A method and apparatus for installing a foam proportioning system into existing fire fighting equipment is provided. The delay calibration technique as disclosed includes estimating a time delay difference according to a phase difference between two adjacent sub-carriers, dividing the estimated time delay difference into a coarse time delay and a fine time delay, and compensating the coarse time delay in time domain and the fine time delay in frequency domain respectively. The smoothing rolls are cambered. Clients access higher time servers to obtain time information. The warm recirculation air surrounds the portion of the fresh air tube to heat it preventing ice from forming. The elements or shells forming the body are made using die-casting, sintering or forging processes with material suitable for these forming techniques.