Abstract: This system is a combination of two techniques which are used together effectively combining their features. One technique is an absolute measurement based on mass accumulation rate and used primarily for calibration purposes. The mass accumulation rate on a filter, when divided by the flow rate of the medium through this apparatus, yields the droplet density. The second technique is a differential measurement with two adjacent sampling inlets, one is unrestricted while the other is inertially filtered to eliminate water droplets. The two flow streams in the differential system are cyclically sampled and the combined flow passes a heated wire grid designed to evaporate droplets encountered. The resulting oscillating signal (with DC component) is fed into a lock-in amplifier whose output is the rms value of the difference signal. The vapor component is thereby nearly eliminated and the signal is processed by the phase-locked signal-to-reference detection technique which produces very high sensitivity.