Both the electronic and mechanical hygrometers need regular calibration. Initial calibrations are usually performed either with precision humidity generators using two-pressure, two-temperature, and divided-flow systems or with secondary standards during manufacturing (refer to ASHRAE Standard 41.6-1982, Stan- dard Method for Measurement of Moist Air Properties). Regular calibrations can be done with a precision aspiration psychrometer or with chilled mirror dew-point devices.