© Arun Microelectronics Ltd 2020. All rights reserved
Page 36
Appendix A
Gauge Principles
A.1
Ionisation Gauges
Ionisation Gauges are thermionic triode devices. The appropriate choice for UHV use is the Bayard-Alpert type. This
consists of a very thin collector wire mounted along the axis of a cylindrical mesh grid. The filament is outside the grid and
usually parallel to it. The grid is voltage biased positively with respect to the filament, and the collector negatively.
A stabilised emission current is established between the incandescent filament and the grid structure. Electrons oscillate on
long paths through the open grid structure, being repelled from the central collector and attracted to the grid. A
proportion of the electrons encounter gas molecules before reaching the grid. These molecules are ionised by the collision
and those within the grid volume are attracted to the collector to form a current, which is proportional to the concentration
of gas molecules over a very wide range.
Pressure may be derived from the ion current by solving the equation:
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 =
𝐼𝑜𝑛 𝑐𝑢𝑟𝑟𝑒𝑛𝑡
𝑆𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦 × 𝐸𝑚𝑖𝑠𝑠𝑖𝑜𝑛 𝑐𝑢𝑟𝑟𝑒𝑛𝑡
where the units for the two currents are the same and the sensitivity is a quoted constant for a particular gauge-head and
gas species.
The impact of electrons on the grid structure generates soft X-rays; some of these impinge on the collector and release
photoelectrons. These form a small current in the same direction as the ion current. When this
‘
photocurrent
’
becomes
significant in relation to the
‘
true
’
ion current, the gauge ceases to function as a reliable pressure transducer and is said to
have reached its
‘
X-Ray limit
’
.
A.2
Pirani Gauges
The Pirani Gauge is a thermal conductivity gauge. A tungsten filament in the vacuum space is heated from a constant
voltage source and is incorporated in a Wheatstone bridge. The electrical resistance of the filament depends on its
temperature and this, in turn, depends on the rate at which heat is conducted away from the filament by residual gas. The
thermal conductivity of a gas depends on its pressure (below about 1 millibar) and the nature of the residual gas. The Pirani
Gauge unbalances the Wheatstone bridge and the voltage across the bridge represents pressure over the range of 0.5
millibar to about 1 x 10
3
millibar.
The lower pressure limit is determined by the heat loss due to radiation becoming significant compared to that due to
thermal conductivity. The radiant heat loss depends on the emissivity of the filament. A new filament is bright but can
become blackened by deposits from decomposed rotary pump oils and the lower limit of pressure readings will rise. It is
possible to clean filaments.
A.3
Capacitance manometers
Capacitance manometers operate by measuring the deflection of a thin, circular, radially tensioned membrane between the
vacuum space and a reference volume at a pressure substantially below the operating range of the transducer. The
deflection is measured as a modulation of the electrical capacitance between the membrane and a fixed plate and
converted to a voltage proportional to the pressure difference across the membrane.
Summary of Contents for NGC3
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