Over the past 20 years there have a number of instances where nuclear power plant operators have discovered gas voids-typically air but occasionally other gases such as undissolved hydrogen- in fluid systems whose function is important to reactor safety. These systems have included emergency core cooling systems, decay heat removal systems, and containment spray systems. The amount of gas has, in some cases, been sufficient to call into question the operability of the systems; had they been needed. The automatic initiation of a system with a gas void present may lead to gas binding of its pumps, or destructive water hammer. The sources of the gas have been various and not readily controlled. The need for licensees to manage gas accumulation has been formally identified in an NRC generic letter (1). The letter points out a need for continuous monitoring, to detect and quantify gas voids in these systems, thereby to ensure their availability in accordance with design basis requirements. The letter further notes that periodic functional tests of the critical systems will not provide the required assurance of operability, if a periodic test finds a system's functionality questionable because of gas accumulation, the question of how long its operability has been compromised is unanswered. The system described in this paper-the Linewatch Gas Void Detection System- addresses these issues definitively It provides the means to detect the onset of void formation in any one of multiple pipes in multiple systems on a continuous basis and, following void formation, the means to quantify the amount of these voids, again continuously.