This is the piece (shown in the website) where different gases are mixed (as necessary) especially in an operating theatre context ; they are also used by scuba divers. The gases can be switched on and off as required without any need to touch the cylinder. If there is no regulator the gas pressure is that of the cylinder and directly on the blender.

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Note added at 6 hrs 54 mins (2004-10-05 21:02:57 GMT)
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http://www.perfusion.com.au/CCP/Perfusion Equipment/Blenders...

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Note added at 6 hrs 56 mins (2004-10-05 21:05:24 GMT)
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http://www.ohmedamedical.com/index.cfm?act=product&subact=pr...
The M2100 Air/O2 Blender blends compressed air and oxygen, delivering pressurized gas at precise concentrations. The blender is suitable for a wide variety of applications, including routine respiratory therapy procedures and more sophisticated life-support procedures.


http://www.rsvinfo.com/managing/spag2/spag2.html