Annulus condition is a key information to follow up integrity of flexible risers in service. Traditional ways to measure water level in annulus are performing offshore inspection or monitoring with "passive" systems. This paper presents ISAGM (Instrumented System for Annulus Gas Monitoring) -an innovative "active" monitoring solution that enables permanent surveillance of this important informationwith the particularity of covering a wider range of riser applications thanks to a deep understanding of the kinetic behavior of the annulus with regards to pressure and gas flow. The system is autonomous, capable ofmonitoring several risers in parallel and based on the injection of nitrogen at low pressure (below 1barg). As it is not relying on the gas diffusion process from the riser, it allows a more accurate and reliable data interpretation compared to the traditional passive way. First of all, ISAGM description and principle are introduced elaborating on the two first world commercial applications, including one for a Malaysian field. Then, the results of tests on a reference bench rig and flexible pipe are presented. Finally, the challenge of performing annulus free volume measurement of riser interconnected annuli with high diffusion rate is presented through test and simulation results.