142 



COMPUTATION FROM TEST DATA 



9 s 



> o 



> 5 



ZO 2 

 O =3 



tr o 

 o w 



Q- O 

 OIL 



100 



FREQUENCY KC 



Figure 9. Receiving response ot QB No. Ill transducer. 



The first step in computing the impedance from 

 the values on the chart is to compute the unknown 

 parallel resistance, which is 



R = 



Ro-Ri 



(2) 



IMPEDANCE 

 CJ)/^//yiTMt stall. 



Project Wo. /,y ? Haie^ 



Tevice: Qtslf>. Q&zL//J. 



Measuring conditions: 

 Location of devio 

 Grounding effects 



Bridge and Auxiliaries; 



Qsc. I7B . 



Bnd E e SA 



'-VV' rfipips.-? Copy \n £-~-£>- Sheet No. F. / 



'•7, i / 

 Y.C. 4 Leakaee Resistance 



Q&b^p 



Bal._v/_Uobal 



r 3/ ATMS 



g3ga««y. 



6^ 



K-m - >30,r wigShieldsto Case > 5,-yn tj 



j ■, Current through Fridge 1/ 



R^ =' Measured value of Shunting Resistance 



Res ii 



■ balanced 



of Rridg- 

 against Resistance of Unknown. 

 Note: Check in first column under R b indi- 

 cates unknown has been shunted by h\,. 

 ~Cp ■ Value of Bridge Capacitance balanced 



against Capacitance of Unknown. 

 ♦Cp - Value of Fridge Capacitance to Reson- 

 ate with Inductance of Unknown. 



From the computed shunt resistance R and the 

 shunt capacity C (given in microfarads on the chart), 

 the equivalent series resistance r and series reactance 

 x are then computed by means of the formulas 



R 



i + (wRcy- 



and 



R-C 



i + (uRcy- 



(3) 

 (4) 



This process, carried through numerically at 25 kc, 

 gives 



« - 1 ' 007 1, X J 8L5 - 3,450 ohms, 



226. 



3,450 

 1 + 394 



= 8.7 ohms. 



and 



68,200 

 1 + 394 



= — 173 ohms. 



This computation is repeated at other frequencies, 

 and an impedance characteristic is plotted for the in- 

 strument, as shown in Figure 11. The threshold then 

 is computed by means of equation (18) in Chapter 4. 

 This computation at 25 kc is as follows: 



T = 10 logr- 194.9 - R t! 

 = 10 log (8.7) - 194.9 + 80.8 

 = — 104.7 db vs 1 dyne per sq cm. 



60 

 | 50 



40 



30 



20 



-10 



-20 

 -30 

 -40- 



REACTANCE 



RESISTANCE 



200 ? 



-400 i 



600' 



800 



20 



30 40 50 60 

 FREQUENCY KC 



70 80 90 



1000 



Figure 10. Sample data sheet giving 5A impedance bridge 

 readings obtained in measuring the impedance of the OB 

 No. 11 1 transducer. 



Figure II. Impedance characteristic of QB No. Ill 

 transducer. 



