A turbidimeter with a pair of movable and stationary elements interconnected by a vertically-aligned hinge pin. Interior walls of these elements, when fully closed, define a cylindrical through passageway and an annular shelf which extends outwardly from its upper end. Otherwise, the stationary element, when the movable element has been pivoted away from it, defines one-half of the through passageway bisected lengthwise and a semi-annular shelf. Prior to use, a transparent cuvette can be slideably inserted sideways into the bisected through passageway and suspended there by protrusions on a cap threadedly engaged with the cuvette. The protrusions include a cap plate and an annular support shoulder which, in use, rests on the shelf. With its cuvette insertable sideways, an upflow sample cell, complete with supply and drain fittings fluidly connected to the cuvette, can be readily moved into and out of the turbidimeter to facilitate calibration procedures.