An optical see-through head-mounted display device includes a see-through lens which combines an augmented reality image with light from a real-world scene, while an opacity filter is used to selectively block portions of the real-world scene so that the augmented reality image appears more distinctly. First, a voltage on at least one end of a to-be-measured capacitor is switched in response to a first set of clock signals such that a level of an integrated voltage is adjusted from a start voltage level to an end voltage level in a first integration period, wherein a capacitance of the to-be-measured capacitor is relevant to a difference between the end voltage level and the start voltage level. Simulation models of fluid saturation of the reservoir are also formed for comparable times.