When it is stated that a pistol sprayer consumes say two cubic ft. of free air per minute, it means when working almost continuously, but as the periods of actual work do not represent more than two-thirds of the actual number of working hours, there is a margin left. Nevertheless, it would be unsafe to state a lower figure, for in some cases where convenient feeding of the work to the operator is arranged, the consumption of air is continuous.
It should also be remembered that the air is used for other purposes, i.e., a small amount is used to provide a pressure feed of colour to the instrument in many cases, and where a heater is employed, a small amount of air is bypassed through the pistol to keep this warm when spraying is momentarily stopped. Although the practice of heating the air slightly increases the ultimate efficiency of a given compressor, yet it is unwise to count too much upon this fact.
In submitting the following figures as to air consumption, the Airostyle pistol has been taken as a type:—
For work with a tip and needle of 1 m/m. dia., 1 cub. ft. per minute; 1½ m/m. dia., 1¾ cub. ft.; 2 m/m. dia., 2½ cub. ft.; 2½ m/m. dia., 3 cub. ft.; 2½-3 m/m. dia., 3½-4 cub. ft.; 4½ m/m. dia., 5 cub. ft.
These consumptions naturally vary slightly with the adjustment of the nozzle of the pistol, but they are from actual tests and so may be taken as authoritative.
CHAPTER VI.
Types of Spraying Apparatus.
We now come to a consideration of the different appliances by means of which the actual spraying is effected, and in order to avoid any suspicion of partiality the review of instruments, etc., in use will be dealt with alphabetically.