Observing window device for liquid tanks

An observation window device for a liquid tank having a window opening formed in a side wall thereof, said window device comprising a generally cup-shaped transparent pane member including a side wall having an outer periphery with an abutment flange therearound, said side wall of the cup-shaped member being provided with slit means, said pane member having an open end extending into the interior of said liquid tank and including an integral lower limit level indicating portion for defining the lowest level of liquid in said liquid tank, pane retainer plate means engaged in said slit means and held in abutting engagement with an inside surface of said side wall of said liquid tank for resiliently urging said pane member inwardly of said liquid tank to place said abutment flange in fluid-tight contact with an outside surface of said side wall of said liquid tank and rotation-restraining means integrally formed with said pane retainer plate means for holding said pane member and said pane retainer plate means in place against rotation thereof relative to said liquid tank, said slit means including a pair of slits in opposition in said side wall of said cup-shaped pane member, said pane retainer plate means comprising a retainer plate including a pair of spaced, parallel grip legs respectively in fitting engagement in said slits and a bridge portion interconnecting said grip legs at the top thereof, said tank having a removable cover member, said bridge portion of said pane retainer plate being held in abutting engagement with said cover member so that said retainer plate is prevented from inadvertent disengagement from said pane member.