Chapter 3 Configuring for Measurements 57
2-Wire Ohms Two-wire ohms is most commonly used when the resistance of the test leads
is much less than the value being measured. If the lead resistance is large
compared to the resistance to be measured, readings will be inaccurate. For
example, suppose you are measuring a 1
W resistor located 10 feet away. If
you use 24-gauge copper wire to make the connections, the 20 feet of leads
contribute about 0.5 ohms to the measurement. This makes the total
measurement 1.5 ohms--an error of 50%. Some other factors that may cause
high lead resistance are loose or dirty connections, kinked or damaged wires,
or a very hot environment. You can enhance the accuracy of 2-wire ohms
measurements with the NULL math operation (refer to NULL in Chapter 4
for more information). Figure 13 shows the front connections for 2-wire
ohms measurements. You specify 2-wire ohms measurements using the
OHM command. For example, to specify 2-wire ohms measurements on the
lk
W range, send:
OUTPUT 722;"OHM 1E3"
4-Wire Ohms The 4-wire ohms mode eliminates the measurement error caused by test lead
resistance. In 2-wire ohms, the voltage measurement is made across the
combined resistance of the test leads and the unknown resistance. In 4-wire
ohms, the voltage is measured across the unknown resistance only, not the
combined resistance. This is important when the test lead resistance is high
in comparison to the resistance being measured. Figure 14 shows the front
connections for 4-wire ohms measurements. You specify 4-wire ohms
measurements using the OHMF command. For example, to specify 4-wire
ohms measurements on the 10MW range, send:
OUTPUT 722; "OHMF 10E6"
10MW 12.000000MW 1W 500nA
100MW 120.00000MW 10W 500nA
1GW 1.2000000GW 100W 500nA
Table 14: Resistance Ranges
OHM(F) Range Full Scale Reading Maximum Resolution Current Sourced
Figure 13. 2-Wire ohms measurement connections
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