How are the various quantities measured?
100,000 cts digital multimeters 35
How are the various quantities measured? (continued)
5. Resistance
measurement
The multimeter measures resistance (opposition to the flow of current) in
ohms (Ω). For this purpose, it sends a weak current through the measurement
leads to the circuit being tested.
The input (+, COM) must not have been overloaded by the accidental
application of a voltage on the input terminals with the switch set to Ω or T°.
Connecting
the multimeter
Selection of range: automatic or manual
"Active" protection: by PTC thermistor
Measurement voltage: approx. 1.2V
Max. open-circuit voltage: 4V typical
Because the measurement current of the multimeter takes all possible paths
between the probe tips, the resistance measured in a circuit is often different
from the nominal resistance.
The measurement leads can add from 0.1Ω to 0.2Ω of error to resistance
measurements. To test the leads, touch the probe tips together and note the
resistance of the leads.
To eliminate the resistance of the leads from the measurement, keep the
probe tips together, press the Meas… function key, then REL, and integrate
this measurement as REF.
A MATH secondary measurement is active in resistance measurement.
All measurements made then indicate the resistance between the probe tips.
In the 50 M range, in order to avoid the
influence of the mains and guarantee the
stated specifications, it is best to
disconnect the multimeter from the Wall
Plug.
For measurements greater than 10 M,
a shielded lead is recommended.
For a 2-wire link, use very short wires
(<25cm) and twist them together.
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