LVIT Linear Position Sensor Subsea SS-7 Series

What Is the SS-7 and What Makes It Subsea-Rated?

The SS-7 Series LVIT (Linear Variable Inductance Transducer) from Alliance Sensors Group is a subsea-rated in-cylinder position sensor designed for hydraulic cylinder feedback at depths up to 12,000 feet or in pressure balanced oil filled (PBOF) chambers. Built entirely in 316 stainless steel with a subsea-grade Macartney connector, the SS-7 provides the same contactless inductive sensing technology used across ASG’s in-cylinder LVIT family — adapted for the extreme pressure, corrosion, and reliability demands of offshore oil and gas operations.

The sensor employs a 7 mm diameter inductive probe housed in a compact 1-inch (25.4 mm) diameter sensor head. It mounts via 3/4-16 UNF or M18 x 1.5 male O-ring port threads, fitting standard port bosses on subsea hydraulic cylinders and actuators. Measuring ranges span 25 mm to 900 mm (1 to 36 inches), covering the full stroke spectrum of subsea valve actuators, BOP cylinders, and ROV hydraulic systems.

Why Choose LVIT Technology for Subsea Cylinder Position Sensing?

No Magnet Ring Required — Simpler Subsea Cylinder Design

Magnetostrictive sensors — the traditional choice for in-cylinder position feedback — require a permanent magnet ring assembled to the piston inside the cylinder. In subsea service, this magnet ring must survive extreme hydrostatic pressure, resist corrosion from seawater-contaminated hydraulic fluid, and maintain its magnetic properties over years of unserviced operation at depth. The SS-7 eliminates this requirement entirely. Position sensing is accomplished through a target tube mounted in a gun-drilled blind hole in the cylinder rod, with no magnets, no waveguides, and no delicate internal components that could fail at depth.

For subsea equipment manufacturers, removing the piston-mounted magnet simplifies cylinder assembly, reduces the bill of materials, and eliminates a failure mode that cannot be repaired without recovering the equipment to surface — an operation that can cost hundreds of thousands of dollars in vessel time alone.

Robust Inductive Coil vs. Magnetostrictive Waveguide

Magnetostrictive sensors use a precision waveguide and time-of-flight measurement principle that is inherently sensitive to mechanical shock. In subsea hydraulic systems — where blow-out preventer rams slam closed under emergency conditions and ROV manipulators experience repeated impact loading — waveguide-based sensors are vulnerable to damage that is invisible from outside the cylinder. The SS-7’s inductive coil sensing approach has no waveguide to fracture or deform. The simple, robust coil construction withstands 1,000 g shock and continuous vibration across the 5–2,000 Hz range, providing a mechanically tougher sensing element for equipment that must operate reliably without maintenance intervention for years at a time.

Subsea Applications for the SS-7

Blow-Out Preventer (BOP) Cylinder Position Feedback

BOPs are the last line of defense against uncontrolled well events, and reliable ram position feedback is critical to confirming that the preventer has fully closed. The SS-7 provides continuous analog position output from inside the BOP’s hydraulic cylinders, giving the control system unambiguous confirmation of ram position without relying on pressure inference or proximity switches. The 316 stainless steel construction and subsea connector survive the pressure, temperature, and corrosion conditions on the ocean floor, and the contactless sensing principle is unaffected by the hydraulic medium inside the cylinder.

ROV Hydraulic Systems and Manipulator Arms

Remotely operated vehicles depend on precise hydraulic actuator control for manipulator operations, tooling deployment, and propulsion. The SS-7’s compact 1-inch diameter head and superior stroke-to-length ratio minimize the space consumed inside ROV hydraulic cylinders, while the robust coil construction survives the repeated mechanical shock that ROV operations generate. For ROV manufacturers and operators, the SS-7 provides position feedback that remains accurate and reliable across multi-year deployment cycles without maintenance.

Subsea Valve Actuators and Christmas Tree Controls

Subsea production equipment uses hydraulic actuators to control valves on Christmas trees, manifolds, and flow control assemblies. The SS-7 provides position feedback for these actuators, enabling the topside control system to verify valve position and detect incomplete strokes that could indicate mechanical problems. The sensor’s SenSet™ programmability allows the analog output to be scaled to each actuator’s specific stroke during surface testing before deployment, eliminating the need for exact stroke-to-sensor-range matching at time of order.

PBOF (Pressure Balanced Oil Filled) Installations

For equipment operating in PBOF chambers — where the sensor is submerged in compensating oil at ambient seawater pressure — the SS-7’s 316 stainless steel construction and sealed probe provide the material compatibility and pressure resistance required. The inductive sensing principle is unaffected by immersion in hydraulic or compensating oil, and the absence of internal air gaps or compressible elements means the sensor’s calibration is not affected by the hydrostatic pressure at depth.

SS-7 vs. MR-7: When Do You Need the Subsea Version?

The SS-7 and MR-7 share the same 7 mm LVIT probe technology, the same measurement ranges, and the same contactless sensing principle. The differences are in the materials and connections designed for the operating environment:

SS-7: 316 stainless steel housing, Macartney subsea connector, rated to 12,000 ft depth

MR-7: Aluminum or stainless steel housing, standard industrial connector, rated to 5,000 psig

For industrial and mobile hydraulic applications at atmospheric pressure or in factory environments, the MR-7 provides the same measurement performance at lower cost. For any application where the sensor will be submerged, exposed to seawater, or installed in a PBOF environment, the SS-7’s 316 SS construction and subsea connector are required. If you are unsure which version your application needs, ASG’s application engineers can help determine the right specification based on your operating depth, pressure, and fluid exposure conditions.

Electrical Output and Field Calibration

Analog Output Options

The SS-7 provides DC-in/DC-out operation with multiple analog outputs: 0–3 V, 0–5 V, 0.5–4.5 V, or 0–10 V voltage outputs, and 4–20 mA current output. Reverse-acting outputs are available for all voltage and current options. All outputs connect directly to subsea control modules or topside PLCs through the Macartney 4-pin subsea connector without external signal conditioning.

SenSet™ Field Programmability

The SS-7 includes ASG’s SenSet™ feature, allowing the analog output to be scaled to the actual cylinder stroke during surface testing and commissioning. This is particularly valuable in subsea applications where the sensor cannot be accessed after deployment. Setting the output endpoints during factory acceptance testing ensures the control system receives a properly scaled signal from day one of subsea operation, eliminating the need for post-deployment calibration that would require equipment recovery.

Key Specifications

Measuring ranges: 25 mm to 900 mm (1 to 36 inches) full scale

Linearity error: ±0.15% FSO typical, ±0.25% FSO maximum

Resolution: 0.025% of full scale output

Update rate: 300 Hz nominal

Operating depth: 12,000 feet (3,650 meters) or PBOF

Operating pressure: 5,000 psig (350 bar)

Shock: 1,000 g, 11 msec

Vibration: 5–20 Hz at 0.5 inch p-p; 20–2,000 Hz at 4.2 g p-p

Housing material: 316 stainless steel

Connector: Macartney MCBH4M subsea connector

Port threads: 3/4-16 UNF or M18 x 1.5 male O-ring

Operating temperature: -20°C to 85°C (-40°C to 105°C extended range)

Temperature coefficient: ≤0.015% FS/°C