...entrainment meter according to the pressure compensation method according to DIN EN 12350 (pressure air meter, ASTM C231 Type B) As long ago as the 1940s it was known that the targeted introduction of air voids makes hardened concrete more resistant to freeze-thaw and freeze-deicing salt attacks. Therefore, the air content of the fresh concrete and its correlation to the air content of the hardened concrete must be known prior to the installation of the fresh concrete. Following the early years where only the volumetric measurement (rolling method) had been used, Klein  published the possibility to determine the air content by the application of the Boyle-Mariotte law and presented the first "pressure air-meter". Type A with a constant pressure and Type B with a decreasing pressure were developed from that. Type B has prevailed and, despite unclear reliability and accuracy (see DIN EN 12350-7, point 8.2: Accuracy of the pressure compensation method) found its way into the European standard. The causes of incorrect measured values and the requirements for reliable measured values, as well as their implementation in a further development are shown in the following report.