Grains for each gallon is how your water hardness is measured. You will see the abbreviation for this (GPG) in relation to the maximum hardness removing selection for every water softener. The higher the GPG, the upper the grain potential you may need. Take a look at the T&P Valve https://water-repiping45432.therainblog.com/29004655/the-basic-principles-of-toilet-repair-and-installation