a device for testing a proximity sensor and precisely measuring the rigging gap between the sensor and its metal target . the sensor is stimulated with a signal of a known frequency and sensor impedance or a component thereof , such as inductance , is measured . the measured value is converted into gap width using a lookup table . the gap width may be displayed to the operator or automatically compared to an optimal range of gap widths for a sensor of the type under test . an operator may also test the sensor using a precision tool that has a dielectric spacer of a known width attached to a metal target . while the operator holds the tool with the spacer in contact with the face of the sensor , the present invention measures impedance and converts it into a gap width . the gap width should equal the known width of the spacer if the sensor is functioning properly . if the difference between the known spacer width and the measured width is less than a predetermined tolerance , the difference may be stored as a gap error . this gap error is used to offset subsequent measurements of the gap between an installed sensor and its target before the measurements are displayed .