This hygrometer is designed to measure the humidity of atmospheric air over the vapor pressure range 3–7400 Pa (0.03–74 mbar). The instrument is an adaptation of a microwave refractometer using two cavities operating at 12 GHz. One cavity is exposed to the moist test air and the other is exposed to the same air sample with all the water vapor removed. Both cavities are maintained at the same fixed temperature in a thermostated oven and at the same total pressure. The difference in frequency between the cavities is automatically nulled by a tuning probe in the sampling cavity. The instrument was calibrated by two independent methods. One involved the measurement of the resonance frequency of the sampling cavity as a function of probe penetration and using this in a theoretically derived equation for vapor pressure. The second involved the measurement of the probe penetration as an empirical function of known vapor pressure of a test gas. These two methods yielded results which agreed on the average to better than 0.5% for vapor pressures up to 3050 Pa (30.5 mbar).