A measurement system and method of conducting cavity resonance and waveguide measurements is disclosed. The cavity or waveguide may be used to monitor the amount, composition, or distribution of a material or sample contained in the cavity or waveguide or passing through the cavity or waveguide. Improved means for operating the measurement system to reduce measurement variability, improve measurement accuracy, and decrease measurement response times are described. The invention's broad applications range from measurements of filters, catalysts, pipe, and ducts where the material collected in or passing through the cavity or waveguide exhibits dielectric properties different from the material which it displaces.