Metrohm 797 VA Computrace - Manual (Page 38)

2 Installation
797 VA Computrace / Hardware-Manual 8.797.8001EN
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can also lead to leakage of the MME, i.e. the mercury flow cannot be stopped any
more.
Proceed as follows:
1 Remove Multi-Mode Electrode from 797 VA Computrace Stand
• Unscrew FEP tubing 34 and 42 from the MME, disconnect electrode cable
20 from MME.
• Take Multi-Mode Electrode 21 out of measuring head 24 while gently tap-
ping the MME to knock off any mercury drops on the glass capillary into
the measuring vessel.
• Place Multi-Mode Electrode in a horizontal position in the electrode holder
92 (see Fig. 11-3).
2 Unscrew slotted screw 74
• Using a suitable coin, unscrew slotted screw 74 out of screw thread 76 un-
til the contact surface of the black O-ring at the Plexiglas wall (thin, black
stripe) is just visible below the metal thread.
3 Replace sealing needle 75
If problems with leaks arise owing to a worn, deformed or damaged sealing
needle, it must be replaced. Three spare needles are supplied separately in a
protective plastic package. After unpacking a needle, please avoid any contact
whatsoever with the needle tip. The spare needle 75 is installed as follows:
• Carefully pull old sealing needle out of PTFE membrane of the slotted screw
74.
• Carefully insert new sealing needle without tilting into the hole in the PTFE
membrane of the slotted screw.
NOTE: Do not touch the needle with bare fingers but use the plastic package
4 Dismantle old capillary
• Position Multi-Mode Electrode vertically in the electrode holder (see Fig. 11-
1).
• Undo retaining nut 86 completely by turning anticlockwise and lift up until
the lower part of the glass capillary 87 with the wide opening is visible.
• Gently tap the glass capillary to knock off any residual mercury in the wide
opening into the MME.
• Press the retaining nut downward with one hand and with your other hand
take glass capillary completely out of the mount.
5 Dispose of old capillary
• Connect filling tubing 93 with the tubing coupling 96 to the two gas wash
bottles 95 and the vacuum pump (see Fig. 10).
• Insert glass capillary 87 (capillary end) in the filling cone 94 of the filling
tubing.
• Remove mercury from capillary with the vacuum pump.
6 Replenish mercury if necessary
Proceed as described in section 2.3.2.