Plug in the 100 ohm test resistor so that the unit will believe it is measuring
a sample.
Set a current of 1mA (press ‘1’ and ‘mA’).
Press ‘FWD’. The unit is now passing a current through the resistor and
measuring the voltage. As V=IR you can expect a reading of 100mV. The
unit’s accuracy is quoted at 0.3% so the reading should be between 99.70mV
and 100.30mV. In practice, particularly above 1mV and when warmed up
the accuracy is better than 0.3% so it would not be unusual to find that you
are within 0.1% of the expected value.
Press ‘REV’. The RM3000 is now passing current in the reverse direction.
Again you should receive a reading of 100mV, but this time with a minus
figure.
The voltages made may be slightly offset. In this case press the Zero button
to remove the offset.
The zero button is most effective when pressed while a
current is being passed.
Press the ‘
Ω /
□
’
button. For thin samples the most commonly used
measurement is sheet resistance. This is measured in ohms per square. As
the size of the square is not relevant to the measurement there are no units
such as ‘square centimetre’ or ‘square metre’. The formula for sheet
resistance is quite complicated but can be resolved to 4.5324 x V/I. When
using a 100 ohm resistor you can expect the reading to be approximately
453.24 ohms/square.
Press the ‘
Ω /
□
’
button again and the unit will once again show the reading
in mV. Note that the ohms/square value is absolute and therefore does not
have a positive or negative sign, but ‘F’ or ‘R’ will be shown on the display
to indicate in which direction the current is flowing.
The range of measurement in high resolution is 0-150mV. In Low resolution
it is 0-1250mV. Where practicable the high resolution should be used and a
mV reading of 10-100mV is ideal.
With the unit back in Standby remove the resistor. Attempt to make a
reading by pressing the ‘FWD’ key. This will cause an error and the
message “Contact Limit” will be displayed. By removing the resistor the
RM3000 can’t drive the current – you can therefore equate this to a four
