This potable water expansion tank is used on gas, oil, or electric, direct-fired, residential water heater and hot water storage tank applications to absorb the increased volume of water created by thermal expansion. It also maintains balanced pressure throughout the water supply system by keeping system pressure below the T&P relief valve setting. This field-adjustable carbon steel tank is built with ASME Section VIII construction, tank volume 3.5 gallons, pre-charged to 40 PSI, has fixed FDA-approved butyl rubber diaphragm, and 3/4 inch stainless steel male NPT inlet connection and a 0.302-32 inch charging valve connection (standard tire valve) to facilitate on-site charging of the tank to meet system requirements. An expansion membrane prevents air-to-water contact in the tank ensuring a long system life. The maximum pressure rating is 150 PSI (10 BAR).