Dive Computer - Partial Pressure

[Lateralus]

So I'm getting lost on what to input for partial pressure on my dive computer when diving Nitrox. I took my Nitrox course over 2 years ago and haven't dove till recently so I'm a little rough along the edges.

How do you calculate this? I'm lost after entering the Nitrox mixture.

 

[outofofficebrb]

I can't remember how to calculate it but I set mine to 1.4-1.6. I set primary for 1.4 and have 1.6 on my backup so in case I dip below momentarily. It's not a pain to turn it off/acknowledge (it's usually in my trim pockets and not easily accessible) vs my primary being on my wrist.

I hope this helps!

 

[runsongas]

are you setting pO2 limit for mod or fO2?

fO2 should match what you analyze the tank to, eg ean28, ean32, etc

pO2 max is recommended at 1.4 for most agencies

 

[Got2Go]

Leave it at 1.4, regardless of O2% in the gas.

 

[hroark2112]

The PO2 setting in your computer is just for the alarm. There is no reason to change it from 1.4.

 

[RonR]

This is where you set the point at which the computer alarms to tell you that you have reached your maximum allowable pressure of O2. The higher the % of O2, or the lower your PPO2 setting, the shallower the depth limit will be on the gas.

You don't have to calculate it, you are just setting the limit value. The PPO2 setting is the maximum partial pressure you are saying you will accept before the alarm kicks in. Too much O2 pressure can result in convulsions or other life threatening issues, so the computer tells you to head for a shallower depth.

The partial pressure of O2 is the % of O2 in the mix times the # of atmospheres pressure you are under- including the 1 ATM at the surface and roughly another ATM for each 33' or 10M of water. Values between 1.4 ATM and 1.6 ATM are generally considered acceptable, but as others have said there is no problem with leaving it at a conservative 1.4 ATM.

Some computers allow for setting multiple PPO2 limits for different gases, as a higher limit may be appropriate for a deco mix used while passively stopped, and allow for shorter deco times, while a lower one may be safer for a gas used in an active phase of the dive. But as a recreational diver, just set it to the more conservative value and stay within the depth limits. But you should understand what it is you are setting and why.

Ron

 

[Scared Silly]

As most have said one does not need to calculate the pO2 one just needs to decide on the value which ranges from 1.2-1.6 with most using 1.4.

HOWEVER, what one does need to be able to calculate is the maximum operating depth (MOD). Which requires the pO2 and the fO2. The value is often given by one's computer after setting both the pO2 and the fO2.

MOD == (pO2/fO2-1) * 33.3 feet (10 meters)

 

[GaleForceEight]

I leave mine set to 1.4 ppO2

I know based on my mix what my max depth can be; but just in case I leave the warning for 1.4 since when you get towards 1.6 ppO2 it can go horribly wrong very fast so in my opinion it's better to get the warning with wriggle room to spare as a safety buffer.

 

[boulderjohn]

How do you calculate this? I'm lost after entering the Nitrox mixture.

I am not sure exactly what you are asking, so I i will cover all basis.

1. Partial Pressure is not calculated. It is a standard safe maximum based on what you have been taught. As previous posts have correctly stated, 1.4 is by far the most common value used.

2. The nitrox mixture you enter is not calculated, either. It is measured using an oxygen analyzer. You will set your computer for whatever the analyzer says is in your tank.

Now, those two then work together. The partial pressure of oxygen increases with depth, and you don't want to get past the 1.4 safe maximum. When you input your nitrox mixture, your computer should tell you what your maximum operating depth will be with that mixture. For example, in salt water at sea level, a mixture of 32% will reach a 1.4 partial pressure at 111 feet. It is best to stay above that depth, although it is not like you will suddenly go into convulsions if you stray deeper for a while. Your computer will not be happy if you do, and it will warn you in some way to get shallower.

 

[caruso]

On a single tank 1.6 P02 setting is perfectly safe. There has never been an instance of an 02 toxicity hit on single tank recreational diving regardless of whether the setting is 1.4 or 1.6 P02. All you're doing by using the arbitrary setting of 1.4 is limiting your maximum depth for no good reason.