Abstract: A probe for determining the thickness of an interface gap between two surfaces is disclosed. The probe includes an inflatable bladder, which can be inserted into a gap and inflated, such that the sides of the bladder contact the planes. A distance-measuring device is mounted within the cavity of the bladder and is suitable for measuring the distance between the sides of the bladder. Passive electrical circuitry for use as the distance-measuring device is also disclosed. Passive electrical circuitry has an electrical property that varies as the distance between the passive electrical elements varies. In the case of a probe utilizing capacitive circuitry, three relationally positioned measuring electrodes and a common electrode are mounted within the bladder cavity such that the measuring and common electrical are in facing relationship and move away from each other as the bladder is inflated. When the bladder is inflated with a dielectric fluid, the electrodes are pressed flat against the surfaces.