Section 5: Basic Digital Multimeter (DMM) Operation
Series 3700 System Switch/Multimeter User's Manual
5-36
3700S-900-01 Rev. C / July 2008
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.
NOTE
When dry circuit ohms is enabled, offset-compensated ohms is
automatically enabled (OC+ annunciator). If you do not wish to use
offset-compensated ohms, after setting dry circuit ohms, disable
offset-compensated ohms using the information in
Enabling/disabling
offset-compensated ohms
(on page 5-33).