point probe being on a sample but the RM3000 having a problem driving the
current. This could be because the sample is too insulating, because the
current needles are not in proper contact with the sample, or simply that a
lead is disconnected! Using a lower current will sometimes prevent a
‘Contact Limit’ error – this could be a situation where the sample has
become damaged due to excessive current, or where the contact resistance of
the material is high.
Autorange
Queries for poor results are often raised where the cause is simply that an
inappropriate current has been selected. The Autorange facility is not
designed for use before every measurement, but can be used to select an
appropriate current range for the samples being measured.
When making measurements and the RM3000 is in standby, pressing the
Standby button will initiate the Autorange feature. It will begin at 10nA and
increase the current by a factor of 10 until a suitable level of voltage
measurement is made. At this time it will stop and display the voltage. It is
not necessary to use only the current given for the measurements, but it
indicates the level around which optimum results can be achieved.
Example:
Plug in the 100 ohm test resistor and while the unit is in standby press the
Standby / Auto button. The unit will begin to autorange and should stop at
100uA current (yielding 10mV reading).
Where a sample has very high or very low resistivity you may observe the
following:
Low resistivity – the current will autorange up to its maximum current and
stop (99.99mA). This does not mean that the reading obtained is invalid,
simply that the measured voltage is below what we consider to be ideal.
Where the maximum current is required and the reading is below 10mV it is
more likely that the unit will need to be zeroed (while the probe is in contact
with the measured material and the current is flowing) in order to get good
forward and reverse readings.
High Resistivity – samples with very high resistivity can sometimes take
some time to reach the optimum voltage level. If a contact limit is obtained
when the unit has reached only the 10nA or 100nA level try setting 10nA
