(_b_) Apparatus not requiring pressure regulators must be so arranged that the gas pressure cannot exceed sixty-tenths (6) inches water column. This requires the use of the pressure relief provided for in Rule No. 20 (_a_). (_c_) Apparatus requiring pressure regulators must be so arranged that the gas pressure cannot exceed three pounds to the square inch. The pressure limit of 3 pounds is taken since that is the pressure corresponding to a water column about 6 feet high, which is about, the limit in point of convenience for water-sealed reliefs. 22. AIR MIXTURES.--Generators must be so arranged as to contain the minimum amount of air when first started or recharged, and no device or attachment facilitating or permitting mixture of air with the gas prior to consumption, except at the burners, shall be allowed. Owing to the explosive properties of acetylene mixed with air, machines must be so designed that such mixtures are impossible. 23. PURIFIERS.--(_a_) Must be constructed of galvanised iron or steel not less than No. 24 U.S. Standard gauge in thickness. (_b_) Where installed, purifiers must conform to the general rules for the construction of other acetylene apparatus and allow the free passage of gas. (_c_) Purifiers must contain no carbide for drying purposes. (_d_) Purifiers must be located inside of gasholders, or, where necessarily outside, must have no hand-holes which can be opened without first shutting off the gas-supply. 24. PRESSURE REGULATORS.--(_a_) Must conform to the rules for the construction of other acetylene apparatus so far as they apply and must not be subject to sticking or clogging. (_b_) Must be capable of maintaining a uniform pressure, not varying more than four-tenths inch water column, at any load within their rating. (_c_) Must be installed between valves in such a manner as to facilitate inspection and repairs. _Class B.--Stationary Apparatus for Central Station Service._ Generators of over 300 lights capacity for central station service are not required to be automatic in operation. Generators of less than 300 lights capacity must be automatic in operation and must comply in every respect with the requirements of Class A. 25. GENERAL RULES. GENERATORS.--(_a_) Must be substantially constructed of iron or steel and be protected against depreciation by an effective and durable preventive of corrosion. Galvanising is strongly recommended as a protection against oxidation, and it may to advantage be reinforced by a thorough coating of asphaltum or similar material. (_b_) Must contain no copper or alloy of copper in contact with acetylene, excepting in valves. (_c_) Must be so arranged that generation will take place without overheating; temperatures in excess of 500° F. to be considered excessive. (_d_) Must be provided with means for automatic removal of condensation from gas passages. (_e_) Must be provided with suitable protection against freezing of any water contained in the apparatus.