The Resin filter’s main function is to reduce water’s hardness level. The H2 Respire 150 requires to use water with high water purity or with very low hardness level in order to generate high purity hydrogen gas. Over time, the electro bath will cause scale buildup during hydrogen generation & will start having sediments and minerals in the water. The Resin filter binds to these minerals & reduces these impurities which helps prolong the life of the device. A new resin filter should be replaced every month or every 100 hours of usage.