Both methods are acceptable for aerosol and liquid measurement according to ISO 8573-1 Part 2 The B2 method targets only the center of the airflow. Oil aerosols are registered but oil that sticks to the pipe wall (wall flow) is not detected. Most air compressor manufacturers still prefer this less stringent method. The B1 method examines the entire air flow to measure both aerosols and wall flow. This comprehensive test method was used on the Atlas Copco range of oil-free air compressors. Even so, no traces of oil were found in the output air stream.