INTRODUCTION TO SONAR 



-^VW — 



50,000JX 



lOOOix/V 

 75 V 



^=- 150 V 



lOO.OOOjv 



V ) 0-100 SCALE 



20,000^/V 

 98 V 



71.125 

 Figure 10-3. — Effect of sensitivity on meter 

 accuracy. 



resistors equals their product divided by their 

 sum. It is always less than the value of the small- 

 est resistor.) The total circuit resistance is no 

 longer 150,000 ohms, but 100,000 ohms, and the 

 total current flow is .0015 ampere. By applying 

 Ohm's law you can see that the meter will read 

 75 volts. (If the meter had a — 500-volt scale, 

 you would get a reading of about 90 volts.) 

 • Now use a more sensitive meter, one rated 

 at 20,000 ohms per volt. The internal resistance 

 of the meter is 100 times 20,000 or 2,000,000 

 ohms. The equivalent resistance of R2is approxi- 

 mately 95,000 ohms, total circuit resistance is 

 145,000 ohms, and total current is .00103 ampere. 

 The meter will now Indicate about 98 volts across 

 R2. 



OHMMETER 



The ohmmeter is used to measure resistance, 

 check circuit continuity, and test for grounds. 

 It has a self-contained voltage source. Because 

 the ohmmeter is a resistance measuring device, 

 you must deenergize the circuit under test before 

 you connect the ohmmeter, otherwise serious 

 damage will result to the meter. 



The resistance measuring capabilities of the 

 ohmmeter are controlled by fixed resistors of 

 different values connected in series with the 

 meter's movement. On some ohmmeters, scale 

 selection is accomplished with a selector switch; 

 on others, separate test lead jack connectors are 

 used to obtain the desired scale. 



To ensure an accurate reading, the meter must 

 be zeroed to the scale used. Zeroing is accom- 

 plished by shorting the test leads together and 

 adjusting the variable resistor labeled "ohms 

 zero adjust" to align the pointer with the zero 

 mark on the dial. Whenever you change scales, 

 the meter must be zeroed again. 



MEGOHMMETER 



An ordinary ohmmeter is incapable of measur- 

 ing resistance of millions of ohms, as found in 

 conductor insulation. To test for insulation break- 

 down requires a much higher potential than is 

 furnished by an ohmmeter. An instrument known 

 as a megohmmeter ( called megger ) is used for 

 these tests. The megger supplies the required 

 high potential by means of a handcranked d-c 

 generator. Meggers usually are rated at 500 volts, 

 although higher rated types are available. To 

 avoid excessive test voltage, the megger is 

 equipped with a friction clutch between the hand- 

 craiik and the generator. When the generator 

 is cranked fast enough to exceed its rated value, 

 the clutch slips. As with the ohmmeter, power 

 must be removed from the circuit under test 

 before connecting the megger. 



MULTIMETER 



The multimeter is a multipurpose meter that 

 can measure resistances, a-c and d-c voltages, 

 and d-c milliamps. Its versatility and portability 

 eliminate the need for carrying several meters for 

 test purposes. The usual precautions must be 

 observed when making resistance and voltage 

 measurements. When measuring d-c voltage, pro- 

 per polarity must be observed. The face of the 

 instrument has separate graduated scales to indi- 

 cate the three values that can be measured. Be 

 sure you read the proper scale. 



TUBE TESTER 



Electron tubes often are the cause of equip- 

 ment failure. They may burn out, or their elements 

 may become shorted or broken owing to vibration. 

 Even new, unused tubes are subject to damage, so 

 you should always test a tube before inserting it 

 into a circuit. The test instrument you will use 

 is the tube tester. Its usual application is to 

 measure the mutual conductance of a tube, which 

 is an indication of how well the grid can control 

 plate current. The tube tester also can measure 

 the emission of rectifier tubes, test for shorts, 

 and tell if a tube has become gassy. 



OSCILLOSCOPE 



The oscilloscope is one of the most valuable 

 pieces of test equipment available to the Sonar 

 Technician. With the oscilloscope you can deter- 

 mine a signal's frequency, pulse width and ampli- 

 tude, measure voltages and phase relations, and 



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