A portable level sensor for a flexible bag for collecting a fluid and/or a solid, and a system and method for sensing the content level of a flexible bag using the portable level sensor. The sensor comprises a flexible resistor element with an electrical resistance that changes in dependence upon its degree of flexing, means for attaching the resistor element to an outside surface of the flexible bag and means for periodically polling the electrical resistance of the resistor element and transmitting to a receiver a signal which varies in dependence on the said electrical resistance.