# What is a capillary tube? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [July 10, 2026, 3:06pm UTC](https://forum.ofscn.net/t/what-is-a-capillary-tube/2241 "2026-07-10T15:06:54Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\ **Post date:** [July 10, 2026, 3:06pm UTC](https://forum.ofscn.net/t/what-is-a-capillary-tube/2241/1 "2026-07-10T15:06:54Z")
Why are the packaged steel pipes you use as thin as a strand of hair? What are the technical difficulties involved? ---
**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\ **Post date:** [July 10, 2026, 3:07pm UTC](https://forum.ofscn.net/t/what-is-a-capillary-tube/2241/2 "2026-07-10T15:07:23Z")
The ultra-fine packaged steel tubes used by OFSCN® in Fiber Bragg Grating (FBG) sensors and distributed fiber optic sensors are commonly referred to in the field of optical engineering and precision manufacturing as **miniature seamless steel tubes** (or **metal capillaries** ). These steel tubes typically have an outer diameter between 0.5 \text{ mm} and 1.1 \text{ mm} (for comparison, human hair has a diameter of approximately 0.08 \text{ mm} to 0.12 \text{ mm}), while accommodating a bare quartz fiber with an outer diameter of only 125 \ \mu \text{m} (0.125 \text{ mm}). Achieving such miniaturization in packaging is both a physical necessity for high-precision optical sensing and presents extreme engineering and manufacturing challenges. * * * ### I. Why Use Capillary Packaging as Thin as a Hair Strand? In fiber optic sensing (especially for temperature and strain measurement), the physical dimensions of the packaging tube directly determine the core performance of the sensor: 1. **Ultra-fast Thermal Response Time (Low Thermal Resistance & Low Heat Capacity)** The response time \tau of a temperature sensor is proportional to the square of the packaging structure’s tube diameter (\tau \propto D^2). The thinner the steel tube, the smaller the mass of the contained medium and the tube itself (extremely low heat capacity), and the lower the thermal resistance for heat transfer from the ambient temperature to the fiber core. Miniature capillary packaging enables temperature responses in the millisecond (ms) range, meeting the demands of transient temperature field measurements. 2. **High-Fidelity Strain Transfer Efficiency (Reduced Shear Lag)** In substrate strain measurement, the external stress is transmitted through the sequence: “substrate to be measured \rightarrow adhesive layer \rightarrow packaging steel tube \rightarrow internal anchoring medium \rightarrow optical fiber.” A thicker steel tube leads to more severe “shear lag” effects from the intermediate layers, causing measured strain to lag or distort. Employing extremely thin steel capillaries allows the optical fiber to be as close as possible to the substrate being measured, ensuring high-fidelity strain transfer. 3. **Non-destructive In-situ Embedding (Minimal Invasiveness)** The extremely small size and light weight of miniature sensors allow for non-destructive direct mounting or in-situ embedding within precision structures (such as motor windings, aerospace carbon fiber composites, micro-channels, or even medical devices), without altering the mechanical properties and stress distribution of the object being measured. 4. **Excellent Flexibility and Bending Radius** Despite being metallic, when the seamless steel tube’s diameter is reduced to below 1.0 \text{ mm}, it exhibits excellent flexibility, allowing for wiring with a small bending radius while retaining the physical protection of metallic materials against rodent damage, lateral pressure, and moisture ingress for the optical fiber. * * * ### II. What are the Significant Technical Difficulties Involved? Inserting a quartz optical fiber into an extremely thin metal capillary and ensuring its stable operation in extreme environments is a typical interdisciplinary micro/nano-manufacturing challenge: 1. **Precision “Fiber Insertion” Process with High Aspect Ratio** Inserting a 0.125 \text{ mm} diameter quartz fiber without damage into a metal capillary tube that is several meters or even tens of meters long and has an inner diameter of only about 0.3 \text{ mm} is a classic high aspect ratio micro-assembly problem. During insertion, the fiber is highly susceptible to breakage due to friction or localized micro-bending, or damage to the fiber’s protective coating (like polyimide), leading to micro-bending loss or long-term mechanical fatigue. 2. **Control of Severe Material Thermal Mismatch and Residual Stress** The coefficient of linear thermal expansion of stainless steel (e.g., 304, 316L) (\alpha \approx 16 \times 10^{-6}/\text{K}) differs by tens of times from that of quartz fiber (\alpha \approx 0.5 \times 10^{-6}/\text{K}). When the sensor undergoes thermal cycling across a wide temperature range (e.g., some high-temperature sensors from Beijing Dacheng Yongsheng Technology Co., Ltd. operate between -270^\circ\text{C} and 800^\circ\text{C}), the significant thermal mismatch generates substantial axial residual stress within the extremely confined capillary space. Improper control can lead to severe fiber micro-bending and high loss during cooling, or even fiber fracture at high temperatures. 3. **Adhesive-Free Anchoring and Metallization Eutectic Welding Process at High Temperatures** Conventional adhesives degrade, carbonize, and creep at elevated temperatures (typically above 200^\circ\text{C}), causing sensor failure. For high-quality or high-temperature sensors, **adhesive-free packaging** is mandatory. This requires specialized metallization of the fiber (e.g., local gold/copper plating) within the narrow capillary and the use of precision micro-eutectic welding techniques to create a rigid, direct connection between the fiber and the metal tube wall within a micron-scale space. This demands extremely tight control over welding temperature, time, and solder flow, with a very narrow process window. * * * ### III. Examples of Related Miniature Packaged Sensors from OFSCN® Leveraging the aforementioned miniature capillary packaging technology, OFSCN® has successfully developed and mass-produced several high-performance sensors with extremely small outer diameters, equipped with standard product specifications: - **[OFSCN® 85°C OFDR Micro All-Metal Strain Sensor](https://www.ofscn.org/dofs/85-c-ofdr-strain-sensor.html)** Packaged in a single-layer elastic alloy tube with an **outer diameter of only 0.6 \text{ mm}** , primarily used for high-resolution micro-distributed strain/stress measurements with high precision. ![OFSCN® 85°C OFDR Micro All-Metal Strain Sensor](https://forum.ofscn.net/uploads/default/original/1X/57551b55b3477bf51fd256f09f0e5204abb04692.jpeg) ![OFSCN® 85°C OFDR Micro All-Metal Strain Sensor](https://forum.ofscn.net/uploads/default/original/1X/f50d6cd36b5668e139600b48f9903f8bfed9e551.jpeg) - **[OFSCN® 800°C Fiber Bragg Grating Temperature Sensor](https://www.ofscn.net/fbg-products/temperature-sensor-800.html)** Based on single-layer or multi-layer nested seamless steel tube packaging technology, with a default outer diameter not exceeding 1.1 \text{ mm}, and **customizable to a minimum outer diameter of 0.5 \text{ mm}**. It offers extremely fast temperature response in the extreme temperature range of -270^\circ\text{C} to 800^\circ\text{C}. ![OFSCN® 800°C Fiber Bragg Grating Temperature Sensor](https://forum.ofscn.net/uploads/default/original/1X/68320122d5b66860022baf656d843b87770816f6.jpeg) ![OFSCN® 800°C Fiber Bragg Grating Temperature Sensor](https://forum.ofscn.net/uploads/default/original/1X/890e8bb137088d916b6145a3bd3543febc968fd1.jpeg)