Independent Twins Are Not Doubles

In the U.S., twins is the word usually applied to two cylinders whose gas contents are not connected together in any way. Independent twins systems are especially handy when traveling and only rental single cylinders are available. Doubles is the word usually applied to two cylinders when they ARE connected together via a manifold to form a single gas source. Outside the U.S. the words 'twins' and 'doubles' may be used interchangeably. Describing twins as independent is a bit redundant, but does emphasize that they are not connected. Describing doubles as independent refers to using them with the manifold isolator closed during normal operation. Keeping two different gas mixtures in either twins or isolated doubles is considered an unsafe practice by many because of the risk of inadvertently breathing the wrong mix.