In February's HEJ--in the first 'half' of a two-part article focusing on the design, installation, and testing of medical gas systems for mobile units, medical gas specialist, Geoff Dillow, examined some of the design and construction limitations particular to such units, and how these can the impact on the equipment's compliance with HTM guidance. In this second part, he considers some of the key requirements for effective testing of mobile unit medical gas installations.