Accuracy of LMA 



NCEL's Magnograph equipment was modified by Noranda during FY81 to 

 simplify the calibration procedure for determining LMA. The original 

 equipment used a calibration procedure that required knowing the mass 

 per unit length of the wire rope. Many times this value had to be 

 estimated, generating a source of error. The Noranda modification 

 entailed some circuitry revision along with a new calibration curve. 

 The new method eliminated the need for prior knowledge of the mass per 

 unit length of the wire rope. 



The accuracy of the LMA readings decreased with this modification. 

 Tests to determine the accuracy were conducted on wire ropes of 1-1/8- 

 and 2-inch diameter, 6x25 right-regular-lay, fiber-core construction. 

 Extra wires (about 15 feet) were added to the wire rope by taping them 

 into the valleys between the strands to simulate an increase of mass. 

 Figure 19 shows that significant differences exist between the known 

 change in LMA and the Magnograph readings. Revisions in the calibration 

 curve would make some improvement in the accuracy, but that procedure by 

 itself is not enough to improve the accuracy to meet the manufacturer's 

 specifications. 



Noranda has recently developed a self-calibrating system that does 

 not require a calibration curve. The sensor head determines the mass of 

 the rope by measuring the flux flow through the rope. A given voltage 

 change (1 volt) from the zero condition represents a 10% loss or gain in 

 metallic area. NCEL has tested this system, and its accuracy is ±0.05%. 

 This self-calibrating system will probably be used for all future equip- 

 ment. 



NCEL's Magnograph equipment can be manually calibrated to simulate 

 the self-calibrating process as described in the Appendix. The procedure 

 is to start the calibration process before the sensor head is placed on 

 the wire rope. In the self-calibrating procedure the following steps 

 should be taken by the operator: 



1. With a four-digit read-out Volt-Ohm-Milliamp (VOM) meter, 

 plug the test leads into the LMA input receptacles of the 

 Magnograph chart recorder. 



2. Turn on the recorder and electronic sections. 



3. On the electronic section, adjust the LMA zero potentiometer 

 until volt is obtained (have the VOM on the 2-volt scale) . 



4. Now place the sensor head on the wire rope and adjust the 

 LMA gain potentiometer until +1.000 volt is obtained on the 

 VOM. 



5. Then go back to the LMA zero potentiometer and adjust until 

 volt is obtained. 



By using this new method to calibrate the Magnograph unit, LMA accuracy 

 is well within the ±0.5% accuracy by area specified by the manufacturer. 



