Operation Manual

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
(100kΩ through 100MΩ ranges) for offset-compensated ohms. Normal ohms
measurements will instead be performed.
With dry circuit ohms enabled, 1kΩ 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 Ω4 func-
tion after you change measurement functions (i.e., DCV). When Ω4 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 1Ω, 10Ω, 100Ω, and 1kΩ ranges for the Ω4 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 0Ω. 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 MΩs. Also, this oxide film changes the contact resistance
over time and with changes in the environmental conditions (such as temperature and
humidity).
Typically, the Ω4 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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