Sea Technology

FEB 2017

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www.sea-technology.com February 2017 / st 13 ing between 0.0 VDC (shorted) and 30 VDC (open circuit). When evaluating such failures, cables are often removed and inspected piece by piece. In an extreme case of leakage, cable conductor con- tamination is evident. Hydrotesting is employed to ensure successful performance at operational depths. Testing verifies that the water-block cable and sensor-to-cable over-mold assemblies resist leak- age during underwater applications. The performance of the unit under test (UUT) is verified prior to being subjected to the hydrotest. The UUT is mounted into a flange that will couple with a pressure vessel, and the cables are inserted into feed through adapters. The UUT assembly is placed into the pressure vessel and the water added to the desired starting level. The water is allowed to stabilize to room tem- perature to avoid unstable pressure levels over extended soak times. A hydraulic pump is connected to the pres- sure vessel and is energized until the desired test pressure is achieved, and then the desired test duration begins. The UUT may be checked during the hydrotest by monitoring characteristics such as operating bias voltage, capacitance and/or continuity as appropriate for the sensor's perfor- mance characteristics. Summary Piezoelectric sensors have proven to provide a high dy- namic range and are well suited to measure both vibration and acoustic levels of dynamic pressure in the presence of high hydrostatic pressures. In any application involving underwa- ter sensing, consideration must be given to electrical isolation, sensor hermeticity and waterproofing of the electrical connection to the cable. Preamplified ICP® constant cur- rent sensors from PCB® provide an easy-to-use two-wire system with a high signal-to-noise ratio and the ability to drive high fre- quencies over long-distance cables. The cable should also include a water-block treatment to avoid ingress of water after an abrasion event or direct cut and to provide support to the internal wires from the hydrostatic forces at opera- tional depths. Finally, sensor survival at depth is a high-risk endeavor easily minimized through hydrotesting each sensor/cable assembly prior to use. Monitoring of bias voltage during testing can offer an understanding of performance degrada- tion during hydrostatic loading. References For a list of references, contact Bob Metz at bmetz@pcb. com. ST Bob Metz holds a B.S. in aeronautical and astronautical engineering from The Ohio State University and an M.B.A. from State University of New York. He has been an instrumentation engineer for 21 years, focusing on acoustic, vibration, pressure and force measurement using piezoelectric sensors, and is currently a director of the Aerospace and Defense Division of PCB Piezotronics Inc. "Sensor survival at depth is a high-risk endeavor easily mini- mized through hydrotesting."

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