Distributed Fiberoptic Sensor for Simultaneous Humidity and Temperature Monitoring Based on Polyimide-Coated Optical Fibers

Abstract

Along temperature, humidity is one of the principal environmental factors that plays an important role in various application areas. Presented work investigates possibility of distributed fiberoptic humidity monitoring based on humidity-induced strain measurement in polyimide (PI)-coated optical fibers. Characterization of relative humidity (RH) and temperature response of four different commercial PI- and one acrylate-coated fiber was performed using optical backscattering reflectometry (OBR). The study addresses issues of temperature-humidity cross-sensitivity, fiber response stability, repeatability, and the influence of annealing. Acrylate-coated fiber exhibited rather unfavorable nonlinear RH response with strong temperature dependence, which makes it unsuitable for humidity sensing applications. On the other hand, humidity response of PI-coated fibers showed good linearity with fiber sensitivity slightly decreasing at rising temperatures. In the tested range, temperature sensitivity of the fibers remained humidity independent. Thermal annealing was shown to considerably improve and stabilize fiber RH response. Based on performed analysis, a 20 m sensor using the optimal PI-coated fibers was proposed and constructed. The sensor uses dual sensing fiber configuration for mutual decoupling and simultaneous measurement of temperature and RH variations. Using OBR, distributed dual temperature-RH monitoring with cm spatial resolution was demonstrated for the first time.

Review of Optical Humidity Sensors

“…Humidity-induced strain measurement was investigated by Pavol Stajanca et al [ 228 ] using polyimide-coated optical fiber and commercial optical backscattering reflectometry (OBR), which is a polarization-diverse implementation of OFDR that allows ultra-high spatial resolution. The team demonstrated a 20 m sensing distance with a dual-sensing fiber configuration for mutual decoupling and simultaneous measurement of temperature and RH.…”

Review of Fiber Optical Sensors and Its Importance in Sewer Corrosion Factor Analysis

“…Optical Backscatter Reflectrometry (OBR) was used to create a sensor for simultaneous measurement of humidity and temperature. Fibers with polyimide and acrylate coatings were used in the designed sensor [159]. Data from fiber optic distributed temperature sensors installed in sewer networks was analyzed using an automated tool with a smart algorithm [160].…”

Distributed Humidity Sensing in Concrete Based on Polymer Optical Fiber

“…Moreover, thousands of fiber sensors can be multiplexed due to the large bandwidth of the optical fiber. Optical fiber-based humidity sensors have progressed very fast during the past years and currently, the humidity information can be obtained from the fiber bending loss, absorption loss, or humidity induced strain [2][3][4][5][6]. Special fiber structures, such as fiber tapers, waist-enlarged bitapers and long-period gratings, have been used to improve the response [7][8][9].…”

“…For example, an optical time domain reflectometer (OTDR) can be used to measure the humidity-induced bending loss over a certain distance [3]. The distributed strain sensing technique can also be used to acquire the humidity information by measuring humidity-induced strain in the fiber [5,6]. The reported response of such a sensor can reach up to 38.5 µε for 1% relative humidity change, though at the expense of the response time [6].…”

“…This type of sensor has also been applied to warn of corrosion under insulation by measuring the water ingress [11]. Note that references [5,6] reveal that fibers with a standard acrylate coating suffer from humidity-induced strain. This can cause errors for other fiber measurements, particularly for distributed acoustic sensing [12] which demonstrates very high sensitivity.…”