AVO AVOMETER 8 Manual de Instruções - Página 8

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OPERATION
INSULATION
RESISTANCE MEASUREMENT
High resistance
measurements
may
be
made using
an
external de
voltage of the
order of
140/
160V.
The
left-hand
switch should
be set at
n
with the right-hand
switch
at
INS
and
the
meter
leads
should
be
connected
to
the
battery. The
pointer
should be
brought
to
zero
on
the
ohms scale by
means
of the adjuster
marked
n
x 1
Ok.
To
test, connect
the
unknown
resistance
in series with
the
meter and its value will be that shown
on
the
ohms
scale
multiplied
by
1
00,000. Resistances up to
200
megohms
can. therefore.
be
read on
this
range.
DECIBEL MEASUREMENTS
The graph on Page 7
can
be used to determine the dB
values
corresponding to
r.m.s.
voltage values
across
a 600
ohm
resistive load.
A dB value is defined
as
the
number
of
decibels above
or below
a reference level of 1 mW
in
600
ohms at 1kHz.
Zero
dB, therefore, would
indicate a
power
level of 1mW;
10dB,
10mW, 20dB,
100mW.
Because dB
are defined
with
respect
to
a 600
ohm
load, power
levels
correspond
to
voltage
levels. Decibels can
be measured in
terms
of r.m.s. voltages across a 600 ohm resistive
load.
For example, 0.775V r.m.s.
indicates
OdB and 7.75V
r.m.s.
indicates
20dB.
Whilst these measurements must be
made
with
a
sine waveform to
avoid
waveform
error, any
fre-
quency may be
used
within the range of the Model
8
Mark 5. The
decibel
and
ear
response curves have their
closest correlation
at
1kHz.
6
Power
levels
can be read
along
the top of
the graph.
If
the
r.m.s. voltage is
measured
across a resistive
load
other
than
600 ohms
the correction
factor given below
must be
added
algebraically to
the
dB values read from the
graph. The following formula
should
be
used
for deter-
mining
the correction
factor.
C
.
0
600
orrect1on
Factor
=
1 log,
0
-
10R
where R is the load in ohms. If
R
is greater
than
600 ohms
the
Correction
Factor
is
negative.
POLARITY REVERSE
CONTROL
If
de voltage is
required
both positive and negative to a
reference
point,
or
the
direction of current flow may be
reversed, in order to simplify the
matter of
lead alteration,
a
polarity reverse
press
button (REV. M.C.) is provided.
It
should
be noted that the polarity marked on the terminals
is for
normal use and does not
apply
when the red
section
of
the REV.
M.C. button can be seen.
OVERLOAD PROTECTION
One
of the most
attractive
features
of the
instrument is
the
provision of
an
automatic cut-out
which
gives a
very
high degree
of overload
protection, and imparts
to the
user
a
feeling of confidence.