First, unscrew the D-ring thumbscrew on the bottle bottom and remove the white cover so you can place the 1 CR2477 battery correctly in its place noting the + side facing the + sign direction. Make sure that the seal/gasket sitting on the base of the sensor has the FLAT side down and the upper side (facing up towards sensor stick) is tucked neatly into the groove/ridge around edge of the white circular sensor base. Replace the bottom cover screwing the D-ring thumbscrew tightly into place. Make sure the bottle bottom ring is also tightened into place.