Abstract: Apparatus for determining the smoke density of flue gas by providing substantially uniform smoke spots, which includes a piston-actuated pump for withdrawing gas from a smokestack through a cooled probe so that cooled gas from the stack is drawn through filter paper and is then cooled further for the removal of condensate. The cooled, dried gas then flows from the system during the compression stroke of the piston of the suction pump through a by-pass provided with a one-way value. The present invention provides the uniformity required to facilitate optical comparison between the obtained spots in the described filter paper and standard reference spots.