An oxygen concentration system for generating an oxygen enriched gas, comprising: an oxygen concentration apparatus for generating an oxygen enriched gas by adsorbing nitrogen to remove the nitrogen from the air; and an ultrasonic apparatus for measuring the concentration of the oxygen in the oxygen enriched gas and flow rate of the oxygen enriched gas, the ultrasonic apparatus comprising: two ultrasonic transmission-reception devices (218, 222); characterised by two housings (25, 26) for enclosing one of the ultrasonic transmission-reception devices each and including ports to provide an inlet and an outlet for the oxygen enriched gas; and a conduit for connecting between the housings, the conduit and the housings being made of same metallic material; wherein the ports extend perpendicular to the conduit; wherein the ultrasonic apparatus is secured at one point to a housing of the oxygen concentration apparatus by a screw so as to allow a longitudinal deformation of the conduit; and wherein the distance between the first and second ultrasonic transmission-reception devices (218, 222) and the respective ends of the conduit satisfies the following relation: 0 < D < f • r 2 / c where: D: the distance (m) between the first and second ultrasonic transmission-reception devices (218, 222) and the respective ends of the conduit, f: frequency of the ultrasonic waves in the sample gas (Hz) r: inner radius of the conduit (m), and c: velocity of the ultrasonic waves (m/sec). an oxygen concentration apparatus for generating an oxygen enriched gas by adsorbing nitrogen to remove the nitrogen from the air; and an ultrasonic apparatus for measuring the concentration of the oxygen in the oxygen enriched gas and flow rate of the oxygen enriched gas, the ultrasonic apparatus comprising: two ultrasonic transmission-reception devices (218, 222); characterised by two housings (25, 26) for enclosing one of the ultrasonic transmission-reception devices each and including ports to provide an inlet and an outlet for the oxygen enriched gas; and a conduit for connecting between the housings, the conduit and the housings being made of same metallic material; wherein the ports extend perpendicular to the conduit; wherein the ultrasonic apparatus is secured at one point to a housing of the oxygen concentration apparatus by a screw so as to allow a longitudinal deformation of the conduit; and wherein the distance between the first and second ultrasonic transmission-reception devices (218, 222) and the respective ends of the conduit satisfies the following relation: 0 < D < f • r 2 / c where: D: the distance (m) between the first and second ultrasonic transmission-reception devices (218, 222) and the respective ends of the conduit, f: frequency of the ultrasonic waves in the sample gas (Hz) r: inner radius of the conduit (m), and c: velocity of the ultrasonic waves (m/sec). two ultrasonic transmission-reception devices (218, 222); characterised by two housings (25, 26) for enclosing one of the ultrasonic transmission-reception devices each and including ports to provide an inlet and an outlet for the oxygen enriched gas; and a conduit for connecting between the housings, the conduit and the housings being made of same metallic material; wherein the ports extend perpendicular to the conduit; wherein the ultrasonic apparatus is secured at one point to a housing of the oxygen concentration apparatus by a screw so as to allow a longitudinal deformation of the conduit; and wherein the distance between the first and second ultrasonic transmission-reception devices (218, 222) and the respective ends of the conduit satisfies the following relation: 0 < D < f • r 2 / c where: D: the distance (m) between the first and second ultrasonic transmission-reception devices (218, 222) and the respective ends of the conduit, f: frequency of the ultrasonic waves in the sample gas (Hz) r: inner radius of the conduit (m), and c: velocity of the ultrasonic waves (m/sec). D: the distance (m) between the first and second ultrasonic transmission-reception devices (218, 222) and the respective ends of the conduit, f: frequency of the ultrasonic waves in the sample gas (Hz) r: inner radius of the conduit (m), and c: velocity of the ultrasonic waves (m/sec).