Abstract: The apparatus of the invention comprises a measurement chamber fed with gas under pressure by a feed nozzle, a measurement chamber having an outlet nozzle spaced apart from the feed nozzle and including a pressure take-off point disposed between the feed nozzle and the outlet nozzle. According to the invention, a separating partition extends inside the measurement chamber overlying the pressure take-off point and co-operates with the wall of the measurement chamber to determine a volume which is closed in airtight manner at its outlet nozzle end and which is open at its inlet nozzle end in the vicinity of the inlet nozzle.