Operation - Aim-TTI BS407 Bedienungsanleitung

Switching On
Switch on the instrument using the Operate switch on the front panel. If there are no digits shown
in the display then the batteries may be completely discharged. Connect the unit to the supply
mains; the Charge lamp should light to show that charging is taking place. If display operation
does not resume after a few minutes charging time the unit may be faulty; switch off, disconnect
from the supply and seek service.
Battery Charging
When the unit is connected to the supply mains the red Charge lamp will be illuminated and the
battery will be charging. Resistance measurements may continue while charging with some small
reduction in accuracy because of the temperature rise within the unit. Charging normally takes
about 12 hours but if the 2000µΩ range is used the 250mA test current considerably reduces the
charging current and it will take longer to fully charge the battery. A taper charge characteristic is
employed to quickly raise the charge level of a completely discharged battery and reduce the
current once fully charged; nevertheless, the unit should not be left on continuous float charge as
this will reduce battery life.
The Ni-MH cells used do not suffer from any memory effect and are not harmed by either
complete discharge or partial re-charge.
While the unit is not in use the batteries will self-discharge. This effect is greatly increased at
higher temperatures. If the unit has been stored for so long that the batteries have become
completely discharged it may require two or three charge–discharge cycles for full capacity to be
restored.
The batteries are not replaceable by the user. If the capacity becomes noticeably low then the
unit should be sent to a qualified service facility for the batteries to be replaced. The cells must
be handled and disposed of in accordance with local safety and environmental requirements.
Principles of Operation
This instrument measures resistance using the ratiometric method: a test current is passed
through both a precise internal reference resistance and the unknown resistance. The magnitude
of the test current is chosen according to the range selected; it is a compromise between
sensitivity to noise and thermal emfs and minimising self-heating of the unknown. The voltages
developed across the two resistances are compared using a ratiometric Analogue to Digital
converter which calculates the result for the display.
The accuracy of this method depends on the accuracy of the reference resistors and on sensing
the exact voltage drop across the unknown, not on the magnitude of the test current. The two
main sources of error are voltage drops in the connections and thermal emfs.
The problem of voltage drops is overcome by the 4 terminal connection. The current flows in the
force leads and the instrument can tolerate any voltage drop in these leads up to its compliance
limit. There is no voltage drop in the sense leads because of the very high input impedance of the
voltage measuring circuit in the unit. The resistance is measured between the physical points
where the sense leads connect to the unknown.
An emf is produced at any junction between dissimilar metals. The magnitude of this emf
depends on the materials and the temperature. In any closed circuit which starts and ends on the
same metal (as in the sockets of the instrument) there will be a equal number of junctions
between dissimilar metals. If all the pairs of junctions are at the same temperature then the net
emf around the circuit is zero, but if there are temperature differences then the sum of the emfs
is not zero - this difference is the thermal emf of the circuit. It is often of the order of a few tens of
microvolts which is sufficient to influence the reading on low resistance ranges.
The instrument cannot distinguish between this emf and the voltage drop across the test sample
caused by the measuring current. However if the direction of the test current is reversed then the
thermal emf will add to one reading and subtract from the other; the true resistance can be
calculated as the average of the two readings. This is the purpose of the Polarity switch.
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Operation