1.
Instability
or
zero offset is much higher than expected.
2. The reading is sensitive to (and responds to) temperature
changes. This can be demonstrated by touching the circuit,
by placing a heat source near the circuit
or
by a regular
pattern of instability (corresponding to heating and air-
conditioning systems or changes in sunlight).
3. To minimize the drift caused by thermal emfs, use copper
leads to connect the circuit to the Model 197.
A
banana
plug is generally sufficient and generates just a few
microvolts.
A
clean copper conductor such as
#lo
bus wire
is about the best for the application. The leads to the input
may be shielded
or
unshielded, as necessary. Refer to
Shielding.
4.
Widely varying temperatures within the circuit can also
create thermal emfs. Therefore, maintain constant tem-
peratures to minimize these thermal emfs.
A
cardboard
box around the circuit under test also helps by minimizing
air currents.
5. The REL control can be used to null out constant offset
voltages.
4.0V
4.0V
5.0V
2.7.6
Resistance Measurements
The Model 197 can make resistance measurements between
1m3 and 22OM3. The 2M3, 20MQ and 200MQ ranges
autorange when the
M3
button is selected. The Model 197
provides automatic selection of 2-terminal
or
4-terminal
resistance measurements. This means that
if
the ohms sense
leads are not connected, the measurement is done &terminal.
If
the sense leads are connected the measurement is done
4-terminal. For 4-terminal measurements, rated accuracy can
be obtained as long as the maximum lead resistance does not
exceed the values listed in Table 2-3. For 2-terminal
or
4-terminal measurements on the 2003 range, zero must be set
by the REL function to obtain rated accuracy. For best results
on the 2003, 2k3 and 2Ok3 ranges, it is recommended that
4-terminal measurements be made to eliminate errors caused
by the voltage drop across the test leads that occurs when
2-terminal measurements are made. To make resistance
measurements, proceed as follows:
Range
200
2k
20 k
,
200 k
200M
1.
Connect the test leads to the HI and
LO
INPUT
terminals.
If four-wire measurements are to be made, connect an ad-
ditional set of leads to the OHMS SENSE terminals.
NOTE
The Model 1641 Kelvin test lead kit is ideal for
low resistance 4-terminal measurements.
2. Select the ohms function.
3.
Select a range consistent with the expected resistance.
If
desired, use the autorange mode for automatic range selec-
tion.
4.
If
readings are to be made on the 2OOQ range, zero the in-
strument to obtain rated accuracy. To zero the instrument
for resistance measurements: Short the test leads together
after disconnecting them from the measured circuit:
if
4-wire measurements are to be made, short
all
four test
leads as shown in Figure 2-6, allow the reading to settle
then press the
REL
button.
5.
For 2-wire measurements connect the instrument as shown
in Figure 2-4. For 4-wire measurements, use the connec-
tions shown in Figure 2-5.
CAUTION
The maximum input voltage between the
HI
and LO INPUT terminals is
450V
DC or
peak AC for
10
seconds per minute,
350V
RMS
continuous. Do not exceed these
values or instrument damage may occur.
Table
2-3
shows the current output for
various resistance ranges.
CAUTION
Incorrect readings will result if the
resistance being measured is part
of
a live
circuit.
6. Take the reading from the display.
Table
2-3.
Resistance Ranges
tesolutior
lm3
1
OmQ
1
OOmQ
1Q
10
0
100
Q
10
k3
I-S
hort
2mA
2mA
400pA
40 pA
400 nA
400 nA
4 PA
Across
Unknown
Maximum Test Lead
Resistance for
(1
Count Error
(Q)
10
32
100
320
Ik
3.2k
10k
2-8
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
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