Aim-TTI BS407 Bedienungsanleitung Seite 10

The most effective way to remove thermal emfs is by removing the temperature differences; take
care when connecting samples not to hold the connections points in the fingers - do not touch
plug contacts when connecting the leads. Wait until thermal equilibrium is established - it can
take many minutes for the effect of even the most brief contact with the fingers to dissipate.
The effect of thermal emfs could also be eliminated by the use of an AC measurement technique
but this results in measuring the resistive component of impedance (including the effect of any
core losses etc.) rather than the true resistance value obtained by the DC method used in this
instrument.
Setting Zero
To set the zero reading it is necessary that the force current is flowing (so the reference
resistance is experiencing normal conditions) and that there is no voltage between the sense
terminals. The ZERO ADJUST control can then be set for a reading of zero to compensate for
the internal offset voltages of the instrument.
The most direct way to do this is to use one cable to connect FORCE + to FORCE – and a
second cable to join SENSE + to SENSE –, select the range required and use the ZERO
ADJUST control to obtain a reading of zero. Note that this reading is signed: the exact zero is at
the point where the minus sign in the display appears on alternate readings. These cables are
then removed and the test sample connected as required. This method zeros the internal offsets
within the instrument.
The zero adjustment can also be used to remove the effect of small external thermal emfs from
the measurement. Connect the test cables to the unknown ready to take a measurement and
select the required range. Depress the Set Zero button and set the reading to zero. Release the
Set Zero button and allow the reading to settle at the resistance value. This is the most
convenient method in normal use.
Note that the zero cannot be set with the sense terminals open circuit.
Connecting the Unknown Resistance
The Kelvin clips supplied with the instrument make the force and sense connections on opposite
sides of the conductor. This suits wire-ended components and long thin objects.
When measuring low resistance physically large samples, separate wires and clips may be
needed for the two sets of connections which must be made in a way that reflects the required
measurement. The sense terminals should be connected to the sample at the exact points
between which the resistance is to be measured; the force terminals should be connected to the
sample outside the sense terminals. They should be sufficiently far from the sense connections
to allow for the region where the current spreads out across the cross-section of the sample.
When measuring components the test connections should match the application connections as
closely as possible; when measuring the properties of material samples, consistency of
connection is the most important thing. This may require the construction of a suitable jig.
The green Force On lamp lights when the test current flowing through the unknown produces a
voltage drop within the compliance specification of the instrument. A valid reading can then be
obtained provided that the sense connections are properly made (if they are not then the display
will normally show the overrange indication). Note that when measuring very large inductances
(such as high voltage transformers) there may be a delay before the lamp lights as it takes some
time for the test current to rise to the full value (the rate of rise of the current depends on the
inductance and the source emf of the instrument). Similarly it takes time for the current to reverse
when the Polarity selection is switched.
It is very important when attaching connections to avoid touching any metallic part of the circuit,
because the heat conducted from the fingers will generate thermal emfs that take a significant
time to decay.
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