nuckolls.bob(at)aeroelect
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 Posted: Sun Nov 14, 2010 4:41 pm Post subject: LR3C and overvoltage (Clarification) |
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<snip>
. . . populated with fresh AA cells. Measure
the terminal voltage on the holder. If your
meter reads 12.8 volts, it's close enough
for government work. If it's something else,
make note of that difference. Let's say it's
12.5 volts. Okay, any reading in that range
is 0.3 volts HIGHER THAN WHAT IS DISPLAYED
ON THE METER so your correction factor is to
add 0.3 volts to the reading.
One reader asked why you couldn't check the
meter calibration with one cell. Two reasons:
(1) Many meters measure 15.0 and 1.5 on different
scale settings which may not exactly track each
other for %of error. In the example I gave, a
single cell would have been measured on the 2
to 4 volt full scale settings while 15.0 volts
was measured on 20 to 40 volt full scale settings;
same settings as those used to investigate the
situation in the airplane. If Radio Shack had
an 9-cell holder, it would be better yet for
qualifying your investigative instrument.
(2) Instruments have several kinds of errors.
Linearity, scale factor and offset. See instrumentation
chapter in the 'Connection. By doing your calibration
in the same neighborhood as the voltage of interest,
the effects of linearity error are washed out.
Effects of offset and scale factor are largely
combined together in the hypothesized 0.3 volt
error factor. This is a good example of how an
instrument with terrible accuracy characteristics
is still useful as a reasonably precise
'transfer standard' once its shortcomings are known.
Now, go measure the bus on your airplane.
Apply any observed calibration offset to the
reading and THAT becomes your benchmark for
evaluating the veracity of the other displays
in the airplane.
<snip>
Bob . . .
[quote][b]
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