From: Wright, B. M., and C. B. McKerrow. “Maximum forced expiratory flow rate as a measure of ventilatory capacity.” British Medical Journal 1959; 2.5159, page 1043.

“The general appearance of the instrument, which weighs about 2 lb. (900 g.), and its method of use are shown in Fig. 2. The subject is asked to take a deep breath, as for a vital capacity, to place the mouthpiece in his mouth, and then to blow into the instrument as hard as he can. It is not necessary to try and empty the lungs, and it is undesirable to do so, because it is unpleasant and tiring, but a certain amount of “follow through” is required. After a couple of practice blows, three attempts in succession are recorded and the average is taken. This is statistically a more satisfactory procedure than simply taking the highest reading,although logically the latter would be preferable, since a maximum if being measured. Most subjects will give three readings with agree with 10% after the practice tries, but the observer must ensure that the subject is really trying his hardest. As with all ventilatory tests of this kind, including the M.V.V., failure to produce reasonably concordant results usually indicates a lack of co-operation.”