POLYETHYLENE CABLIO SHEATH THICKNESS 



563 



Fig. 4 — (Left) Removing test strip from end of cable; (right) performing 

 micrometer measurements on test strip. 



CALIBRATION OF THE TEST SET FOR SHEATH THICKNESS MEASUREMENTS 



Calibration of capacitance into thickness was difficult because the 

 capacitance is not a simple function of polyethylene thickness. It de- 

 pends also on the curvature of the sheath surface, the size and shape of 

 the probe, the amount of flooding and the height and shape of the cor- 

 rugated metal. For a given probe, it depends chiefly on the thickness, 

 the flooding and the sheath curvature. The flooding sometimes varies 

 from a thin film to an e.xcess that overfills the corrugations. The surface 

 curvature is not unifonn because the soldering of the metal overlap of 

 Stalpeth cable generally produces a flattened sector and the capstan at 

 the soldering operation results in an elliptical shape. Changes in the sur- 

 face curvature and in the amount of flooding can be compensating or 

 cumulative in varying the capacitance. 



To deteiTiiine whether a correlation between jacket thickness and 

 capacitance existed, extensive spot checks for three sizes of cable were 

 made. Marked points on cable were measured for capacitance and then 

 with a micrometer. A slight error can exist because the micrometer 

 measurement is only one spot in the center of an area which is effective 

 to capacitance. This condition is sho\\Ti by Fig. 6. Also, it is difficult to 

 determme accurately the surface curvature associated with the capaci- 

 tance measurement. 



The relation of thickness to capacitance conditions in the samples is 



