Abstract: The reader apparatus includes a clamp colocated with a photoelectric cell and lamp. A scale, e.g. calibrated in millimeters extends laterally from the photoelectric cell/lamp axis. A gas detector tube whose column of test particles is stained from one stain-starting end to a region part way therealong is clamped in the reader with its starting end of stain juxtaposed with the scale. The tube is manually moved axially until a portion of the lightest-in-color region is directly between the photoelectric cell and lamp. The transmitted light indicator is adjusted to align with one extreme on its scale. Then the tube is manually moved axially until a portion of the darkest-in-color region is directly between the photoelectric cell and lamp. The transmitted light indicator is adjusted to align with the other extreme on its scale. Finally the tube is slid axially until the indicator coincides with a preselected intermediate tick-mark, e.g. indicating a light transmittance value half-way between the two extremes.