When Measuring Resistance; Two-Wire Configuration; Correcting For Test Lead Resistance; True Rms Measurements - Fluke 45 User Manual

When Measuring Resistance

Two-Wire Configuration

The meter measures resistance in a two-wire configuration using a resistance ratio (some-
times called ratio-ohms) technique. Two-wire resistance measurements are simple to set
up and yield good results for most measurement conditions.
The full-scale voltage for each resistance range is shown in Table 4-5. The
test lead is positive with respect to the COM lead.

Correcting for Test Lead Resistance

The resistance of the test leads can introduce error when measuring low resistances. Typi-
cal test leads may add as much as 0.5
To correct for this error using the relative modifier (REL):
1. Insert test leads in the
2. Turn the meter on and press
3. Select the manual range mode by pressing
to select the desired range. This is necessary because autoranging is turned off
(and the meter is locked in the range it is in) when the relative mode is selected.
4. Touch the test leads together. The display will show the resistance of the test leads.
5. With the test leads still touching, press
the relative base and the meter should show 0 .
6. As long as the relative modifier remains selected, the resistance readings shown on
the display will be the resistance measured minus the relative base, which in this case
is the resistance in the test leads.
Medium and Fast Reading Rates
Range Typical Full Scale Voltage
300
3 k
30 k
300 k
3 M
30 M
300 M
Open circuit voltage is 3.2 V (maximum) on the 100
1.5 V (maximum) on all other ranges.

True RMS Measurements

The meter measures the true rms value of ac voltages and currents. In physical terms, the
rms (root-mean-square) value of a waveform is the equivalent dc value that causes the
same amount of heat to be dissipated in a resistor. True rms measurement greatly
simplifies the analysis of complex ac signals. Since the rms value is the dc equivalent of
the original waveform, it provides a reliable basis for comparing dissimilar waveforms.
to readings.
and COM input terminals.
to select the resistance function.
Table 4-5. Ohms Test Voltage
Range
0.25 100
0.24 1000
0.29 10 k
0.29 300 k
0.3 1000 k
2.25 10 M
2.9 100 M
, 300
When Measuring Resistance
, , or . Then press
. The resistance in the test leads becomes
Slow Reading Rate
Typical Full Scale Voltage
0.09
0.10
0.11
0.11
0.12
0.65
2.75
, 30 M , 100 M , and 300 M
Applications 4
input
or
ranges, and
4-9