Coated Fiber Optic Temperature Sensors for Cryogenic Accuracy
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Solution Overview
Problem
Fiber Bragg Grating (FBG) sensors lose accuracy at cryogenic temperatures below 20 K, making them unsuitable for precise temperature measurements in environments like liquid hydrogen.
Innovation Solution
A multi-layer coating for FBGs comprising a polymeric or metallic first layer, a UV-curable epoxy adhesive second layer, and a UV-transparent PTFE heat-shrinkable tube sheath, which maintains sensor accuracy and structural integrity at cryogenic temperatures by exploiting high thermal expansion coefficients and providing mechanical protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If FBG sensors are used at cryogenic temperatures below 20 K, then temperature measurement capability is maintained, but measurement precision deteriorates significantly
Solution Approach 1:
The patent changes the thermal expansion parameter of the coating materials to resolve the measurement precision deterioration at cryogenic temperatures. By selecting materials with high thermal expansion coefficients (PTFE: 100-200×10^-6/K, epoxy: 50-100×10^-6/K, polyimide: 20-50×10^-6/K), the coating's dimensional changes at low temperatures compensate for the FBG's thermal contraction, maintaining measurement precision down to 20 K and below.
Solution Approach 2:
The patent employs a multi-layer composite coating structure comprising PTFE outer layer, epoxy adhesive middle layer, and polyimide inner layer. Each layer contributes different properties: PTFE provides high thermal expansion and low temperature stability, epoxy provides strong adhesion and flexibility, and polyimide provides thermal stability and fiber protection. This composite structure resolves the contradiction by combining materials with complementary thermal expansion characteristics.
2Measurement precision
If coating materials with high thermal expansion coefficient are applied to FBGs, then measurement precision at low temperatures is improved, but adhesion between layers deteriorates
Solution Approach 1:
The patent applies local quality by assigning different functional properties to different layers of the coating. The PTFE outer layer provides high thermal expansion for precision, the epoxy middle layer provides strong adhesion to both PTFE and polyimide, and the polyimide inner layer provides thermal stability and fiber protection. This localized functional distribution resolves the adhesion problem while maintaining measurement precision.
Solution Approach 2:
The epoxy adhesive layer serves as an intermediary between the PTFE outer layer and the polyimide inner layer. It provides chemical bonding to both materials, ensuring reliable adhesion despite the different thermal expansion coefficients. The epoxy's flexibility and bonding capability mediate the mechanical stresses between layers, preventing delamination at cryogenic temperatures.
3Measurement precision
If multi-layer coating is applied to maintain accuracy at cryogenic temperatures, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent segments the coating into three distinct functional layers, each with specific thickness and material properties. This segmentation allows independent optimization of each layer's function while maintaining overall system performance. The layered structure is applied through a systematic process that manages complexity through modular construction.
Solution Approach 2:
The multi-layer coating structure serves multiple functions simultaneously: thermal expansion compensation, mechanical protection, adhesion, and flexibility. By integrating these functions into a single coating assembly, the patent avoids the need for separate components, thereby managing device complexity while achieving high measurement precision at cryogenic temperatures.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The sensor achieves high sensitivity and robustness, allowing accurate temperature measurements down to 20 K with the ability to be bent without damage, suitable for multiplexed temperature and deformation sensing in cryogenic applications.
Implementation Method 1
a second coating comprising a polymeric material adapted to be cured with UV light
Implementation Method 2
a sheath comprising a UV light transparent heat-shrinkable tube... which acts as a mechanical protection of the FBG
Data Source
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Figure 5
AI summary
The present invention discloses a temperature sensor 1 comprising a Fiber Bragg Grating 2, a first coating 3 comprising a polymeric or metallic material such as polyimide, a second coating 4, arranged externally to the first coating 3, comprising a polymeric material such as an epoxy adhesive and a cylindrical sheath 5 comprising a UV light-transparent heat-shrinkable tube 6 of high thermal coefficient at 20 K, as it is the case of polytetrafluoroethylene, arranged externally to the second coating 4, and wherein the cylindrical sheath 5 has suffered a thermal process so that it exerts a pressure or throttling in the radial direction on the outer surface of the second coating 4. This temperature sensor 1 is adapted to provide temperature measurements at cryogenic temperatures without loss of accuracy. Systems 7 including one or more temperature sensors 1 and manufacturing processes of the temperature sensors 1 are also disclosed.