System Switch/Multimeter User's Manual Section 5:
Basic Digital Multimeter (DMM) Operation
-900-01 Rev. A / August 2007 5-
With offset-compensated ohms enabled, it will be “remembered” by the 4W function
after you change measurement functions (i.e., DCV). When 4W is again selected,
offset-compensated ohms will be enabled.
dmm.offsetcompensation is a common ICL command and is shared with
fourwireohms, dryciruit, threertd and fourrtd. To activate dmm.offsetcompensation,
select the available function desired, set dmm.offsetcompensation = dmm.ON or
OFF. The function will retain the dmm.offsetcompensation state even if the function is
changed.
Dry circuit ohms (DRY+)
Standard resistance measurements have open-circuit voltage levels from 6.4V to 14.7V,
depending on the selected range. Dry circuit ohms limits open-circuit voltage to between 20mV
and 27mV. This allows you to perform resistance measurements that require low open-circuit
voltage. Dry circuit ohms can be used on the 1, 10, 100, 1k and 10k ranges (maximum
resistance of 2.4k for the 4W function only. Also, offset-compensated ohms (OC+) can be
used with dry circuit ohms to cancel the effect of thermal EMFs.
Measuring contact resistance (oxide film build-up)
The ideal resistance between switch connectors, or relay contacts is 0. 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 milli-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 4 function of the Series 3700 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 accurately
measure the resistance introduced by the oxide film, dry circuit ohms should be used to prevent
oxide film puncture.
Measuring resistance of voltage-sensitive devices
Dry circuit ohms should be used for any device that could be damaged by high open circuit
voltage. If you are not sure the slightly degraded accuracy is a consideration, it is good practice
to use dry circuit ohms to measure low resistance.
Enabling/disabling dry circuit ohms
Dry circuit ohms is an attribute set on the 4-wire ohms (4W) function.
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