Introduction
This guide walks through the complete setup of the MME Pro electrode for use with the 757 VA Computrace. The accompanying videos demonstrate each step of the process, including installing the needle and capillary, filling the electrode with mercury, adjusting the needle valve, and verifying proper electrode operation.
Before beginning, ensure you have all required components and accessories available. Follow the videos in sequence to properly prepare the electrode for operation with the 757 VA Computrace.
Video
Topics Covered
- To put the MME pro into operation, the following are required: Multi-Mode Electrode pro (MME pro), collection tray, electrode stand, a new needle, a new capillary, scissors, mercury, syringe with needle, the key to the MME, and the mercury catcher "Mercury Catcher" for collecting scattered mercury droplets.
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Inserting the needle (0:06)
- The following are required for inserting the needle: Multi-mode electrode pro (MME pro), a new needle, the key for the MME and scissors.
- Turn the needle holder counterclockwise until it can no longer be turned out, and then carefully pull it out using the grey mounting ring.
- Cut open the plastic packaging of the new needle with scissors. Insert the needle into the hole in the membrane and push it in until the white mark is reached.
- Insert the needle holder with the new needle into the MME pro and turn it clockwise until the black sealing ring is just visible. This prevents mercury from leaking out.
- Do not turn the needle holder in too far. The needle valve will be adjusted later on the measuring stand.
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Installing the capillary (01:07)
- The following items are needed for inserting the capillary: Multi-mode electrode pro (MME pro), electrode stand, a new capillary, the key for the MME and scissors.
- To prepare, unscrew the retaining screw for the capillary, loosely reinsert it into the thread but do not tighten it.
- Do not touch the needle with your bare fingers during this process.
- Insert the capillary into the MME pro and, using gentle twisting motions, push it down through the seal until it stops. Then, loosely tighten the retaining screw by hand. To secure the capillary, place the key on top and use one finger to turn the retaining screw clockwise until the key falls off. Do not re-engage the key.
- The application areas of the standard capillary (6.1226.030) and the silanized capillary (6.1226.050) can be found in the corresponding data sheet for the capillaries.
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Filling the electrode with mercury (03:06)
- The following tasks must be carried out over the plastic tray!
- In addition to the MME pro, the following are required for filling the mercury: collection tray, electrode stand, mercury, syringe with needle, the key for the MME, and the mercury catcher for collecting any spilled mercury droplets.
- Slowly pour mercury into the electrode. It should be filled to the "max. fill" mark (max. 6 mL) at most.
- Using the key, tighten the screw cap until it stops. The opening of the mercury reservoir will then be securely closed.
- Small amounts of spilled mercury can be collected using the mercury catcher (part number 6.2406.000).
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Adjusting the needle valve (04:29)
- The needle valve is adjusted in the measuring station. Before inserting an electrode with a new capillary, be sure to empty the measuring vessel (any solution entering the vessel can render the capillary unusable)!
- Step 1: Pressurize the electrode (required pressure 1.0 to 1.2 bar). In the "Computrace control" window, select electrode type HMDE. Do not activate the electrode with "New drop". Once a large mercury droplet has formed, add approximately 20 mL of water and a few drops of KCl solution to the measuring vessel.
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Step 2 (coarse adjustment): Turn the needle holder clockwise until no more mercury flows out.
It has proven useful to check after each half turn, by observing the formation of new drops, whether the capillary is already sealed by the needle. -
Step 3 (fine adjustment): Slowly rotate the needle holder counterclockwise until a new droplet forms.
Close the capillary again with a small clockwise turn (approx. 10°). -
Step 4: Rotate the needle holder another 90–100° clockwise.
The electrode is now ready for use. Droplet formation can be controlled by the software.
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Verifying electrode function in DME, SMDE, and HMDE operating modes (08:09)
- DME – Dropping Mercury Electrode. The classic dropping mercury electrode.
- To check the DME function, select the DME electrode type in the "Computrace control" window and activate the electrode with "New drop". Drop formation is correct if the drop falls from the capillary at regular intervals.
- SMDE – Static Mercury Drop Electrode. A dripping mercury electrode, like the DME, but with a static surface, like the HMDE.
- To check the SMDE function, select the SMDE electrode type in the "Computrace control" window and activate the electrode with "New drop". Drop formation is correct if each stroke of the knock-off mechanism removes an equally sized drop from the capillary.
- HMDE – Hanging Mercury Drop Electrode. The hanging mercury drop electrode; a single hanging drop of mercury serves as the electrode.
- To check the function of the HMDE, select the HMDE electrode type in the "Computrace control" window and activate the electrode with "New drop". Drop formation is correct when the old drop is knocked off and a new drop is formed. The drop should not increase in size over time. Reactivating the electrode will produce a new drop.
Additional Support
If you encounter any issues or need additional help, please submit a support ticket through Metrohm Technical Support.
For more detailed training on any of these topics, explore our Metrohm Custom Training options.