Using capacitive sensors at long ranges (10-20x their plate diameter) for long term environmental sensing can be limited by slow but significant measurement drifts that can often far exceed the small capacitance variations of interest, which can be around 0.01% or less. We propose a differential capacitance measurement method that rejects the quasi-constant drift currents for single plate capacitive sensors by averaging the absolute slope values of adjacent charge-discharge voltage ramps of the plate capacitance, under constant current. Compared analytically and in simulations with period modulation techniques using astable multivibrators, our method shows much better rejection of drifts due to quasi-constant charge migration and improved random noise attenuation, while preserving the measurement sensitivity. We also provide an implementation example that avoids errors caused by some types of ramp distortions and improves noise reduction.