The inspiration for FDG came from pneumatic gauging, an earlier example of a non-contact gauging technique. This features a jet of air moving outwards from a nozzle mounted perpendicular to the surface to be gauged. The pressure profile within the nozzle will be affected by the proximity of the deposit surface, whence the thickness of a deposit on a substrate can be determined from the pressure profile. Pneumatic gauging, at one time quite widely applied in engineering metrology, has more recently been used to measure rates of recession of volatile coatings from model heat transfer surfaces exposed to air flows in analogue estimations of convective heat transfer rates and to measure the thickness of soft deposits. The method is thus based on the ability of the coating to modify, or partially obstruct, a flow of air; it requires only that the coating be sufficiently rigid to resist significant deformation by impingement of the gauging fluid during the few seconds needed for attainment and reading of a steady pressure in the air supply system.
It occurred to Dr. W R Paterson that the old technique of pneumatic gauging might be exploited, perhaps with adaptations, in the study of the soft films typical of fouling and cleaning problems of the sort that he had studied in the past. He took his idea to Dr. D I Wilson, whose group was engaged in the experimental study of whey protein fouling. From their discussion arose the ideas of using the process fluid, rather than air, as the working fluid, and of using a "sucking" rather than "blowing" action for the jet. The decision to exploit a suction flow, whereby liquid is drawn into the nozzle, was intended to be particularly suitable for applications where sterility is an important consideration. As a precaution, on their behalf Paterson discussed their ideas with Dr. N Macleod, an authority on the existing technique of pneumatic gauging. Macleod saw no objection to their ideas, encouraged them to proceed and made various helpful suggestions. The key one was that it would prove advantageous to reverse the usual pneumatic gauging mode of imposing a selected flow rate of working fluid and measuring the consequent gauge pressure drop; instead, they should measure the flow rate, having imposed a selected pressure drop e.g. by using a siphon. Paterson and Wilson accepted both the encouragement and the suggestion. In their novel modification of this system, process liquid is sucked into a nozzle presented to the surface region to be gauged. Instead of measuring the pressure profile in the nozzle apparatus, the nozzle-surface clearance is deduced from a measurement of the discharge rate of liquid under the constant suction head provided by a siphon tube. This novel online gauging technique can provide thickness profiles of soft deposits non-destructively in their native environment, giving the capability to measure the deposit thickness distribution over a surface during a fouling or cleaning experiment. It therefore yields much more information than could previously be measured, with much reduced experimental times.