Abstract: A hermetically encased instrument includes a tubular housing with a front window glass, and a rear opening and a peripheral groove in its rear inside face, an instrument unit movably located in the housing and having a front meter observable through and limited in forward movement proximate the window glass; and a locking and sealing assembly mounted on the rear of the instrument unit and springs urging the asesmbly rearwardly, the assembly including a circular closure plate with a rearwardly facing stepped periphery engaging a rubber O-ring and a sealing plate coaxial with the closure plate and retained thereon by screws, the forwardly pressing of the assembly and the tightening of the screws loading the springs and distorting the O-ring into sealing engagement with the groove and the surrounding faces of the closure and sealing plates and housing to releasably lock the instrument unit in the housing and hermetically sealing the housing rear opening.