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www.sea-technology.com February 2017 / st 11 underwater use include electri- cal isolation and integral over- molded cable assemblies. Elec- trical isolation is important when monitoring noise in the presence of electric-drive vessels. The cable attachment mechanism to the sensor is vital and should be hydrotested under pressure for leaks and functionality. Ex- amples include pressure sensors and accelerometers with water-blocked cables. Underwater vibration and pressure environments are very harsh, so a robust sensor design is required that exceeds the test levels. With respect to accelerometers, there are a number of important sensor characteristics that benefit from a shear mode accelerometer design, which clamps the sensing crystals between a center post and seis- mic mass. For example, a compression memory ring applies a pre- load force required to create a rigid linear structure. Under acceleration, the mass causes a shear stress to be applied to the sensing crystals. The advantage of this design is that by isolating the sensing crystals from the base and housing, the shear accelerometer excels in rejecting thermal transients and signal noise resulting from base bending effects. This is a very important feature when attaching them to relatively thin-walled vessel hull models during wave slap applica- tions, such as splash down testing of space capsules. The shear geometry also lends itself to small size, which minimizes mass loading effects on the test structure. To as- sist in weight reduction, most low-mass accelerometers are made from titanium. The accelerometer is assembled using a laser welding process to ensure good weld penetration and resulting hermetic seal. Piezoelectric pressure sensors are suited for dynamic pressure measurements, including turbulence, cavitation or explosive blast. These measurements require a fast response or rise time, ruggedness and high stiffness in order to obtain a high-frequency response. Piezoelectric pressure sensors are available in various shapes and thread configurations to allow suitable mounting for various types of pressure mea- surements. Quartz crystals are used in pressure sensors for dynamic pressure measurement to ensure stable, repeatable opera- tion. With an active surface diameter of 0.15 in. (3.2 mm) and resonant frequency between 250 and 500 kHz, well above those typically experienced in flow or cavitation, the AC coupled nature and high strength of these piezoelectric crystals allow them to measure small changes in the back- ground of very large static loads, more than 100 to 1,000 times their measurement range. For a general purpose pressure sensor with built-in elec- tronics, flush mounting of pressure sensors in a plate or wall is desirable to minimize turbulence, avoid a cavity effect or avoid an increase in a chamber volume, especially im- portant when measuring dynamic pressure on the external surface of an underwater vessel. Piezoelectric pressure sensors can also measure shock waves and bubble energy associated with underwater ex- plosion testing. For example, sensors can be structured with a volumetrically sensitive, omnidirectional tourmaline crys- tal suspended and sealed in an insulating, oil-filled vinyl tube. Integral ICP® microelectronics, by PCB Piezotronics Inc., can provide a clean, nonresonant, high-voltage output through long cables in underwater environments. They can be supplied with a sealed cable of appropriate length up to 300 m, ready to operate. Physical configurations can allow for mounting for a free field measurement or direct mount- ing to underwater structures. Tourmaline is a naturally occurring piezoelectric ma- terial in nature, and when used in underwater sensing an (Clockwise) South Bay water-block cable cross-section view. Orion crew module drop test. PCB Piezotronics Inc.'s piezo- electric pressure sensors and accelerometers.