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Figure 6.15: UHV multiple metal-to-ceramic electric feedthrough<br />
This is important when using thermocouples, for example, as solder joints could falsify the<br />
measuring voltage. The feedthroughs are cooled with water for high amperages.<br />
With respect to their insulating resistance, feedthroughs with glass-to-metal fusings are suitable<br />
for high-voltage and weak-current feedthroughs for electronic devices. Feedthroughs with<br />
ceramic insulation offer greater mechanical stability and temperature resistance than glass.<br />
In addition, ceramic (e.g. aluminum oxide) can also be produced in an insulating form that is<br />
suitable for high voltage. This is why ceramic feedthroughs are superior to glass feedthroughs<br />
for high voltages and high performance. Only rigid metal-to-ceramic connections should be<br />
considered for the most rigorous electrical, thermal and vacuum technology requirements.<br />
The voltage level must be taken into account for electric feedthroughs, because gas discharges<br />
and flashover can occur in the vacuum if there are small clearances between conductors<br />
with high voltage differentials. In the vulnerable pressure range between 10 - 3 und 10 mbar,<br />
appropriate clearances must be provided between high-voltage conductors. Potting with cast<br />
resin can also be useful in this regard.<br />
Feedthroughs for liquids and gases<br />
Problems can occur with these feedthroughs if media at extremely high or low temperatures<br />
(liquid air) must be advanced into the vacuum vessel. Feedthroughs can be employed in which<br />
the thermal conductivity between flange and tube is reduced far enough by a sufficiently long,<br />
thin-walled stainless steel tube that the flange remains at room temperature, with normal<br />
elastomer seals being used to seal the flange.<br />
Sight glasses<br />
Sight glasses are primarily used to observe the interior of the vacuum chamber, even during<br />
the process. Consequently, normal window glass of an appropriate thickness is usually<br />
employed and installed in a sight glass flange by means of elastomer seals (ISO-K and ISO-KF<br />
sight glasses). The glass is metalized and soldered into the sight flange (CF sight glasses)<br />
for UHV applications and high temperatures.<br />
Page 137<br />
<strong>Vacuum</strong><br />
<strong>Technology</strong>
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