Agar-Coated Fiber Bragg Grating Sensor for Relative Humidity Measurements: Influence of Coating Thickness and Polymer Concentration

The importance of humidity control is spreading through a broad spectrum of applications, from food and pharmaceutical industries to biomedical and chemical fields. During the last decades, the growing interest in fiber Bragg gratings (FBG) led to new approaches and applications for the humidity measurement by means of the deposition of hygroscopic materials on gratings. The influence of the coating material and its features such us coating thickness (tk) and concentration (wt%) on static and dynamic metrological properties have been investigated in this paper. In this paper, four agar-coated FBG sensors for humidity measurements have been fabricated, and the influence of tk and wt% on the sensors’ sensitivity and response time has been experimentally assessed. Sensitivity increases with tk and wt%: for instance, at 1 wt% it was <inline-formula> <tex-math notation="LaTeX">$0.0024\,\,\text {nm}\cdot {\%}^{\mathrm {-1}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$0.0052\,\,\text {nm}\cdot {\%}^{\mathrm {-1}}$ </tex-math></inline-formula> for <inline-formula> <tex-math notation="LaTeX">$\text {tk}=87\,\,\mu \text{m}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\text {tk} = 212\,\,\mu \text{m}$ </tex-math></inline-formula>, respectively. Results showed that also the response time increased with tk and wt%: for instance, at 1 wt% it was 56.7 s and 107.6 s for <inline-formula> <tex-math notation="LaTeX">$\text {tk} = 87\,\,\mu \text{m}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\text {tk}=212\,\,\mu \text{m}$ </tex-math></inline-formula>, respectively.