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should not be left in the instrument, since it might
cause damage by leaking electrolyte. If it is
impossible to obtain readings on the 'Ω x 1' and
'Ω x 100' ranges, the 1 A fuse located in the
battery compartment should be checked.
Note:- A 15 V battery may age in such a manner
that although it indicates a potential of
15 V, its internal resistance has increased so much
that some loss of accuracy can occur on the high
resistance ('Ω x 10k'). If errors are suspected on
the high resistance range, remove the battery and
check its short circuit current on the 100 mA d.c.
range. If the reading is below 15 mA it should be
discarded. Do not short circuit for more than two
seconds.
INSULATION RESISTANCE MEASUREMENT
High resistance measurements may be made
using an external d.c. voltage of approximately
150 V. The left-hand switch should be set at 'Ω'
with the right hand switch at 'ins', then the
instrument test leads should be connected to the
d.c. voltage source. The pointer should be
brought to zero on the ohms scale by means of
the adjuster knob marked 'Ω x 10k'.
To test, connect the unknown resistance in series
with the meter and its value will be that shown
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on the ohms scale multiplied by 100 000.
Resistance up to 200 megohms can therefore be
read on this range.
DECIBEL MEASUREMENT
The graph on page 14 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 1 kHz. Zero dB, therefore, would indicate a
power level of 1 mW; 10 dB, 10 mW; 20 dB, 100
mW etc. 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, 0,775 V r.m.s. indicates 0 dB and
7,75 V r.m.s. indicates 20 dB. Whilst these
measurements must be made with a sine
waveform to avoid waveform error, any frequency
may be used within the range of the Model 8mk7.
The decibel and ear response curves have their
closest correlation at 1 kHz.
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
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