Nitrox and partial pressures

[maudiver216]

Hello all,

I went diving on Saturday, April 26th with Jupiter Dive Center on an afternoon dive. I was given two cylinders of nitrox with approximately 36% of oxygen each and I set my Oceanic Pro Plus 3 computer accordingly as well as setting a conservative partial pressure of 1.40 as I was taught in the nitrox class. Both dives were expected to bottom out at a maximum depth of 75 feet at deepest portion of the reef. My first dive went well. After a 1 hour surface interval, we descended once more to the reef. However, my computer was beeping at me every time that I went below 60 feet, showing me that I was reaching my partial pressure of 1.40. I tried to stay above 60 feet the rest of the dive, but I was concerned that I would not be able to descend further to get a closeup view of the reef. What would be the reason that I was reaching the PP02 of 1.40 at 60 feet, while the other divers (who i assumed were breathing the same 36% mixture as everyone had either rented the cylinder from the shop or had them filled there) were able to descend below me to about 70+ feet? This really took out the fun out of the second dive.
I would be grateful with any assistance with the mechanics behind this.

Thanks!

 

[ps444mike]

might wanna check your math.. 1.4 and 36 % is upto 95'

 

[johndiver999]

Almost for sure, your computer has a setting that automatically resets the nitrox mix to 50% as a default for the second dive. It is supposed to protect the manufacturer from liability, I assume, but it is a royal PIA.

There is a default 50 setting or something, I think you want to turn that option off, which will mena that the computer will ASSUME that you are using the same mix as the subsequent dive that day, which is more often than not, a correct assumption. Regardless, it is good practice to make sure you check what mix the computer is set at before each dive.

 

[inquis]

From your dive computer manual:

When the FO2 50% Default is set On and FO2 is set for a numerical value, 10 minutes on the surface after that dive the FO2 will be displayed as 50 and further dives will be calculated based on 50% O2 for oxygen calculations and 21% O2 for Nitrogen calculations (79% Nitrogen), unless FO2 is set before the dive.

In other words, it reset your mix to EAN50 behind your back, which has a 1.4 PO2 (alarm limit) at 60 ft. I would turn off that asinine "feature".

 

[broncobowsher]

Computer "safety" feature that sucks.

 

[rongoodman]

Good excuse to toss the computer and buy a Shearwater. You're lucky it didn't get mad and lock you out for a couple of days.

 

[tarponchik]

Good excuse to toss the computer and buy a Shearwater. You're lucky it didn't get mad and lock you out for a couple of days.

Why overpay? Cressi Leo 2.0 is 2.5x less expensive than the cheapest Shearwater comp and if you set it to 1.4/36% it will keep this settings untill you make a change.

 

[rongoodman]

With a proprietary algorithm which will lock you out if you make it sad.

 

[tursiops]

A great example of forgetting to RTFM.

 

[lostsheep]

Why overpay? Cressi Leo 2.0 is 2.5x less expensive than the cheapest Shearwater comp and if you set it to 1.4/36% it will keep this settings untill you make a change.

I own several shearwaters and did not overpay for any of them