A spirit level, bubble level, or simply a level, is an instrument designed to indicate whether a surface is horizontal or vertical. The secret - the vials made of block acrylic glass are firmly epoxy-locked in the frame. A perfect system. The vial is firmly cast in the high strength aluminum frame with non-shrink epoxy. No re-adjustment, no realignment, no tensioning against springs made of steel or fatigued plastic, no welding or clamping stresses.