Naksen, P., P. . Saisui, T. . Chomphu, W. . Chaksuma, W. . Anutrasakda, and P. Jarujamrus. “Barcode-like paper sensor for water hardness detection”. Journal of Science and Science Education (JSSE), vol. 4, no. 1, June 2021, pp. 51-61, https://so04.tci-thaijo.org/index.php/JSSE/article/view/251650.