A Device to Measure the Size of Volatile Droplets Utilizing a Hot-film Sensor

A device has been developed that is useful for the size measurement of volatile liquid droplets. In this device, droplets are sampled into a tube, where they are allowed to collide with a hot-film sensor, which has originally been designed for use in a constant-temperature anemometer. Upon a collision of each droplet, a pulse is formed in the voltage applied to the sensor, and the diameter of the droplet is obtained directly from the pulse area; no calibration with any standard is necessary. The accuracy of the hot-film method was proved through test experiments in which water droplet sizes measured by the present device were compared with those measured by a microscopic photographic or a gravitational settling method. By evaluating the detection efficiency of droplets, the number concentration of droplets was also obtainable. The device was shown to be applicable to ethanol droplets as well as to water droplets. A field test to observe a fog event was carried out, and it was found that the variation of the liquid water content could be tracked with a measurement interval of about 3 min.