This paper proposes an experiment in which students build microdevices for conveniently electrolyzing water and use image recognition and processing technology to measure the volume of gas generated during the reaction in a noncontact way. Taking advantage of three ratio relationships between (i) gas volume and the area of bubbles, (ii) the factual size and the size on the picture, and (iii) the area on the picture and the pixel number, the experiment converts the measurement of trace gas volume into measurement of bubbles' pixel number on the picture, which simplifies the complex problem. The device includes pencil refills as the electrodes, compressing the reaction space in the interval between two glass slides to squeeze the gas generated into flat-cylinder shaped bubbles with a fixed height. The students use a smartphone to take pictures and use software to process the images. Briefly, edges of bubbles are recognized and enhanced by PowerPoint, followed by a cutout process by Photoshop to get the bubbles' image without the background; a histogram tool is then used for analyzing the pixel number of bubbles to finally get the volume of hydrogen and oxygen. Results of the survey among participating students proved the pedagogical effectiveness of this project.