POLYETHYLENE CABLK SIIEATII THICKNESS 571 



Tf = Thickness fo flat polyethylene sample at same recorder 

 meter reading as for T. 



Ki = Ratio of actual cable sheath thickness to flat sample thick- 

 ness at zero meter reading. 



7v2 = Constant to change slope of TpKi curve. 



■^ (at +35 meter) i 



For + meter readings K2 = 



For — meter readings K2 



J F-tVi(at + o5metor) — ■/ 

 J (at — 35 meter) -i 



i FjK.l(at- 35 meter) — -/ 



To = Thickness in thousandth's of an inch of cable sheath at 

 zero meter reading. 

 The A'l factor accounts for the dimensional differences between the 

 capacitor formed by a flat thickness of polyethylene on a flat plate com- 

 pared to the actual capacitor constmction of cable at zero meter. Both 

 have the same capacitance of 1.20 uuF at zero meter reading. Ko ac- 

 counts for changes resulting from the curved surfaces of cable. 7li and K2 

 are different for each cable diameter. 

 Since zero meter is used as a reference point, the formula becomes: 



T = (TfK, - To) K. -f- To 



ACCURACY CHECK UNDER OPERATING CONDITIONS 



A check* was made of the accuracy of calibration and of the response 

 under operating conditions of applying the sheath to the cable. The 

 upper graph in Fig. 12 was obtained with the test set probe tracking at 

 a cable sheathing speed of 50 feet per minute. The probe was shifted to 

 different octant locations on the circumference for lengths of the cable 

 as indicated on the graphs. The track of the probe was marked on the 

 sheath surface and the sheath then removed, cleaned of flooding com- 

 pound and the micrometer measurements of the thickness taken at 

 sbc-inch intervals along the length. The lov»-er graph is a plot of the thick- 

 ness obtained by micrometer. The ability of the test equipment to track 

 and respond to the thickness variations is apparent from comparison of 

 the two graphs. 



APPLICATION OF TEST EQUIPMENT FOR EXTRUSION CONTROL 



The test set is placed at some distance after the extruder to prevent 

 the probe from marking the plastic polyethylene. The machinery of the 



* Test and measurements by courtesy of J. L. O'Toole, Bell Telephone Lab- 

 oratories. 



