Aim-TTI BS407 Bedienungsanleitung Seite 11

Taking the Reading
After setting zero, release the Set Zero switch and allow the reading to settle at the resistance
value. Note this reading and depress the Polarity switch; ideally (if there are no external thermal
emfs) this reading will be the same as the first. If it is not, take the numerical average of the two
(ignore the sign); this value is the true resistance. Note that if the sample is not in thermal
equilibrium the reading will be changing because of both thermal emfs and the temperature
coefficient of the unknown.
It is possible to check the results by depressing the Set Zero button, noting the zero reading and
then computing the two differences between this value and the reading of each polarity (taking
the signs into account). These two values should agree within a digit; if they do not, then the
thermal emfs are changing too rapidly for a reliable measurement to be made.
Contact Resistance Measurements
The contacts of relays, switches etc. are often covered by oxides or corrosion products. If the
voltage in the circuit being switched is not high enough to break down this insulating film the
contact resistance measured will be much higher than that measured in a high power conditions.
In order to obtain a reading that reflects the operation of the component in these "dry circuit"
conditions it is necessary to ensure that the test equipment does not subject the sample to a high
open circuit voltage. International standards define the "dry circuit" measurement voltage as
being not more than 20mV.
This instrument contains a suitable clamp circuit activated by the front panel 20mV Clamp push
button. This places an internal electronic shunt across the force terminals and controls the
resistance of this circuit to maintain a voltage of 18mV (±2mV) between these terminals. When
the contact being measured closes, its resistance must be sufficiently low for the voltage drop
across it (at the measuring current of the range selected) to be less than the clamp voltage. The
internal shunt then switches off, the green Force On lamp lights and the correct resistance
reading is displayed.
Note that battery drain is higher when the clamp is engaged as the measurement current is
always flowing, either through the internal clamp or the external contact.
The 20mV clamp facility does not operate with the 2kΩ or 20kΩ ranges as the maximum
measurement voltage on these ranges is greater than 20mV.
Applications
To measure the temperature rise of the windings of a transformer or motor first measure the
resistance with the item cold. Then disconnect the meter and operate the device for the required
period of time. Disconnect all supplies and re-connect the meter and measure the winding
resistance in the hot condition. Knowing the temperature coefficient of the winding material these
two resistance readings can be used to calculate the temperature change.
Measurement Notes
If the reading drifts continuously this may indicate either a change in a thermal emf caused by a
change in temperature or a real change in the resistance of the item being measured. If the
sample is physically small this may be caused by the heating effect of the test current. Using a
higher range reduces the test current but also reduces the measurement resolution.
Random fluctuations in the reading of more than a digit may indicate poor connections to the
sample under test, particularly the sense leads. It can also be caused by magnetic fields
intersecting the test circuit; move the leads to keep them close together and away from any
transformers or motors.
If the three readings (zero, normal and reverse polarity) do not correlate then either there is a
poor connection, or a thermal emf is changing rapidly or the actual value being measured is
changing rapidly.
Semiconductor junctions will appear open circuit in both directions because the measurement
voltage of this instrument is too small to cause any noticeable conduction.
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