An apparatus for testing flexible containers traveling along a production line, the apparatus comprising: a compression assembly (14, 16) in line with the production line, the compression assembly (14, 16) having a flexible section (56, 58) for directly contacting and applying a predetermined compression over a predetermined distance to a plurality of containers as they travel by an inspection station; a sensor assembly comprising a load cell roller provided in direct contact with a container while a container is in the inspection station, wherein the sensor assembly is fixed relative to the production line, the sensor assembly being arranged to sense the force applied by the compression assembly to the container, the sensor assembly generating a signal that varies in accordance with the internal pressure of the containers as they pass by the sensor assembly; and a processing circuit configured to receive the signals from the sensor assembly and to determine the acceptability of the internal pressure of the containers; characterised in that: the load cell roller is provided in direct contact with a flexible lid of the container while the container is in the inspection station wherein the sensor assembly is configured to contact the flexible lid of each of the plurality of containers for detecting the internal pressure of the containers; wherein the load cell roller of the sensor assembly directly contacts the flexible lid of the container and the sensor assembly further comprises a load cell directly in contact with the load cell roller so that loads imposed on the load cell roller from the flexible lid of the container are directly transferred to the load cell. a compression assembly (14, 16) in line with the production line, the compression assembly (14, 16) having a flexible section (56, 58) for directly contacting and applying a predetermined compression over a predetermined distance to a plurality of containers as they travel by an inspection station; a sensor assembly comprising a load cell roller provided in direct contact with a container while a container is in the inspection station, wherein the sensor assembly is fixed relative to the production line, the sensor assembly being arranged to sense the force applied by the compression assembly to the container, the sensor assembly generating a signal that varies in accordance with the internal pressure of the containers as they pass by the sensor assembly; and a processing circuit configured to receive the signals from the sensor assembly and to determine the acceptability of the internal pressure of the containers; characterised in that: the load cell roller is provided in direct contact with a flexible lid of the container while the container is in the inspection station wherein the sensor assembly is configured to contact the flexible lid of each of the plurality of containers for detecting the internal pressure of the containers; wherein the load cell roller of the sensor assembly directly contacts the flexible lid of the container and the sensor assembly further comprises a load cell directly in contact with the load cell roller so that loads imposed on the load cell roller from the flexible lid of the container are directly transferred to the load cell.