sea_dragon:
i was wondering what does partial pressure and safty factor....means. it is in the dive mode... can some one give me a better understanding... i am trying to understanding my cressi Archimedes II, 3-gauge console a little better...
A Nitrox course would answer all of these questions with a bit more detail, but I'll give it a try.
As you know from your OW class, every 33' or 10m doubles the pressure on your body. Thus at 33' you're breathing twice as much Oxygen. If on land you're breathing 21% oxygen, then at 33' you're breathing 42% oxygen. Make sense?
During the 1940s, medical practitioners started seeing problems with using 100% oxygen for long periods of time (problems = death in this case). It turns out that Oxygen becomes toxic in two ways: Pulmonary Toxicity and Central Nervous System Toxicity. The people died in the hospital because the high doses of oxygen caused the internal lining of the patients lungs to oxidize (i.e. rust and bleed if you will). The patients drowned from their own blood. To prevent this from happening, studies were done to determine how much oxygen over a particular time period would be acceptable. These studies helped us address the issues of pulmonary toxicity in diving which is discussed in a Nitrox course.
The CNS toxicity problems were another issue. Since Oxygen is metabolized, our bodies are affected by oxygen in many different ways. Because Oxygen under pressure doubles every 33', thus increasing the percentage of Oxygen in the gas with breath, we needed to determine acceptable limits of Oxygen in the gas we breath.
They use the term Partial Pressure of O2, or ppO2, to describe the percentage of Oxygen in a mix under pressure. The standards are ppO2 of 1.6 as the max, and a ppO2 of 1.4 for average dives. Mind you, I'm skipping over a ton of information provided in a basic Nitrox class, and would advise taking the class to fully understand this topic.
Your computer gives you the option of adjusting the ppO2 limits you are comfortable with. For instance, for Tech dives we train to keep the working ppO2 of our gas mixes around 1.1 to 1.2 to be more conservative, and use a ppO2 of 1.6 for our resting deco stops. These are a personal preference (and GUE standards) for these types of dives. For your concerns, these options in your computer are there to help you customize your preferences for the type of diving you want to do.
Does this help answer your question?
To fully understand the impact of what I'm attempting to describe, you should really look at taking a Nitrox course or doing some serious reading from those that truly understand this sort of thing. You'll find that what might work for one person, may not work for another. And what might work for you one day, may not work for you the next. This has to do with our limited understanding of how high doses of Oxygen impact us, and from the research done in this field. Like I said, it's not a topic best learned on the internet, and there are many levels of detail to understand IMO.
~ Jason