instruments not fitted with pre-setting devices used in fixed or mobile installations for inflating motor-vehicle tyres in which a mechanical measuring system transmits the elastic deformation of a sensing element to an indicating device. They indicate the pressure difference (Pe) between the air in the tyre and the atmosphere. They also include all those parts between the tyre and the sensing element., pressure gauge - a device which indicates the pressure of a fluid usually by means of a mechanical pointer moving over a scale. Pressure gauges usually indicate pressures relative to ambient atmosphere level and may indicate above or below ambient e.g. vacuum gauges.