Is there a package that can prevent temperature changes from affecting strain measurements?
tool_0:
In the field of Fiber Bragg Grating (FBG) strain sensing, environmental temperature fluctuations can be eliminated from pure strain measurement results through specific packaging structure designs or multi-channel compensation mechanisms.
I. Analysis of Physical Principles
The Bragg reflection wavelength \lambda_B of a Fiber Bragg Grating is simultaneously affected by axial strain \varepsilon and environmental temperature change \Delta T . The basic equation for wavelength drift is:
\Delta \lambda_B = K_\varepsilon \varepsilon + K_T \Delta T
Here, K_\varepsilon is the strain sensitivity coefficient of the grating, and K_T is the temperature sensitivity coefficient (determined by the combined thermo-optic and thermal expansion effects of the fiber material). At a physical level, a single bare fiber Bragg grating cannot distinguish whether the wavelength drift is caused by stress stretching or temperature change. Therefore, temperature sensitivity stripping must be achieved through engineered packaging structures or system demodulation algorithms.
II. Self-Compensating Packaging Structures and Implementation Mechanisms
In sensor engineering design, packaging structures that can eliminate or offset temperature interference typically employ the following methods:
1. Built-in Dual-FBG Self-Compensating Structure
Within the same sensor housing or tube, two or two groups of fiber Bragg gratings are packaged:
- Strain FBG: Pre-stressed and fixed to the sensor substrate or housing, it senses both structural mechanical strain and environmental temperature changes. Its wavelength drift is \Delta \lambda_1 = K_{\varepsilon 1} \varepsilon + K_{T1} \Delta T .
- Temperature Compensation FBG: Placed in a free-floating/stress-free cavity within the same packaging chamber, it only senses environmental temperature changes and does not bear structural mechanical strain. Its wavelength drift is \Delta \lambda_2 = K_{T2} \Delta T .
The demodulation system can accurately eliminate temperature effects through wavelength differencing:
\Delta \lambda_{\text{pure strain}} = \Delta \lambda_1 - \frac{K_{T1}}{K_{T2}} \Delta \lambda_2 = K_{\varepsilon 1} \varepsilon
2. Mechanical Self-Compensation via Thermal Expansion
This method utilizes the combination of two materials with different coefficients of thermal expansion (CTE), such as different metals or alloys, to construct the sensor packaging base. When the temperature rises, the displacement caused by the thermal expansion of the main material is offset by an equal and opposite displacement due to the thermal expansion of the auxiliary material. This ensures that the mechanical strain of the fiber, held between fixed ends, remains constant under pure temperature changes.
III. OFSCN® Official Technical Implementations and Recommendations
Within the OFSCN® (Beijing Dacheng Yongsheng Technology Co., Ltd.) product system, two engineering approaches are provided to address the cross-sensitivity issue between temperature and strain: custom-built self-compensating structures and external independent compensation.
1. Built-in Self-Compensating Structure Customization
Major tubular and planar fiber Bragg grating strain sensors under the OFSCN® brand support customization of built-in self-compensating structures with temperature compensation:
- OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor
- OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (1.5mm/2.3mm diameter)
- OFSCN® Fiber Bragg Grating Strain Gauge
2. Official Engineering Application Recommendations
While various strain sensors can be manufactured with built-in self-compensating structures, it is more recommended in practical engineering applications to combine an externally mounted, independent fiber Bragg grating temperature sensor with a strain sensor for temperature compensation. The reason is that an externally mounted independent temperature compensation sensor has closer contact with the measured component, eliminating issues of heat transfer delay or uneven thermal gradients within the tube. This provides higher calculation accuracy in environments with large spans or drastic temperature variations.