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 connect
ed
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 r
es
istance 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 mego
hms
can. therefore. be
read
on this range.
DECIBEL
MEASUREMENTS
The graph on
Page
7 can
be
used
to determine the dB
va
lu
es
corresponding to
r.m
.s.
voltage
va
l
ues
across a 600
ohm
resis
tive 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
ter
ms
of
r.m.s. voltages across a 600 ohm resistive load.
For exampl
e,
0.775V
r.m.s.
indicates
OdB
and 7.75V r.m.s.
indicates
20dB. Whilst these measurements must
be
made
with a
si
ne 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 t
he
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 fact
or.
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
sec
tion
of
the
REV.
M.C. button can be
seen.
OVERLOAD
PROTECTION
On
e
of
the most attractive features
of
the instrument is
the provision of
an
automatic
cut-out
which gives a
ve
ry
high degree of overload protection, and imparts to the
user a feeling
of
confidence.
To Next Page
Loading...
