I. Introduction -- II. Summary -- III. Basic principles -- The Atmosphere -- Structure -- Composition of air -- The gas laws -- The mole -- Weight-volume relationships -- Concentrations -- Derivation of mass-ratio relationships -- Physical volumes and gaseous volumes -- Water vapor -- IV Air flowrate and volume measurements -- Primary standards -- Spirometers -- Frictionless pistons -- Aspirator bottles -- Pitot tubes -- "S-type" pitot tube -- Intermediate standards -- Wet test meters -- Dry test meters -- Rotary displacement meters -- Secondary standards -- Rotameters -- Orifice meters -- Capillary orifice meters -- Critical orifices -- Heat transfer anemometers This report contains in a condensed form the basic mechanical and theoretical principles of air flow measurement devices commonly used in air pollution control monitors. The first part is devoted to a review of the composition and properties of the atmosphere. Air, to a large extent, behaves as an ideal gas. Simple temperature, pressure and volume relationships hold for most ambient air conditions. Volume ratios such as parts per million (ppm) or parts per billion (ppb) are shown to be easy to calculate. The mole-volume concept is described in such a manner as to make weight-volume conversions from parts per million to micrograms per cubic meter ([mu]g/mp3s), and vice versa, easy to perform. The role of water vapor in the air is also explained in absolute and relative humidity terms. The latter part of this report describes in sequence the physical and mechanical features, as well as the theoretical and applied air flow relationships, of primary, intermediate, and secondary standards. The primary standards which are described and discussed include spirometers, frictionless pistons, aspirator bottles, and pitot tubes. The intermediate standards include wet and dry test meters, and the "Roots" type volume displacement meters. The secondary standards include velocity type meters such as rotameters, orifice meters, hot wire anemometers and pressure transducers. The Summary Table which follows includes pertinent relationships of the various meters and appears in this section for convenience.