3.1. Rotary Piston Meters
3.1.1. Meters should include upstream and downstream static pressure tappings of 3 to 5 millimetre diameter to permit measurement of the pressure drop : the pressure measured upstream will constitute the reference pressure. ...
3.2. Turbine Meters
3.2.1. The meters must incorporate a pressure tapping permitting, if necessary indirectly, the determination of the pressure immediately upstream of the turbine wheel as a reference pressure.
3.2.1.1. If there is a device for throttling the gas flow upstream of the rotor, the meter may incorporate, as well as the pressure tapping required by item 3.2.1 of this Chapter, another pressure tapping immediately before the throttle to permit determination of the pressure drop across the throttling device.
3.3. Pressure Tappings
3.3.1. These pressure tappings must be provided with means for closing them.
3.3.2. The pressure tappings for the reference pressure must be clearly and indelibly marked "pr" and the pressure tapping should be indicated as "p".
