Metrohm 757 VA Computrace - Manual (Page 167)

7.9 Diagnostic procedures
757 VA Computrace – Software
157
8. Select SMDE and click on .
9. A small drop is formed and removed in intervals. The drop
size has to be reproducible and the drop has to be removed
in every interval.
10. If the drop is not removed reproducibly, adjust the tapper
mechanism (see Hardware Manual).
11. Select HMDE and click on .
12. A small new drop is formed and stays at the end of the capil-
lary. The drop should remain for at least 1 min. Otherwise the
capillary and/or the needle should be changed (see Hardware
Manual).
Perform a linearity test with the dummy cell
For testing the linearity of current measurement, the dummy cell
of the 757 VA Computrace Stand is used with the test method
Test757_L.mth . Proceed as follows:
1. Click on or MAIN WINDOW / Mode / Determination .
2. Click on or MAIN WINDOW / Window / Working method
specification to open the WORKING METHOD SPECIFICATIONS
window.
3. Click on or MAIN WINDOW / File / Load method .
4. Select the method file Test757_L.mth in the OPEN window and
click <OK> . The method is loaded into the WORKING METHOD
SPECIFICATIONS window.
5. Connect the dummy cell at the 757 VA Computrace Stand: At-
tach electrode cable AE to clamping screw AE, attach elec-
trode cable RE to clamping screw RE, attach electrode cable
WE to clamping screw WE-L.
6. Click on or MAIN WINDOW / Window / Monitor to open the
MONITOR window.
7. Start the measurement by clicking the icon in the MAIN
WINDOW or the button in the MONITORING window.
8. Enter the Sample ID (used as part of the determination file
name) in the PLACE SAMPLE window and click <OK> .
9. At the end of the measurement, a curve is printed out. This
curve should satisfy the following conditions:
• The plotted diagonal must be straight.
• At -200 mV, the current should be -1.6…-2.4 µA.
• At +200 mV, the current should be +1.6…+2.4 µA.