In oxygen cylinder there is helium also filled with oxygen . Why not nitrogen is filled in place of helium?
Oxygen cylinders or tanks are used widely in industry as in a medicine or by divers for breathing under water. Use of such oxygen for breathing requires some limits: high pressure and high concentration of pure oxygen are harmful and dangerous for life. That is why oxygen must be diluted with another gas. Helium is used for this purpose because: 1) it is inert and not reactive (as nitrogen); 2) it is less soluble than nitrogen (it does not penetrate blood) 3) high concentrations of nitrogen are toxic or cause painful conditions after rapid penetration into bloodstreams