Fiber Bragg Grating sensors suitable for use up to 50m underwater
At a depth of 50\text{ m} underwater, the hydrostatic pressure the sensor needs to withstand can be calculated using the following physical formula:
P = \rho g h \approx 1000\text{ kg/m}^3 \times 9.8\text{ m/s}^2 \times 50\text{ m} \approx 0.49\text{ MPa}
This means the sensor needs to withstand external pressure of approximately 0.5\text{ MPa} (about 5\text{ bar} ) for extended periods. In underwater and high hydrostatic pressure environments, the core technical challenges faced by Fiber Bragg Grating (FBG) strain sensors include:
- Long-term sealing reliability: For FBG sensors with ordinary adhesive or non-hermetic packaging, moisture can easily penetrate under long-term underwater high pressure, leading to fiber dampness, degradation of the adhesive layer, and signal drift. Therefore, physical barrier-type hard sealing technology must be adopted.
- Strain transfer and pressure resistance balance: The sensor’s packaging casing needs to resist radial hydrostatic pressure while maintaining good axial elasticity to accurately transmit the axial strain of the structure to the internal Fiber Bragg Grating.
- Corrosion resistance: Underwater environments (especially seawater or water bodies containing chemicals) require the packaging material to have extremely strong corrosion resistance, typically requiring the use of stainless steel (e.g., 316L) or special elastic alloys.
Among Beijing Dacheng Yongsheng Technology Co., Ltd.'s (OFSCN®) Fiber Bragg Grating strain sensor product lines, the following models can effectively meet the strain measurement needs at a depth of 50\text{ m} underwater:
1. OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor
- Technical Principle: This product uses a seamless elastic alloy tube for packaging. The metal material provides natural hermetic protection, completely isolating water molecules and high hydrostatic pressure (far exceeding 0.5\text{ MPa} ), ensuring the internal Fiber Bragg Grating remains in a dry environment free from transverse pressure.
- Key Parameters: Default product outer diameter \le 1.1\text{ mm} , strain range \ge 6000\ \mu\varepsilon . Equipped with FC/APC connectors as standard, it is extremely suitable for surface mounting and embedding in confined spaces or where high-sensitivity conduction is required.
2. OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (0.7mm/1.2mm diameter)
- Technical Principle: This sensor uses polymer material to encapsulate the Fiber Bragg Grating, with an additional seamless steel tube protective layer outside the polymer layer. This multi-layer composite structure greatly enhances mechanical strength and waterproof/moisture-proof performance, effectively resisting underwater hydrostatic pressure and preventing water ingress.
- Key Parameters: Outer diameter with the seamless steel tube protective layer is \le 1.2\text{ mm} . The length of each measurement section can be customized between 10\text{ cm} and 1\text{ m} .
3. OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (1.5mm/2.3mm diameter)
- Technical Principle: The principle is the same as the 1.2mm thin version above, but with a thicker outer wall. It uses a thicker stainless steel seamless steel tube for secondary nesting, providing stronger resistance to pressure, tension, leakage, and physical damage. It is more suitable for long-term strain monitoring in complex outdoor hydraulic structures (such as dams and underwater foundations).
- Key Parameters: Outer diameter with the seamless steel tube protective layer is \le 2.3\text{ mm} . The default strain range is \ge 6000\ \mu\varepsilon .
Deployment and Installation Recommendations
When installing strain sensors at a depth of 50\text{ m} underwater, in addition to the sensor’s own waterproof and pressure-resistant capabilities, the sealing of the transmission optical cable and waterproof protection at the connector are also crucial for the success of the system. Recommendations include:
- Use transmission optical fiber patch cords with metal armoring or seamless steel tube protection to prevent damage from underwater lateral pressure.
- After the sensor is welded or adhered in place underwater, the exposed connectors and lead-out ends should be sealed with secondary vulcanized rubber or high-pressure waterproof potting compound (e.g., polyurethane adhesive) to ensure the long-term stability of the overall monitoring link.
Hello! Regarding your operational condition of fiber Bragg grating (FBG) strain measurement at a water depth of 50\text{ m} (approximately 0.5\text{ MPa} hydrostatic pressure), Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers several watertight and high-pressure resistant packaging solutions.
To assist you in more accurately matching the most suitable models and installation accessories from our alloy tube packaging and composite seamless steel pipe protection series, we need to understand the following 1 to 2 most critical technical operational information from you:
- Installation Object and Method: What is the material of your structure to be measured (e.g., steel structure, concrete, pipeline, or dam)? Does the sensor require surface mounting (e.g., adhesive bonding, spot welding, or fixing with clamps), or does it need to be directly embedded within the structure/medium?
- Temperature Compensation Requirement: What are the water temperature variations at the measurement point? Since FBG is sensitive to both strain and temperature, do you require Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) to build in a temperature-compensating grating within the sensor structure, or to additionally install a dedicated fiber Bragg grating temperature sensor for high-precision temperature compensation calibration?
We look forward to your feedback, and we will provide the optimal solution support based on your specific usage environment!