INCREMENTAL SHEATH THICKNESS MEASUREMENTS 



355 



1.3 Measurement Difficulties 



Some of \hv prol)l(Miis encountered in tiie nianut'adufe of the new 

 cable were related directly to the lack of reliable methods for measurinp; 

 thickness of the plastic sheathing. Under manufacturing conditions, 

 where sheath thickness cannot be adequately controlled, excess material 

 must be used to assure meeting minimimi thickness reriuirements. 



Before the new method was de\'eloped, measurements were made by 

 destructive testing of end samples. One or two circumferential strips 

 weie taken from each cable length and microm(>ter measurements were 

 perfonned on each strip, at four to eight points. Unfortunately, the 

 actual sheath thickness ^'aI•ies in a random way along the cable length, 

 even between points only a few inches apart. It was evident that a 

 method, based on a few point measurements, extrapolating long-cable 

 properties which are describable rather in statistical terms only, left 

 much to be desired. 



1.3 Preliminary Considerations 



The following methods of cable sheath measurements were considered: 



A. Use of an X-ray machine. 



B. Ultrasonic echo method (radar technicjues). 



C. Capacitance measurements. 



For practical reasons as well as for anticipated lack of accuracy, the first 

 of these methods was rejected. The success of the second method was 

 judged doubtful, the main reason being the presence of corrugations and 

 of an irregular layer of the filling compound under the polyethylene 

 sheathing, obscuring delimitation of the reflecting boundary surface. The 

 third method, at first, also had discouraging aspects. In the case under 

 discussion only grounded capacitance measurements are involved, since 

 the metal core cannot possibly be insulated from the corrugating and 

 forming machinery. The recjuired long-time capacitance-to-ground sta- 

 bility and accuracy of the measuring system were estimated to be of the 



CABLE CORE 

 SUPPLY (IN A 

 CORRUGATED 



GROUNDED 

 STEEL JACKET) 



APPLICATOR 

 OF THE 

 BONDING 

 CEMENT 



POLY- 

 ETHYLENE 

 EXTRUDER 



WATER COOLING 

 TROUGH 



TESTING 



ASSEMBLY WITH 



MEASURING 



PROBES 



COMPRESSED 



AIR DRYING 



POSITION 



FINISHED 



CABLE 

 TAKE-UP 



Fig. 2 — Block diagram of the polyethylene extrudinti process. 



