Although any inert gas can be used to pre-charge the accumulator, we recommend to use high purity dry nitrogen gas only for pre-charge. Never use shop air or oxygen to avoid any risk of explosion. Accumulators are normally shipped with pre-charge pressure of 25 psi for shipping purpose only unless otherwise specified by the customer. For optimum and reliable operation of accumulators, it is important to fill and maintain the correct nitrogen pre-charge pressure. We recommend checking pre-charge pressure regularly. Charging & gauging kits are designed to fit on a commercially available nitrogen bottles. Charging instructions are provided with charging kit for safe operation. Servi Fluid Power's general recommendations for nitrogen pre-charge varies by application. For Energy storage, precharge should be 90 percent of the minimum system operating pressure. For shock absorption, the precharge should be 60 to 75 percent of the normal working pressure. For pulsation dampening, the precharge should be 60 to 80 percent of the normal working pressure. These recommended pressures must be adjusted taking operating temperatures into consideration. Pre-charge pressure must be checked during first week in operation. Subsequent maintenance should be done 3 to 6 months depending on the severity of the application. High cycling and high temperature applications require maintenance every 3 months at a minimum.