practice, particularly above 1mV the accuracy is better than 0.5% so it would not
be unusual to find that you are within 0.2% of the expected value.
Press ‘REV’. The HM21 is now passing current in the reverse direction. Again you
should receive a reading of 100mV, but this time with a minus figure.
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. Therefore when using a 100 ohm resistor you can expect
the reading to be approx 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.
Sometimes the voltage readings can be slightly offset. This offset can be eliminated
by ‘zeroing’ the HM21. With the unit already in Standby, press the Standby button.
The unit will display ‘Zeroing…’ and after a few seconds return to standby mode.
When making measurements if you have different forward and reverse readings it
is worth zeroing the HM2) to make sure that it is not being caused by an offset. If
you still obtain readings which are very different in forward and reverse this can
indicate a poor contact or invalid measurement site.
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 50-100mV is ideal.
Use the ‘INC’ button to increase the current to 10mA. With a 100 ohm resistor we
will expect a reading of 1000mV. Press ‘FWD’. You should encounter an error
message as 1000mV is outside of the high resolution range. The unit will display
‘Out of Range’. Press ‘Low’ and you should obtain the reading of 1000mV.
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 we have made it so that the HM21 can’t
drive the current – you can therefore equate this to a four point probe being on a
sample but the HM21 having a problem driving the current through. 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
