77 



The original design called for 1/16 inch inner diameter 

 stainless steel tubing between the vzater-tight cylinder and isolation 

 diaphragms. The fuselage, or center, tubings were 14 inches in 

 length, while the arm tubings were 44 inches in length. Methanol was 

 first tried as the back-filling fluid, but then discarded because of 

 the poisonous hazard it presented. Standard automobile transmission 

 fluid was next used as the back-filling agent. Although much safer, 

 it easily contaminates laboratory tools and work areas. 



Hundreds of attempts were made to back-fill the sensors and 

 eliminate all air in the system. An air bubble of appreciable size 

 could compress under a dynamic load and thereby prevent the 

 transducer from sensing the full pressure exerted upon the isolation 

 diaphragm. The back-fiiiing fluid itself is assumed incompressible 

 so that with no air in the system the fluid transmits the pressure 

 sensed by the isolation diaphragm immediately to the transducer. The 

 final technique used to back-fill the DPG is outlined in Appendix D. 



Once satisfied that air was removed from the system, more 

 problems arose • when the transducers, sensing through the 1/16 inch 

 I.D. tubing filled with transmission fluid, failed to respond to a 

 load faster than five to twelve seconds. It was decided that the 

 frictional head loss caused by the high viscosity of the fluid and 

 long, small diameter tubing might be responsible for such large 

 response times. It was also considered possible that small bubbles 

 might still be lodged in the system or that impurities in the fluid 



