Abstract: An instrument for measuring a gap between it and an imposing surface comprising a gas conduit having an inlet and an outlet, a sensing orifice, a gas pump connected to the inlet for applying an alternating pressure to the gas conduit so that the puffs of gas are directed from and back to the sensing orifice. The timing frequency rate is determined by the adding of a new dimension to the well-known Helmholtz resonance frequency parameters when used as feedback to an amplifying-oscillator circuit. This frequency pitch rather than its amplitude is now used to determine the gap distance, and this frequency can be perfectly transmitted to eternal observers. This frequency pitch can be counted over longer periods of time to improve its sensitivity and minimize system noise.

Abstract: An instrument for measuring a gap between it and an opposing surface, comprising a gas conduit having an inlet and an outlet, a sensing orifice in the outlet, a gas pump connected to the inlet for applying an alternating pressure to the gas conduit so that puffs of gas are directed from and drawn back to the sensing orifice. A sensor is in fluid communication with the conduit responsive to a physical property of the gas responsive to a condition determined by the gap size and a circuit is responsive to the sensor to provide a signal respective to the physical property.

Abstract: A gage for measuring the width of a gap. The gage includes a gaging conduit with an inlet and an outlet. Pressure means supplies gas to the inlet. A hot-wire anemometer is placed in the gaging conduit, and means is responsive to the condition of the anemometer to measure the size of the gap between the outlet of the gaging conduit and the surface as a function of the gas flow rate in the gaging conduit. In a preferred embodiment, a reference conduit is provided which has an inlet and an outlet, and which receives gas under pressure from the pressure source, and a reference orifice through which gas flows when exiting through the reference conduit. A hot-wire anemometer is placed in this reference conduit, the said means being responsive to differences in conditions of the two anemometers.