UFC 3-570-06
JANUARY 31 2003
Figure 7-7. Typical Displayed Readings Using Digital Voltmeter
g
g
1. 0egative Volts
N 25
1.21. 025
1. 025
1
1
0. 865
0.80. 855
0. 845
Instant-OFF
1. 02
0.6
1. 022
1. 025
0.4
1. 025
Interrupter Set at 1 Second Off and 4 Seconds On
1. 025
Number of Readings at 1/4 Second Intervals
0.2
1. 025
1. 025
01. 025
1. 025 5
1
9
13
17
21
25
29
33
37
41
45
49
Limitations of a meter using a needle to display the measurement
(D'Arsonval movement) preclude their use for this application. The needle
swing is a relatively slow movement which may be slower than the initial
depolarization which occurs after interruption; by the time the needle
swing catches up with the actual measured potential, significant
depolarization may have already occurred. If used, this technique would
yield a measurement value that is less than the actual instant-OFF, which
can be measured using other methods.
Typical digital voltmeters take hundreds or thousands of readings a
second, but normally update the display only after a change in the reading
or about every of a second under constantly changing conditions. The
actual instant-OFF measurement must be determined by viewing the
digital display and manually recording the measurement. Since the
display is updated only after a change, and is only updated about every
second under changing conditions, the display must be interpreted.
Normally, the display is constant (not changing) while the current is on.
When the current is interrupted, the meter must recognize the change,
average the measurement and update the display. This first blink contains
an average of all the ON, all the OFF, and any spiking (either positive or
negative) which has occurred since the last display update or about of a
second. This first blink is not the instant-OFF: It contains part of the ON
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