Containers selected for a physical check must be measured prior to opening the container by an intermediate gas measuring expert. He/she is authorised to mark the container as safe to enter. Customs lays down several requirements the measurement must meet. The allowed measuring techniques are laid down. Measurements need to be checked within two hours prior to entering the container and regulations have been drawn up about which gases and vapours require checking. These gases and vapours are added to the EWS measuring process.