Fluke 825A - Page 18

825А
d.
FROM
50,
000
MEGOHMS
to
250,
000
MEGOHMS.
To
determine
the
value
of
an
unknown
resistance
be-
tween
50,000
megohms
and
250,
000
megohms,
proceed
as
follows:
(1) Perform preliminary procedure as stated in
paragraph 2-3.
(2) Set RANGE switch to 500 and NULL switch to 0.1.
(3)
Connect
unknown
resistance
between
input
ter-
minals.
(4) Adjust voltage readout dials for a convenient
meter deflection.
(5) The resistance in megohms may be calculated
by
substituting
the
meter
reading
in
volts
(Em,
0
to
0. 1 volt on bottom scale) and the voltage readout dial
setting (E) into the following equation:
Rx = 10 Tl megohms
2-9. NOTES ON MEASURING VOLTAGES
a. ADJUSTMENT OF 500 VOLT SUPPLY. The 500
volt DC reference supply may be adjusted (paragraph
2-3, step c) at any time deemed necessary without heed
to the position of the switches and without removing any
input or output connections. However, until the instru-
ment has warmed up to an equilibrium temperature
. (about 1/2 hour), it should be adjusted prior to each
Specific measurement for best accuracy. When making
prolonged measurements, allow one hour warmup time
to insure that 500 volt reference supply does not shift
during the final warmup phase.
b. GROUND LOOP PRECAUTIONS. Ground loop cur-
rents should be avoided to assure accuracy when making
measurements. Potential differences are often found at
different points on power system grounds. When this is
the case, current may flow from the power system
ground thróugh the 825A and the equipment under mea-
surement and back to the power system ground. To
avoid this when system being measured is grounded, do
not connect 825A input binding posts to chassis ground
post.
c. USE OF SHORTING LINK. А 0.47 uf capacitor
(C1) is connected from the lower input binding post to
the chassis ground post to reduce the effect of circu-
lating AC currents from the transformer. In some
cases, it is possible for C1 to acquire a charge. For
example, C1 will become charged through leakage re-
sistance over a period of time, if there is no external
connection to the input terminals and the controls are
set as follows: range to 500, null to any null range,
polarity to +, and voltage readout dials to several hun-
dred volts. This condition may cause an error on low
level measurements (under 5 volts) due to C1 discharg-
ing through the Kelvin-Varley divider. Connecting the
Shorting link from the lower input post to the ground
post will discharge C1 and thus prevent an inaccurate
indication.
d. RECOMMENDED NULL RANGES. Certain null
ranges are recommended for use with each setting of
the RANGE switch for the following reasons. With the
2-6
RANGE switch at 500 volts, the last voltage readout
dial (E) changes the reference voltage in steps of 0.01
volt. Therefore, the unknown voltage would have to be
an exact multiple of 0. 01 volt if a null is to be obtained
on the 0. 1, 0.01, or 0.001 volt NULL range. Further-
more, it is unlikely that an unknown voltage of a few
hundred volts will be stable within 10 millivolts. Fi-
nally, the regulation of the reference supply is approxi-
mately 0.002595, ог +0. 0125 volt for a 10% change іп
line voltage. Although this is more than adequate when
the instrument is used in the recommended way, a badly
fluctuating line voltage may cause the 825A to meter
the regulation of its own 500 volt reference supply. For
example, when measuring 500 volts, a line change of
10% may cause the 500 volt reference supply to change
as much as 12. 5 mv. Although this is small, the 825A
will indicate full scale for a change as little as 100 mv,
10 mv, or 1 mv if attempting to use the 0. 1, 0.01, or
0. 001 null ranges.
e. EFFECT OF AC COMPONENTS, An AC component
of several times the unknown DC may be present on the
unknown and the 825A will always indicate well within
the specifications for frequencies over a few hundred
cycles. An AC component may have an adverse effect
if it is of a low frequency or if it has a frequency that
is a multiple or submultiple of the chopper frequency.
A double section low pass filter (R201, C201, R202, and
C202) is used at the input of the null detector to reduce
any AC present on the DC being measured. At lower
frequencies, this low pass filter is less effective and
the magnitude of the AC component is more significant.
If this frequency is below 100 cycles, the accuracy may
no longer be with specifications. For example, a 60
cycle AC voltage that is 10% of the input voltage will
cause an error of approximately 0. 01% which is well
within the specifications. Also, since the input attenu-
ation is less for the more sensitive null settings, the
accuracy may be affected only on the more sensitive
null settings. When the frequency is very close to a
multiple or submultiple of the chopper frequency (ap-
proximately 94 cycles), the meter needle will oscillate
at the difference frequency. If AC components that
affect the accuracy are ever encountered, additional
filtering will be required. For an AC of a single fre-
quency, a twin- T filter is effective and has the advan-
tage of low total series resistance. For an AC variable
frequency, an ordinary low pass filter may be used. In
either case, high quality capacitors of high leakage re-
sistance should be used.
f. MEASUREMENT OF NEGATIVE VOLTAGES. Be-
cause of a polarity switch, voltages which are negative
with respect to ground as well as the more commonly
encountered positive voltages may be measured with
equal facility. If the upper input post is connected to
the metal case, either at the 825A or at the source under
measurement, the accuracy of the voltmeter may be
reduced. However, with the polarity switch, the upper
input post never has to be connected to ground. If the un-
known voltage is grounded, always connect the grounded
side to the lower input post (middle post) and use the
polarity switch to obtain the proper result.