Gauge having a surface follower with peripheral vent

Apparatus to be utilized in performing a measurement of a traveling sheet or the like, comprising

• a surface follower body adapted to be constrained during the measurement for movement along a surface follower path intersecting the path of the sheet and urged by a force toward one surface of the sheet,
• the body having a gas confining wall, the wall having at least an edge portion adapted to be located next to the sheet surface, the edge portion having thereat uniformly distributed pressurized gas supply means with an outer periphery for pressurizing a gas bearing region that is contiguous to the sheet surface and bounded at the sheet surface substantially by the orthographic projection of the periphery on the sheet suface,
• the pressurization producing sufficient pressure against the sheet and the wall to levitate the body against the urging force,
• the gas supply means having gas retaining wall means with substantially only knife edge thickness next to the periphery and the sheet surface so as to minimize the area of the sheet surface over which high velocity gas flows in escaping from the gas supply means and the gas bearing region,
• the body being so shaped that the flow of gas from the gas supply means is substantially unconfined in the region of the sheet surface outside the gas retaining wall means in order to terminate the gas bearing region substantially at the periphery and so that the pressurization is effective without producing substantial amounts of high velocity gas flow over a substantial area of the surface of the sheet within the gas bearing region, whereby the body is maintained at a distance from the sheet surface that is substantially constant and relatively unaffected by variations in the roughness of the surface.