Aitken's Nometer

Aitken's Nometer, or dust meter (from Greek konia-dust and metron-measure), is used for counting dust particles in a specific volume of air (see Dust). The device is based on the condensation principle: dust particles contained in the air being tested, upon a sudden sharp decrease in air pressure, cause the condensation around themselves of the smallest droplets of water vapor and can thus be counted. A. k. consists of a piston (P) and a small chamber (A). The movement of the piston can create specific degrees of air rarefaction in chamber A, measured on scale S. The connection of the chamber with a pump or surrounding air (at normal pressure) is achieved by a three-way cock JR. The wall of the chamber is lined on the inside with filter paper saturated with moisture. The air being studied is aspirated through cock H, after which the rotation of cock R and the proper setting of piston P create the necessary rarefaction. Water vapor evaporating from the filter paper settles around dust particles falling on a plate divided into square millimeters, located at the bottom of the chamber. Then, under a microscope, the number of shining droplets that have settled is counted; knowing the degree of rarefaction and the volume of the chamber, the calculation is made, usually per 1 cubic cm. Disadvantages of the device: 1) weak suction effect, due to which larger dust particles may escape aspiration; 2) insufficient accuracy in determining the degree of rarefaction obtained; 3) especially small dust particles may escape counting, as the device is not adapted for sufficiently strong microscope magnifications; 4) the aggregation of the smallest dust particles is not excluded and 5) the precipitation of not only vapors around dust particles but also of the smallest particles of tar, etc., carried in the air is possible. A more perfect device is considered to be Owens' counter (see Owens' counter).