Abstract: The constant pressure flowmeters (CPFMs) were used to calibrate vacuum leaks and vacuum gauges in some national laboratories.The errors of measurement of CPFM and CPFM with a reference chamber were theoretically analyzed in the paper.The variation of temperature and pressure in the measurement process is one of the main parts of measurement errors.With the aid of a reference chamber,the error of measurement contributed by the variation of temperature in the variable chamber can be reduced.