Oxygen economizing devices have been extensively studied, both at rest and during muscular exercise, in an attempt to increase the autonomy of a portable oxygen apparatus. The aim of this study is threefold: first, to suggest a simple method to verify in a simple way the technical accuracy of a demand flow oxygen delivery device; second, to suggest how we can monitor in a simple way the clinical efficacy of an economizer; and third, to remember that we can utilize an oxygen saving device to give a better protection than nasal prongs against the worsening of HbO2 desaturation induced by exercise.