Abstract: Four curtains are drawn or released together to set the measuring aperture of an air-flow measuring box at the desired area and the linear velocity of flow through the aperture is measured by a single-probe velometer normally held with the probe tip essentially at the center of the aperture, a perimeter sealing strip blocks unmeasured air flow around the back of the curtains. A box-like hood guides the air flow from a ceiling outlet under investigation to the curtained aperture and is provided with diagonal vanes to keep the measurement from being falsified by cyclonic air movement. The box-like hood folds up for storage or transportation.