A feature of a countersink in a hole, such as the diameter, is measured by a gauge having a self centering, spherically shaped probe which shifts laterally upon engagement with the countersink side walls to as to align itself with the central axis of the hole. A super-heterodyne structure is used to translate the signals from one frequency to another so as to avoid interference sources, which may corrupt the communication signals. The vent strap has opposite ends that are fixed when the protection device is in the deflated condition.