Metrohm 791 VA Detector - Manual (Page 34)

3 Operation
791 VA Detector
28
3 Overload test
For this test the working electrode must not be connected to the
791 VA Detector:
• Set the polarization voltage on the 791 VA Detector to the
required value for the following determinations with digital
switch 11.
• Set commutator 3 on the 791 VA Detector to "meas". Over-
load display 2 must light up briefly and then go out again.
• If overload display 2 remains lit up for a longer period of time
this indicates that there is a faulty electrical contact between
the reference electrode and the auxiliary electrode. Eliminate
the cause (e.g. air in the system or in the reference electrode,
blocked diaphragm, faulty cable connection) and repeat the
test.
• If the overload test is OK, set commutator 3 on the 791 VA
Detector back to "Stand-by".
4 Equilibrating the working electrode
• Connect working electrode 28 to connection 18 of the 791 VA
Detector with the 6.2120.020 Cable.
• Set commutator 3 on the 791 VA Detector to "meas".
• Observe equilibration curve: the curve should flatten out
noticeably within a relative short time (10 … 20 min), otherwise
the working electrode must be repolished or refilled.
• In current measuring ranges > 5 nA a constant base current
is usually achieved with one hour. At higher sensitivities the
working electrode should be equilibrated overnight.
5 Zero balancing the base current
(only for recorders)
• Set the current measuring sensitivity on the 791 VA Detector
with knob 8 to the required value.
• Use potentiometer 9 on the 791 VA Detector to adjust the
base current to the zero point of the recorder. If the setting
range of potentiometer 9 is not sufficient then it can be ex-
tended by using commutator 5 ( × 5) or 6 ( Ч 50).