A device is described which is used to measure very small mass losses of a target (typical mass 1 g) which is impacted by sand particles at velocities up to 80 m s^-1. In this adverse environment, ordinary gravitational mass measurement is not sufficiently sensitive, so the target is made part of a resonant system; the resonant frequency of this system is continually tracked with great accuracy, and this is used to obtain information about the mass loss of the target. Tracking is performed by forcing the system to oscillate at a frequency produced by an electronic voltage controlled oscillator (VCO); the phase difference between driving force and amplitude response is monitored with a new electronic device, and the frequency of the VCO adjusted to maintain phase quadrature. Under the conditions described, mass changes of 0.2 mg can be recorded: these correspond to changes in resonant frequency of 2 parts in 10⁵.