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LEARNING OBJECTIVES Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a preview of the information you are expected to learn in the chapter. The comprehensive check questions and answers are based on the objectives and enable you to check your progress through the reading assignments. By successfully completing the OCC/ECC, you demonstrate that you have met the objectives and have learned the information. The learning objectives for this chapter are listed below. Upon completion of this chapter, you will be able to do the following:
INTRODUCTION TO MEASUREMENTS In today's modern Navy, a large part of a ship's, submarine's, or aircraft's ability to complete its mission depends on the efficiency of sophisticated electronic systems. As the technician responsible for these systems, you are the focal point in ensuring their reliability. In the event of a system failure, it is your responsibility to repair the system and to do so in a timely manner. Whether you are troubleshooting a faulty system or performing preventive maintenance, you are required to perform basic electronic measurements on a regular basis. This chapter will acquaint you with various alternative methods of performing measurements and discuss the relative merits and demerits of each method. No discussion of electronic test equipment or electronic measurements would be complete without mentioning the Navy's Metrology Calibration (METCAL) program. Figure 1-1 shows the METCAL structure. Basically, the METCAL program is an elaborate quality control system designed to compare your electronic test equipment with test equipment of much greater accuracy. When you submit your piece of test equipment for calibration, it is compared with the calibration laboratory's equipment (referred to as STANDARDS), which are generally at least four times more accurate than yours. If your equipment does not meet specifications, it is either repaired, adjusted, or rejected with an explanation of why the calibration laboratory was unable to calibrate it. The accuracy of equipment at your local calibration laboratory is ensured by calibration of the test equipment to the standards of the next higher echelon calibration laboratory. The accuracies of test equipment at each higher echelon is increased by a ratio of approximately 4 to 1. Figure 1-1. - Calibration laboratory structure.
METCAL provides assurance that your test equipment is in top-notch shape. Remember, your measurements are only as accurate as your test equipment; be fully aware of the limitations of your test equipment and never use equipment that isn't properly calibrated when performing measurements or adjustments. Q.1 What assures the accuracy of your electronic test equipment? Now that we have discussed the advantages of calibrated test equipment, let's review the reason for all this concern. The fundamental electrical quantities of a circuit are voltage and current and are dependent on the circuit characteristics of resistance, capacitance, and inductance. In addition to these three individual characteristics, don't forget that many electronic components exhibit more than one circuit characteristic at the same time. An example would be a piece of coaxial cable that is engineered by its manufacturer to meet characteristic specifications for impedance, capacitance, and inductance. But let's keep it simple and begin by covering voltage measurements. Operation and use of common test equipment was covered in NEETS Module 16, Introduction to Test Equipment, NAVEDTRA B72 - 16 - 00 - 95. It is recommended that you review this module before continuing. VOLTAGE MEASUREMENTS Most Navy technical manuals provide voltage charts that list correct voltages at all primary test points in a piece of equipment. Voltage measurements, when compared with these charts, provide a valuable aid in locating troubles quickly and easily. However, if the sensitivity of the test equipment differs from that of the test equipment used in preparing the chart, the voltage measurements may not reflect true circuit conditions. You must keep in mind that a voltmeter with low sensitivity used on a low range may disturb circuits under test or provide a false indication. Most technical manuals will tell you what type and model of test equipment was used to prepare the voltage charts. As a rule of thumb, the input impedance of the voltmeter should exceed the impedance of the circuit by a ratio of at least 10 to 1. Technicians have spent uncounted hours of wasted time because they have selected improper test equipment. Q.2 The input impedance of your test equipment should exceed the impedance of the
circuit under test by what ratio? Direct current voltage may be steady, pulsating, or have ac superimposed on it. The average value of a dc waveform depends on the symmetry of the wave and other aspects of the wave shape. It can vary from 63.6% of peak value for a rectified full sine wave to 50% of peak value for a triangular wave. For a superimposed sine wave, the average value can be zero. Regardless of whether the dc is steady, pulsating, or the ac is superimposed on the dc, a rectifier form of measuring device will indicate its average value. Voltages are usually measured by placing the measuring device in parallel with the component or circuit (load) to be measured. The measuring device should have an infinite internal resistance (input impedance) so that it will absorb no energy from the circuit under test and, therefore, measure the true voltage. The accuracy of the voltage measurement depends on the total resistance of the measuring device compared to the load being measured. When the input impedance of the measuring device is 10 times greater than the load being measured, the error usually can be tolerated. If this error cannot be tolerated, a high input impedance measuring device, such as a vacuum tube voltmeter (vtvm), should be used. Alternatively, using two voltmeters in series increases the voltage range and, because of the increase in total voltmeter resistance, provides a more accurate measurement of voltage across the load. If the voltage to be measured is sufficiently high, more than two similar voltmeters can be connected in series across the load to provide greater accuracy; the total voltage measurement is the sum of the individual meter indications. Q.3 What are the advantages of using two voltmeters in series? A common piece of test equipment used in the Navy is the Simpson 260 analog multimeter, as shown in figure 1-2. It is capable of measuring both ac and dc voltages of up to 5,000 volts. Figure 1-2. - Simpson 260 multimeter.
Two obvious advantages of the Simpson 260 are its portability and ease of operation. Among its disadvantages are its low input impedance and the inherent low accuracy associated with D'Arsonval meter movements, which are used in the meter. When performing measurements with any analog multimeter, remember that the most accurate readings are taken with the pointer midscale. You should also be aware of inaccuracies introduced as a result of parallax. PARALLAX is defined as the apparent displacement of the position of an object because of the difference between two points of view. In the case of meters, this means the position of a meter's pointer will appear to be at different positions on the scale depending on the angle from which the meter is viewed. Some of the Simpson 260 and 270 series multimeters have effectively eliminated the problem of parallax by incorporating a mirror on the scale that accurately reflects the position of the pointer of the meter movement. Q.4 At what point on a meter movement are the most accurate readings taken? |
 
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