Sea Technology

FEB 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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Page 12 of 72

12 st / February 2017 electrical charge is generated with a magnitude depending on the hydro- static pressure applied to it and the area over which this pressure acts. Tourmaline responds to hydrostatic pressure if electrodes are applied to its z-axis direction. By contrast, quartz does not possess a hydrostatic response on its x-, y- or z-axes. It is this unique property of tourmaline that makes it suitable for measuring under- water explosions. Leak Testing Leak testing on hermetic sensors de- termines if the structure has any cracks in its welds. The two most common testing methods are a gross bubble test and a helium leak test. A bubble test is typically provided as an end-of- line test for all sensors to ensure their structural integrity. For fine-leak test work, the helium test is performed. In the gross bubble test, sensors are sub- merged into a tank of heated fluorinert liquid. The fluorinert is maintained at an operating temperature of approxi- mately 100° C (212° F). The heated fluorinert causes any trapped gasses inside the sensor to expand and to bubble out of the sensor. The bubbles become visible during the test in the fluorinert liquid. For a more thorough investigation of hermeticity, accelerometers are sub- jected to a fine helium leak test. The fine leak test uses a mass spectrometer, where the accelerometers or their con- nectors are pressurized in helium. The sensor is pressurized to 300 psi with helium and then placed in the mass spectrometer chamber. If it does not pass this test, a sniffing test is used to pinpoint the source of leakage. In the sniffing test, a hose is connected to the mass spectrometer test chamber. The sniff hose uses a probing needle to sniff around all welds, chamber seams, the sensor connector shell and center electrical contact to pinpoint the leak. Getting the Signal Out of the Water Piezoelectric sensors operate from power supplies with either ICP® con- stant current or constant voltage line driver. A typical sensing system in- cludes a quartz ICP® sensor, a simple two-wire connection and basic con- stant current signal conditioner. Signal conditioning consists of a well-regulated 18 to 30 VDC source (battery- or line-powered), a current- regulating diode (or equivalent con- stant current circuit), and a capacitor for decoupling the signal (removing the bias voltage). A voltmeter monitors the sensor bias voltage (normally 8 to 14 VDC) and is useful for checking sensor operation and detecting open or shorted cables and connections. Cables used for signal transmis- sion are the greatest source of failure in underwater sensor design. A water- blocking compound must be used in cables to impede the axial flow of water should the outer cable sheath become cut. There are various degrees of acceptance and tests that can be performed on a cable to determine the effectiveness of the water-blocking compound. Another, and often more impor- tant, reason to water block or void fill a cable cross-section is to provide the crush resistance required to operate as specified at depth. The cable material must also be compatible with thermoset materi- als used for attachment to the sensor housing. Thermoplastic elastomers and polyurethane materials offer good bonding characteristics and are easy to mold. Sensor Configurations, Hydrotesting If the bond is not well made, the first indication is typically in the form of an intermittent bias voltage measur- "Piezoelectric sensors have a wide dynamic range and are AC coupled devices, thus ignoring any ambient or static pressure."

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