Metrohm 757 VA Computrace - Manual (Page 30)

3 Installation
757 VA Computrace – Hardware
26
5 Checking the MME for leaks
• Switch on the dropping mercury electrode by selecting
DME in the COM-
PUTRACE CONTROL window and clicking on . The mercury
drops freely out of the capillary.
• Select
HMDE and click on . A single mercury drop is formed.
Knock this off by gently tapping the MME 17 with your finger and check
that the mercury flow has really stopped. Repeat this operation several
times.
• If mercury continues to flow, turn slotted screw 75 still further in a clock-
wise direction and repeat check.
• If it is not possible to stop the mercury flow, both the glass capillary 88
and the sealing needle 76 have to be replaced (see section 3.3.9).
3.3.5 Filling the capillary using vacuum
Filling of the glass capillary 88 with vacuum is advisable in all cases where difficul-
ties have been found with the method without vacuum described in section 3.3.4.
Filling with vacuum is especially recommended when no ultrapure Hg is available.
To fill the mounted glass capillary 88 (section 3.3.3) with Hg with vacuum, proceed
as follows:
1 Set up filling station
• All actions involving the electrode and the mercury vessels must be
performed in or over the drip pan 92 supplied (see Fig. 10).
• The MME 17 is placed in the electrode holder 93 for filling.
2 Connection for vacuum pump
• For filling the capillary 88, the filling tubing 94 is required. At one end it is
fitted with a filling cone 95 for mounting on the capillary 88, and at the
other end with the tubing coupling 97 for attachment to the line for the
vacuum pump.
• To avoid possible mercury losses, two gas wash bottles 96 are attached
to the filling tubing 94.
3 Vacuum pump
• To draw up mercury a suitable vacuum pump is required (e.g. water jet
pump). The partial vacuum ∆ p should be around 25 mbar.
• A vacuum release tap must be installed at the vacuum pump or in the line
between the gas wash bottle and the pump for slowly releasing the vac-
uum.
4 Mount filling tubing
• Mount filling tubing 94 with filling cone 95 on glass capillary 88.
• Connect filling tubing 94 with tubing coupling 97 to the two gas wash
bottles 96 and the vacuum pump (see Fig. 10).