The measurement of film thickness with a pneumatic hypodermic needle mechanism

An apparatus is described by means of which the measurement of film thickness may be made on supported polymer films in a non-destructive manner. This apparatus is based on the principle of flow of a gas, e.g. nitrogen, through the space formed by the tip of a cut-off hypodermic needle and a given surface. The theory, an extension of a previous one for a swelling measurement apparatus, is presented and appropriate sensitivity expressions are derived. Agreement with the theory is good except when the rate of flow parameter is high, and kinetic energy corrections must then be applied. The r.m.s. error in the location, at a fixed distance away, of a point on a surface is found to be ±0.2 μ. The films are ordinarily cast on optically selected flat-faced glass plates which are mounted on the flat base plate of the apparatus. By lateral sliding action of the support on the base plate, a number of surface locations are made on the film and around the film edge from which the mean film thickness is obtained. The r.m.s. error in the mean film thickness is found to be ±0.4 μ for films ranging from 10 μ to greater than 200 μ in film thickness.