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Technical Information
Application Notes
Measurement of ultra-high vacuum is not a trivial undertaking. There are many
factors that contribute to measurement problems. Some other concerns include
that:
The gauge sensitivity factor is based on nitrogen and very little, if any, nitrogen
is present in UHV systems that are leak tight
There are no convenient primary pressure standards that exist for UHV
calibration.
A grounded conductive wall near the tube raises the sensitivity by as much as
30%. This is typical of gauges mounted in tubulations attached to the system.
However, these three concerns are usually theoretical in nature and most problems
fall under the following areas:
All Ion gauges are pumps, (likewise, all Ion pumps are gauges). The difference
is that gauges are designed primarily to measure pressure and have relatively
small pumping speeds. However, given the right conditions it is possible for the
gauge to pump the gas that being measured, especially if the chamber pump is
small.
Pumping speed for a nude Ion gauge is typically around 0.5 litres/sec. The
pumping speed is affected by the emission current. The higher the emission
current the higher the pumping speed
Ion gauge out-gassing occurs by virtue of operating the filament. Even after the
gauge is degassed the filaments continue to generate gas when on. The major
component is carbon-monoxide, though other gasses may also be liberated.
This gas concentration tends to be higher in the vicinity of the gauge and can
lead to higher than expected pressure readings. The hotter the filament, the
more it out-gasses. Therefore, to minimize out-gassing use yttria-coated
iridium filament, which run significantly cooler than their tungsten counterparts,
and run them at the lowest emission current that gives good readings.
However, it is necessary to determine if the benefits of the yttria-coated iridium
filament overcome the drawback of higher particle generation and shorter life
when exposed to hydrogen and halogen gases. There is a tradeoff between the
gauge pumping and the gauge out-gassing that may compensate one for the
other.
Agilent UHV-24/UHV-24p Ionization Gauge User Manual
21
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