the safety stop making sure I spend at least the last 5 minutes in the 6 to 4 metre area. It is always filled to 210 - 220 bar.

My logic at leaving it on 21% using 33% is if I am diving an air profile it would add extra safety to my dive. I am questioning this as one of the dive masters said my oxygen toxicity reading will be out which I agree with but figured as long as it goes nowhere near and I am talking 75% is the computers warning mark I will be right. Or am I just being stupid as it is like putting the clock forward 10 minutes so you get up on time and in the back of your mind you are always thinking yeah but I have an extra 10 minutes.

It is possible to max out your oxygen clock doing recreational profiles, but it takes a lot of dives per day to get there especially if you're not going below 60ft. The table limits I have in front of me at a ppO2 of 1.0 (.33 x 3atm @ 60ft) comes out to 5 hours over a 24 hour period, but you should check your own tables on this.

Another option is to put your computer into a conservative mode if it has that option. It'll still track the oxygen clock correctly, but also will add the safety buffer with regard to NDLs. Putting my Oceanic into conservative mode when diving EAN32 gives about the same NDLs as diving air in normal mode.

At 20m breathing a 33% O2 mix, your PPO2 will be 1atm. The NOAA oxygen exposure table gives 300min maximum single exposure and 300min maximum 24h exposure for 1atm. It means you would have to spend 225min (3h 45min) at your greatest depth to go over 75% CNS risk. Given your 45min dive duration, it means 5 dives at maximum depth in a single day.

With all this in mind, I think you will be OK regarding oxygen toxicity risk.

How about you put that thing in gauge mode and cut some tables for a while? It will help you understand your question more thoroughly and lets you decide what amount of conservancy to aply to your profile. A lot of divers use tables exclusively and use a puter set to the correct gas to have a record of what really happened on the dive, for comparison to what you thought would happen.
Eric

The rate of DCS using air is already so low that using nitrox only reduces the risk by less than a percentage point. It won't make you "safer" by using air tables or setting your computer to 21%. Use the computer for the correct setting so you will know what your actual ndl is.

There have been no benefits found when using nitrox to stay within air NDL vs air alone at those same NDL.

scubastingray:

There have been no benefits found when using nitrox to stay within air NDL vs air alone at those same NDL.

Click to expand...

not sure if its a sugar pill or placebo effect, I always feel better after a dive using nitrox. i usually dive a 29% blend which keeps me above 130 to 140 feet that'll cover all my diving needs locally.

some folks around here do set there watches to air, then dive nitrox to be conservative.

I hate to bring a shadow to the discussion, but..... God forbid there is an accident. Responders need the info on your puter or depth gauge device to get an idea what happened to you and how to treat. If you are diving 33% and your puter is set on 21%, your incident will become the next thread on SB subjected to endless speculation. Police yourself, do not rely on a false puter input to do it for you.

I cut tables and use a puter as a real time bailout device should the need arise. Puters have many uses, but they should never replace your brain.
Eric

Not sure what dive site in Sydney you could be doing based on the depth and the fact that you are within 5 mins of safety stop depth at all times (only 2 or 3 sites get to this depth anyway). As you will never get close to deco on this site based on what you have written, there is little point in not setting the computer to the right oxygen percentage.

Thanks for all the replies.

I liked waterpirates point. Very valid. Ok so setting my computer to the correct mix it is just in case a problem happens.

clownfish I mainly dive at Bass Point in Shell Harbour in particular The Gutter. You are normally only 2 or 3 hundred metres away from the entry/exit point.

My fins are Mares Quattro's so when in a hurry they tend to move me along quite rapidly .