Apart from the quality of the ingredients, the purity of the rubber mixture is crucial to the quality of the tube. The base material of the Schwalbe AV13 26 x 1.5-2.5' Inner Tube - Schrader Valve is therefore sieved at enormous pressure through seven filters before being extruded. All tubes are inserted into a mould and inflated for vulcanization to ensure constant wall thickness and high air pressure retention. All Schwalbe tubes are inflated and stored for 24 hours in order to test for air pressure retention. Afterwards each, individual tube undergoes a careful visual inspection. This special quality control helps prevent unpleasant surprises and is why, over many years, Schwalbe tubes have become highly respected for outstanding reliability by bicycle dealers.