An improved computer display device is disclosed. A second reset period of variable length between a black calibration phase and an integration phase occurs in a processing cycle of a frame for each pixel, wherein the length of the second reset period is adjusted based on the ambient light conditions. The driving head has two flat portions located on top and bottom of the driving head respectively and the flat portions each have an outer face, an inner face and a recess, all of which are located around the clamping opening and arranged in U shape. The hydrogen evolved is used to determine the concentration of the reducing agent present in the sample.