Why is there nitrogen in a scuba tank?

Answer 1

It varies. Divers use different breathing gases depending upon their needs.

For standard air, it is 21% oxygen and 79% nitrogen.

For nitrox, it can be any higher mix of oxygen, but typically either 32% or 36% oxygen, and the balance nitrogen.

For trimix, you can have almost any combination of oxygen, nitrogen and helium to suit your depth, and manage your risk to oxygen toxicity and nitrogen narcosis.

Answer 2

Because that is the main chemical in the air humans like to breath. Our atmosphere (near the ground anyway) is made up of 71% nitrogen. Compressed air in a Scuba tank is the same air you are breathing right now just without the oils and moisture.

If you replace some of the Nitrogen with Oxygen that is called Nitrox.

http://en.wikipedia.org/wiki/Nitrox

If you replace some of the Nitrogen with Helium that is called Trimix. http://en.wikipedia.org/wiki/Trimix_%28breathing_gas%29

Addition

Air is made up of approximately 79% nitrogen. This percentage stays the same regardless of altitude.