Application of a Relative Humidity Sensor for Monitoring Water Vapor Concentration inside Enclosures

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The presence of water can have adverse effects on the performance of mechanical and electrical hardware. Standing or condensed water can cause shorting or grounding issues in the short term and can accelerate corrosion in the long term. High humidity can also accelerate corrosion as well as cause swelling of electrical components, seals, or composite materials. Water exposure data can be used to actively intervene prior to degradation(e.g. to fix a water leak) or to calculate the remaining life of a system that has been exposed to water or high humidity. The U.S. Army has recognized the importance of environmental monitoring and failure prediction in weapon systems to ensure readiness, to enhance safety, and toimprove weapon performance. Since 2000, Picatinny Arsenal has sponsored the development of specialized environmental monitoring sensors with on-board diagnostics in various new weapon/ammunition systems. In this article we present an effort at the Pacific Northwest National Laboratory to develop an asset health monitor to assess water concentration inside the storage container of an artillery shell. Desiccant is used in the container to maintain the water vapor concentration below 5000 parts per million; a higher water concentration would indicate container leakage or the need to replace the desiccant. Various types of water sensors, including standing water indicators, corrosion indicators, and humidity sensors, were identified and categorized according to type of water exposure, accuracy, and response time. A commercially available relative humidity sensor was identified as a cost effective approach to measuring water vapor concentration inside the container. Relative humidity can be converted to waterconcentration using the water vapor saturation pressure at ambient temperature. Using diurnal temperature simulations, it was determined that the accuracy of the relative humidity sensor would be unsatisfactory in high temperature/low humidity situations where a small change in relative humidity results is a large change in water vapor concentration. To overcome this obstacle, a strategic sampling technique was developed to increase the accuracy of the water concentration measurement that is based on relative humidity sensor data. The relative humidity sensorwas integrated into the Remote Readiness Asset Prognostic/Diagnostic System (RRAPDS), which is an asset health monitor developed by PNNL to measure environmental conditions in munition containers. RRAPDS will be secured to the container with a probe that extends into the container to monitor internal temperature and humidity. RRAPDS includes a microcontroller to process environmental data and execute diagnostic routines. Thispaper will describe the approach that was developed to estimate water concentration using relative humidity sensor data and describe the sampling technique used to maximize the accuracy of the water vapor concentration assessment.