LVDT Linear Position Sensors
Subsea Measurement Applications
Subsea Water Conditions
Subsea environments are one of the most difficult areas for any sensor to perform, especially when performance life of as much as 20 years is expected in an underwater application.
In subsea environments, seawater depths of 15,000 ft can exert external pressures of approximately 7,500 psi on a sensor. Many sensors fail over time in high pressure deep sea environments, causing undue expense related to sensor replacement. Often, the maintenance service cost in replacing the sensor is more than the sensor itself. That’s why many applications require extended sensor life as part of operating specifications.
Seawater also attacks sensor metals at varying water depths, resulting in corrosion accelerated by the different levels of oxygen, temperature, pH, chlorinity, biological activity, electrical conductivity and velocity flow rates present in seawater at various depths. Corrosion can occur in the form of pitting, crevice or intergranular that leads to sensor failure.
Stagnant or polluted waters are additional triggers that often promote sulfate-reducing bacteria that can affect sensor materials’ performance. The high electrical conductivity of seawater promotes macro cell corrosion and increases galvanic corrosion, which accelerates temperature rise as well, further promoting corrosion.
Microbially-induced corrosion also is a very serious problem that affects sensor operation based on different service conditions and materials used in sensor construction, especially low-grade austenitic stainless steels. It is a corrosion process involving material degradation that normally occurs on welded joints and leads to weld failure if not checked and treated in time.
As a result of the effects of pressure and seawater on sensors, subsea applications pose special challenges for reliable operations. Depending upon the temperature, salinity, oxygen levels and seawater depths, the LVDT (linear variable differential transformer), when hermetically sealed and constructed of special alloys, is often the only technology that can deliver accurate and reliable performance in subsea conditions.
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