Air Vs Nitrox

[Hatul]

It makes no difference what gas is in the tank; you get the same number of breaths. A cu ft is a cu ft.

 

[Jim-SAR]

i said, i will be doing 1 dive using 1 tank only.

last time i was using air, other people on the boat were using a nitrox, we almost assend and desend on the same time !! so why nitrox ? we were doing safty stop togeather !!

i though having more training might raise my bottom time, but im kinda hate the idea that im only allowed 20 to 30 min on most of the dives i do!!

This has already been answered/discussed, but I'll try to give a different view to help you separate the issues in question. Bear with me...

First - Take a Nitrox course and get Nitrox certified BEFORE you use it for diving. This will explain everything you've been asking.

Why Nitrox?
Part-1: Gas is made up of 100% mixture of different gases. Normal air is considered 21% O2 + 79% N2, and could also be referred to as EAN21 if you wanted. When you see EAN32, and EAN36, it's just showing you the O2 percentages. The remaining percentage of the 100% mix is Nitrogen at 68% and 64% respectively. While diving, and breathing a compressed gas mixture, these percentages factor into the Partial Pressure of each individual gas. Dalton's Law defines how all of this works ( Partial pressure - Wikipedia, the free encyclopedia ). You multiply the partial pressure by the TOTAL number of atmospheres to calculate your PPO2 (oxygen partial pressure). PPN2 is the Nitrogen partial pressure.

*** This PPO2 is what limits your allowable depth due to Oxygen becoming toxic to the central nervous system at higher partial pressures. This is where the numbers PPO2 of 1.4 and 1.6 come from, and this is how you calculate your MOD (maximum operating depth) for a given gas mixture. If you exceed these PPO2 limits, then you risk a central nervous system hit (CNS Hit), which can cause convulsions leading to losing your reg and then breathing in water during subsequent convulsions. NOT GOOD!!!

Part-2: When we dive, our body tissues absorb Nitrogen. This is caused by the difference is pressure between the PPN2 and our tissue compartment pressures. When the PPN2 is higher, we absorb Nitrogen through our lungs, and it's transported to cells/tissues throughout our bodies. When our PPN2 is lower, the Nitrogen comes out of the cells/tissues, and "hopefully" stays in solution until it reaches our lungs where it can be expelled and exhaled out. The Air and Nitrox dive tables are based on this process of being able to off-gas at an acceptable/safe rate as long as you stay within the limits of the tables.

*** If you ascend too fast, or miss a decompression requirement, then there will be a very high difference between the low PPN2, and the tissue compartment pressures you've built up. When the Nitrogen is expelled from the cells/tissues at an excessive rate, the micro bubbles can join together and form larger bubbles, and if bad enough, those combine to form very large bubbles. The medium bubbles are what cause the "Bends" (Decompression Sickness), and the large bubbles are what cause Arterial Gas Embolisms (AGE) and this is what kills divers.

Part-3: The lower PPN2 results in less Nitrogen being absorbed by our bodies than if we dove the same profile on air. Since the N2 loading is what limits our bottom time, it allows us to stay down longer. Some divers use Nitrox, but still go by the Air tables to provide a greater safety margin against the bends/DCS.

Inhalation Cycles:
This is basically your breathing rate, and as it's been pointed out, your breathing is triggered by the buildup of Carbon Dioxide (CO2) in your lungs. For tanks of the same size, you will consume Air at the same rate you would consume Nitrox.

Do you know you SAC rate? (Surface Air Consumption Rate) This is the key to knowing how much gas you will need for a given dive profile. Assuming that your SAC was calculated from a dive that involved a similar amount of physical activity. If you have your dive log, and you have some ACCURATE entries, then you can use these to get an avarage SAC rate. Then you can plan your dives based on you DCR (Depth Consumption Rate) which accounts for the usage at depth.

Use your log to fill in the formula on this web site: DiveNerd - SAC Rate / RMV Calculator, Imperial Units This gives you the SAC rate once you select calculate, and then a page is displayed showing the math behind it. Here's some formulas you can also use if you don't have a computer handy while you're planning. I'm posting some links instead of typing it all out

Surface Air Consumption Rate

How to Calculate How Long Your Air Will Last on a Scuba Diving Using Your Air Consumption Rate (Imperial, psi) (the tank conversion is CUFT/PRESSURE)


Summary:
Your bottom time is based on Nitrogen loading of your tissues and remaining within the NDL of the gas your breathing, AND/OR it is limited by the amount of gas required for the dive. The Nitrox table may say you can stay at a certain depth for 50 minutes, but if you're an air hog, then the volume of your tank might only last 25 minutes before you have to start your ascent.

Take some classes and get a good diving education, and it will answer a lot of your questions now and in the future.

 

[swimmer_spe]

So, if a diver says that you should get Nitrox so that you can dive longer, but you are an air pig and currently end your dive within the NDL and have less than 1000 psi on a 120CF tank, Nitrox is a waste?

What is a good SAC? What is a bad SAC?

I suck my steel 120 HPs as fast as a DM on AL 80s

 

[dfx]

So, if a diver says that you should get Nitrox so that you can dive longer, but you are an air pig and currently end your dive within the NDL and have less than 1000 psi on a 120CF tank, Nitrox is a waste?

Not a complete waste, as it still increases your safety margin with regards to DCS. You may still bump into your NDL when doing repetitive dives - nitrox would allow you to extend the NDL in those cases.

 

[Jim-SAR]

So, if a diver says that you should get Nitrox so that you can dive longer, but you are an air pig and currently end your dive within the NDL and have less than 1000 psi on a 120CF tank, Nitrox is a waste?

What is a good SAC? What is a bad SAC?

I suck my steel 120 HPs as fast as a DM on AL 80s

Sounds like you should start working on a doubles setup

Average SAC Rates: http://www.scubaboard.com/forums/ba.../37301-what-your-average-air-consumption.html