Research Article Water Vapor Sensors Based on the Swelling of Relief Gelatin Gratings Sergio Calixto 1 and Miguel V. Andres 2 1 Centro de Investigaciones en Optica, Loma del Bosque 115, 37000 Le´ on, GTO, Mexico 2 Departamento de Fisica Aplicada y Electromagnetismo, Universidad de Valencia, Dr. Moliner 50, Burjassot, 46100 Val` encia, Spain Correspondence should be addressed to Sergio Calixto; scalixto@cio.mx Received 4 December 2014; Revised 27 February 2015; Accepted 11 March 2015 Academic Editor: Luigi Nicolais Copyright © 2015 S. Calixto and M. V. Andres. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We report on a novel device to measure relative humidity. Te sensor is based on surface difraction gratings made of gelatin. Tis material swells and shrinks according to the content of water vapor in air. By sending a light beam to the grating, difracted orders appear. Due to the gelatin swelling or shrinking, frst order intensity changes according to the relative humidity. Calibration curves relating intensity versus relative humidity have been found. Te fabrication process of difraction gratings and the testing of the prototype sensing devices are described. 1. Introduction Devices like sensors and transducers devoted to collect the data from the physical world have developed slowly. Tus, new and better sensors should be made. Tese new sensors should be sufciently accurate or suitable for most industrial applications. Te content of water in air (or water vapor) is important [1] in various industries like medical and health industry (humidity controlled hospital operation rooms, incubators, and air conditioning), automotive indus- try (window defogger), food processing industry (food dehy- dration), meteorological industry, semiconductor industry (clean rooms), building and construction industry, and more. Ideal humidity sensors should fulfll requirements like (a) good reproducibility over a wide range, (b) short response time, (c) small hysteresis, (d) good durability and long life, (e) resistance against contaminants, (f) low cost, (g) sensitivity to the measured property only, (h) not infuencing the measured property, and more. However, it is difcult to fnd a humidity sensor that fulflls the mentioned characteristics. For this reason many diferent measurement methods and sensors have been developed, each having certain advantages and limitations and each suitable for some but not all applica- tions. Among the conventional techniques for humidity detection [2] is the mechanical hygrometer where materials expand and contract in proportion to humidity change. Common materials include synthetic fbers and the human hair. Te method is inexpensive but slow and presents nonlinearity and hysteresis. Other techniques are, for example, the chilled mirror hygrometer [2] based on a temperature-controlled refective condensation mirror, infrared [2], and Lyman-alpha instruments and those based on optical [2] fbers and MEMS technology [3, 4]. Tese MEMS-based sensors use electronic devices that cannot be used in fammable environments because they introduce a spark risk. Every type of humidity sensors has found a niche market where its performance characteristics apply. But new requirements for humidity measurements are coming up. Here we present the use of a spark-free optical method that uses difraction gratings as sensors. It is an alternative method to measure relative humidity (RH). In Section 2 we present the material used to fabricate the grating sen- sor. Section 3 describes the difraction gratings fabrication method. Section 4 presents the gelatin grating behavior as a function of relative humidity. Section 5 describes the principles of the measuring method and calibration plots and in Section 6 we conclude the paper. Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2015, Article ID 584324, 5 pages http://dx.doi.org/10.1155/2015/584324