Capacitive Sensor Hermetic Seal for Harsh Environments
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Solution Overview
Problem
Capacitive sensors used in high temperature and corrosive environments, such as turbo machinery, have a limited lifetime due to temperature gradients and corrosive gas flow paths, leading to degradation and reduced accuracy over time.
Innovation Solution
A capacitive sensor device with a platinum rhodium alloy sensor tip, ceramic insulation, and a braze joint forming a hermetic cavity, coupled with a coaxial cable that transitions to copper for reduced electrical losses, and a stainless steel casing for protection, ensuring a reliable and accurate measurement of clearance between objects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a capacitive sensor is placed in a high temperature and corrosive gas flow path to measure clearance, then the measurement function is achieved, but the sensor lifetime is limited to 100-500 hours due to degradation from heat and corrosion
Solution Approach 1:
The sensor is divided into distinct functional segments: the sensor tip (exposed to harsh environment) is separated from the signal processing unit (protected in ambient environment). The tip includes a sensor element, insulation layer, and sealing structure that can withstand high temperature and corrosion, while the electronics remain protected. This segmentation allows the sensitive electronic components to be isolated from harmful environmental factors.
Solution Approach 2:
A hermetic seal structure acts as an intermediary barrier between the sensor tip and the internal cavity. This seal prevents moisture and corrosive gases from penetrating into the sensor cavity, protecting internal components while allowing the sensor tip to remain exposed for measurement. The seal includes a sealing member that creates a moisture barrier, extending sensor lifetime in corrosive environments.
2Measurement precision
If the sensor tip is exposed to high temperature environments for accurate clearance measurement, then measurement capability is maintained, but moisture ingress through cracks in probe tips and insulation causes degradation over time
Solution Approach 1:
A hermetic seal structure acts as an intermediary barrier between the sensor tip and the internal cavity. This seal prevents moisture and corrosive gases from penetrating into the sensor cavity, protecting internal components while allowing the sensor tip to remain exposed for measurement. The seal includes a sealing member that creates a moisture barrier, extending sensor lifetime in corrosive environments.
Solution Approach 2:
The sensor cavity is designed as a hermetically sealed environment that isolates internal components from the external corrosive and humid atmosphere. By creating an inert-protected internal environment, the sensor maintains measurement precision over extended periods without degradation from moisture ingress or corrosion.
3Loss of energy
If a coaxial cable with copper center conductor is used for signal transmission, then electrical losses are reduced, but oxidation of the conductor occurs in high temperature environments
Solution Approach 1:
A hermetic seal structure acts as an intermediary barrier between the sensor tip and the internal cavity. This seal prevents moisture and corrosive gases from penetrating into the sensor cavity, protecting internal components while allowing the sensor tip to remain exposed for measurement. The seal includes a sealing member that creates a moisture barrier, extending sensor lifetime in corrosive environments.
Solution Approach 2:
The sensor cavity is designed as a hermetically sealed environment that isolates internal components from the external corrosive and humid atmosphere. By creating an inert-protected internal environment, the sensor maintains measurement precision over extended periods without degradation from moisture ingress or corrosion.
4Reliability
If the sensor components are hermetically sealed to prevent moisture ingress, then reliability is improved, but manufacturing complexity increases due to multiple sealing joints and assembly steps
Solution Approach 1:
Multiple sealing functions are merged into a single integrated hermetic seal structure. The seal combines the sealing member, insulation layer, and protective casing into one unified component assembly that provides both mechanical support and environmental protection. This reduces the number of separate sealing joints and simplifies the assembly process while maintaining hermetic protection.
Solution Approach 2:
The hermetic seal structure performs multiple functions simultaneously: it provides mechanical support for the sensor tip, electrical insulation, environmental sealing, and structural protection. By designing a multi-functional seal component, the overall device complexity is reduced compared to having separate components for each function.
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 solution extends the operational life of capacitive sensors in harsh environments, maintaining accuracy and reducing oxidation and structural failure, enabling reliable operation for thousands of hours with improved high-frequency signal transmission and cost-effectiveness.
Implementation Method 1
a braze joint is formed between the casing and the metallization to form a hollow, hermetic cavity
Implementation Method 2
an insulation layer supporting the sensor tip between the first and second ends
Implementation Method 3
capacitive sensor device... for measuring clearance between two objects
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
A capacitive sensor device (24) and a method of manufacture are provided. The capacitive sensor device (24) includes at least one sensor tip (200) that includes an electrode (212) positioned at a first end (208) of the sensor tip (200), and a stem member (214) coupled to the electrode (212) and extending toward a second end (210) of the sensor tip (200). The device (24) also includes a coaxial cable (202) including a center conductor (220), the center conductor (220) coupled to the sensor tip (200) at the second end (210), and an insulation layer (204) supporting the sensor tip (200) between the first and second end (210)s. The insulation layer (204) includes a metallization on a portion surrounding the second end (210) of the sensor tip (200). The device (24) further includes a casing (206) surrounding a portion of the coaxial cable (202), the metallization, and the coupling of the center conductor (220) and the sensor tip (200), wherein a braze joint (224) is formed between the casing (206) and the metallization to form a hollow, hermetic cavity.