5.3
Resistance measurement
The DO7PLUS should be in the measurement [STOP] mode. In this mode the measuring
current is switched off and the current terminals are shorted internally. Connect the measuring
leads as described in section 5.1 Before starting the measurement select the current mode
required as described in section 4.6. Select the range required see 4.5 or auto range.
WARNING
the maximum measuring current is 10 Amp on the lowest measuring range ensure
that the current level selected will not damage the resistance being measured
To start the measurement press the
MEASURE
button the measured resistance value will be
displayed.
5.4
Resistance measurement with temperature compensation
Temperature compensation should be used when measuring the resistance of materials
which have a high temperature coefficient. Copper, for example, has a temperature coefficient
of 0.3930%/K. The value of the resistance measured will therefore vary with the temperature
of the copper. To obtain consistent and meaningful results, the resistance values may be
related to a set ambient temperature, normally 20
q
C.
Example
: Copper with a temperature coefficient of 3930ppm/k = 0.3930%/k
To measure with the temperature compensation, proceed as in 5.3 but in addition connect the
temperature sensor probe (see section 9 for accessories details and part numbers) to the
front panel socket. Select the temperature compensation measurement mode as described in
4.9.9. Alternatively, if the ambient temperature is stable, a manual value may be entered, in
this case a temperature sensor is not required. The compensated resistance value will be
displayed in large digits on the DO7PLUS screen and the true measured resistance value will
be shown in smaller digits below the main display. The measured and set temperature will
also be shown.
5.5
Effects of Lead Resistance
The DO7PLUS is a true four terminal measuring instrument and will, therefore, measure
accurately with long lead lengths. The DO7PLUS monitors the lead resistance and will display
an error message if the lead resistance exceeds the resistance values below. It is good
practice to keep the lead lengths to a minimum, this ensure that any magnetic fields and other
environmental noise that may be present have minimal effect. Sometimes long leads can’t be
avoided and in these cases the cable resistance should be kept to a practical minimum and
not exceed the following:
x
Current Leads Maximum resistance 60m
each lead (120 m
total)
x
Potential leads Maximum resistance 1k
each lead (2k
total)
HINT:
The accuracy of the measurement is not affected by different lead lengths, only by the
resistance of the leads.
Temperature
20
q
C
25
q
C
30
q
C
35
q
C
Resistance of Copper
18.000 m
:
18.354
m
:
18.707
m
:
19.061
m
:
% Increase in resistance
0%
1.96%
3.93%
5.89%
Operating Instructions
Digital Microhmmeter Type DO7 PLUS
34
