Making Voltage and Current Dual Measurements
Measuring ac and dc Volts, or ac Volts and frequency, etc. still only require two measurement
probes because both parameters of the dual measurement are made at the same physical point.
Simultaneous measurement of voltage and current on the same circuit will however require a
third connection, see the diagram:
Note that the voltage measured at the multimeter terminals is that across the load plus the
voltage drop in the common lead which is now carrying the whole circuit current. Even if the
resistance of the lead is very low, errors may arise at high currents and low voltages (i.e. low load
resistance) because the lead resistance becomes significant compared with the load.
Measurement Update Times
As discussed at the beginning of this section, the reading update rate is reduced in Dual
Measurement Mode because the two readings are made alternately. However, the reading rate is
not simply halved because enough time must be allowed for each measurement to fully settle to
the different conditions before the display is updated; unless this is done neither display will
reliably show the true measurement value. The settling time depends on the differences between
the main and secondary measurement range, function, and signal level; the delay is longest when
both displays show an ac measurement. Note that the settling times allowed assume a steady
state signal; varying signals will give unpredictable readings.
The table below summarises the measurement time (i.e. the time for the measurement to settle
and the display to be updated) for each parameter in all the permitted display combinations.
Main Secondary Measurement Time
Any Function None
.25s
Vdc, Vac, Idc, Iac, Hz Same as Main
.25s
Vac, Iac Hz
.25s
Hz Vac, Iac
.25s
Vac, Iac, Vdc, Idc, Hz
.5s
Vdc, Idc, Hz Vac, Iac
.5s
Vac Iac 4s †
Iac Vac 4s †
† This time is extended to 8s if the measurement value is <1000 counts to allow for the increased settling
time required by the True RMS converter at low input levels.
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