Inspectors shall be classified into four grades which shall be designated respectively first grade, second grade, third grade and fourth grade.

Inspectors of each grade shall receive such compensation as is or may be fixed by the civil service commission in accordance with the provisions of law.

Inspectors of the third and fourth grades shall be appointed from the list of eligibles for each respective grade as ascertained by open competitive examination and submitted by the board of civil service commissioners.

Any inspector having satisfactorily served five years as inspector of the third grade shall on recommendation of the commissioner be admitted to a noncompetitive promotion examination by the board of civil service commissioners and upon successfully passing such examination shall be promoted to the second grade.

Similarly any inspector having satisfactorily served five years as inspector of the second grade shall on recommendation of the commissioner be admitted to a noncompetitive promotion examination conducted by the board of civil service commissioners, and, upon successfully passing such examination, shall be promoted to the first grade.

Appointments of inspectors of the first and second grades shall not be made otherwise than after noncompetitive promotion examination.