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3-22
Basic DMM Operation
Model 2750 Multimeter/Switch System User’s Manual
NOTE
The OCOMP annunciator will flash when the instrument is on an invalid range
(100k
Ω
through 100M
Ω
ranges) for offset-compensated ohms. Normal ohms
measurements will instead be performed.
With dry circuit ohms enabled, 1k
Ω
is the highest offset-compensated ohms
range that can be selected.
For buffer recall, there is no way to distinguish between a normal ohms reading
and an offset-compensated ohms reading. The OCOMP annunciator (off, on, or
flashing) has no significance for recalled resistance readings that are displayed.
Buffer operation is covered in Section 6.
With offset-compensated ohms enabled, it will be “remembered” by the
Ω
4 func-
tion after you change measurement functions (i.e., DCV). When
Ω
4 is again
selected, offset-compensated ohms will be enabled.
Dry circuit ohms (DRYCKT)
Standard resistance measurements have open-circuit voltage levels from 5.4V to 12.8V,
depending on the selected range. Dry circuit ohms limits open-circuit voltage to 20mV.
This allows you to perform resistance measurements that require low open-circuit voltage.
Dry circuit ohms can be used on the 1
Ω,
10
Ω,
100
Ω,
and 1k
Ω
ranges for the
Ω
4 function
only. Also, offset-compensated ohms (OCOMP) can be used with dry circuit ohms to can-
cel the effect of thermal EMFs.
Dry circuit ohms applications
Measuring contact resistance (oxide film build-up)
The ideal resistance between switch connectors, or relay contacts is 0
Ω
. However, an oxide
film may be present on the switch or relay contacts. This oxide film could add resistance
on the order of several hundred M
Ω
s. Also, this oxide film changes the contact resistance
over time and with changes in the environmental conditions (such as temperature and
humidity).
Typically, the
Ω
4 function of the Model 2750 or a standard DMM is used to measure low
resistance. However, if standard resistance measurements are performed, the relatively
high open-circuit voltage may puncture the oxide film, and render the test meaningless.
Dry circuit ohms limits voltage to 20mV to minimize any physical and electrical changes
in a measured contact junction. This low open-circuit voltage will not puncture the film,
and will therefore provide a resistance measurement that includes the resistance of the
oxide film.
Oxide films may also build up in connections on a semiconductor wafer. In order to accu-
rately measure the resistance introduced by the oxide film, dry circuit ohms should be
used to prevent oxide film puncture.
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