Nitrox and partial pressures

[tarponchik]

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

Sure. Women who own a dozen of $28K+ Prada purses did not overpay too.

 

[lostsheep]

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!

OP, I commend you for actually learning your computer, WAY too many people omit this.

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.

If this is true, that is insane. Lying to your computer (even if it’s supposed to be more conservative) is bad enough, your computer lying to you is unacceptable.

Sure. Women who own a dozen of $28K+ Prada purses did not overpay too.

That’s an odd strawman but okay.

I’m not a woman and don’t know anything about purses. But I do know that my shearwaters don’t lie to me, they use a standard buhlman algorithm that matches desktop planning software , they are easy to read, they have pretty colors to choose from, they are customizable, and will grow with me. Perhaps the most important thing about shearwaters is they make you look cool, you know, kinda like an expensive purse

ETA: the whole point of the previous statement is I feel like a real computer is worth every penny.

 

[tarponchik]

ETA: the whole point of the previous statement is I feel like a real computer is worth every penny.

And the more penny you've spent the more real if feels, right?

 

[lostsheep]

And the more penny you've spent the more real if feels, right?

Now you’re just being obtuse or trolling.

No you’re completely wrong and clearly don’t know me, I’m a cheap bastard. I fought getting a shearwater for years because the cost was high. Then I took a class that required a computer with user set GFs , gas switching, and detailed dive logs for post dive analysis. That pushed me over the edge and I broke down and bought one. I now own three if you count the CCR controller.

A Perdix is most likely overkill for the OP but some of the lower end models are not and the price isn’t outrageous.

I don’t know why you have an axe to grind against shearwater but know that you are clearly in the minority, most everyone I know loves their shearwater.

You may have the last word…troll on.

 

[tursiops]

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.

True, but....

Sure. Women who own a dozen of $28K+ Prada purses did not overpay too.

And the more penny you've spent the more real if feels, right?

....you sound really jealous.

 

[johndiver999]

The discussions alway have to migrate to the same themes, with computers, BC's, pony bottles etc.

There is nothing wrong with the computer and a new diver who does a couple tourist dives certainly doesn't need a more expensive computer. It is simply an issue with learning how to use his computer; I've had the same damn thing happen to me more times than I can remember.

You just gotta set the defaults on the Oceanic computer correctly.

 

[tursiops]

The discussions alway have to migrate to the same themes, with computers, BC's, pony bottles etc.

There is nothing wrong with the computer and a new diver who does a couple tourist dives certainly doesn't need a more expensive computer. It is simply an issue with learning how to use his computer; I've had the same damn thing happen to me more times than I can remember.

You just gotta set the defaults on the Oceanic computer correctly.

In short, RTFM.

 

[aedwards92]

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.

Is this an Oceanic thing or do you know of other brands that do the same?

 

[tarponchik]

Now you’re just being obtuse or trolling.

No you’re completely wrong and clearly don’t know me, I’m a cheap bastard. I fought getting a shearwater for years because the cost was high. Then I took a class that required a computer with user set GFs , gas switching, and detailed dive logs for post dive analysis. That pushed me over the edge and I broke down and bought one. I now own three if you count the CCR controller.

A Perdix is most likely overkill for the OP but some of the lower end models are not and the price isn’t outrageous.

I don’t know why you have an axe to grind against shearwater but know that you are clearly in the minority, most everyone I know loves their shearwater.

You may have the last word…troll on.

I am as cheap bastard as you are, but this is all about priorities. I'd rather save to go on an expensive trip with a cheapo comp than buy a bunch of pricey comps only to cut my travel expenses as result. I never care how I look or what the guys on the boat think about my gear, so I always go for the lowest price when picking up stuff. I do not need any extra functions in my comp (so in this case I believe you simply paid for the brand, just like those who buy Prado do); I do not need extra pockets or a 2nd tank strap on my BCD, and I still dive in a shortie I bought used for $50 in 2015 and booties I bought in 2002. But I travel the world.

I do not have anything against Shearwater except that you can find comps that do the same thing for a lower price. Sometimes price correlates with quality but most often it does not. And many people will simply buy anything if it is new and expensive.

 

[boulderjohn]

I do not have anything against Shearwater except that you can find comps that do the same thing for a lower price.

Pray be specific.

You named a computer with an algorithm that most experienced divers consider to be defective and without many of the features of a Shearwater. It has a lower price, but it does not do the same thing.

Or did you mean that for the average OW diver doing relatively shallow dives well within NDLs (like the ones in this case), any cheaper computer will do? If so, then I can see your point.