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Vacuum Measurement Varian, Inc. Vacuum Technologies
Vacuum Measurement
Varian, Inc.
Vacuum Technologies
Introduction 266-269
Transducers 270-283
Gauge Controllers 284-294
Active Gauges 295-303
Cables 304-305
Calibration and Service 306-310
Technical Notes 311-313
265 The Broadest Gauging Line in the Industry
Selecting a Vacuum Measurement System
266
Varians broad line of vacuum gauge controllers and gauge
tubes are the most reliable, accurate, and economical means
of measuring, monitoring, and controlling pressures in a
variety of ranges. The vacuum gauge controllers and tubes
are specifically designed for industrial use where simplicity in
operation and rugged design for challenging environments
are important. In addition, their accuracy and efficiency meet
the demanding requirements for research applications.
Vacuum gauges can be grouped by pressure range.
Useful ranges are: Rough Vacuum Atmosphere down to 1mTorr (1mbar) High Vacuum 1mTorr and lower
Within the high vacuum range: Ultra High Vacuum (UHV) pressures below
1 x 10
-9
Torr (mbar) Extreme High Vacuum (XHV) pressures below
1 x 10
-12
Torr (mbar)
Varian supplies products for all but the XHV range.
Roughing Gauges
There are many applications for rough gauges, from
the simple monitoring of the high vacuum pump foreline
and determination of high vacuum crossover points, to the
control of the pump down and vent sequences of a load
lock system.
Thermal roughing gauges, of which Varian supplies several
types, are the most cost effective choice when absolute
accuracy and gas species independence are not required.
A thermal type gauge, since it measures heat loss to the
surrounding gas and not the actual force exerted by it,
is dependent on several properties of the gas, such as the
specific heat and thermal conductivity.
Since the properties of gases vary, the calibration of a thermal
rough gauge is different in nitrogen versus argon for
example. In practice, this gas sensitivity is not a practical
problem, as calibration curves can be determined for any gas
of interest. Compared to a typical capacitance manometer, a
thermal gauge is smaller and can cost 1/10th as much and
still provide excellent performance in a real system.
The traditional thermocouple (TC) type gauge is a very low
cost solution for monitoring forelines, chamber evacuation,
and crossover to high vacuum pumps. TCs are characterized
by response times of several seconds and operate over the
range of 1mTorr to 2 Torr (0.001mbar to 2mbar). However, in
large chambers where pump down is inherently slow and a
need for good measurement at high pressures exists, or when
monitoring foreline pressures, TCs (such as the 531 or 536)
are a sound, cost-effective choice.
A convection-enhanced thermal gauge maintains the sensing
element at a constant temperature. Vacuum Measurement
267
Varian, Inc. Vacuum Technologies
V
acuum
Measurement
This provides two benefits: Excellent response and sensitivity over the entire roughing
range up to atmosphere. Fast response time (typically 0.2 seconds).
Varians Convectorr, operated by convector controller, is such
a gauge. It provides excellent performance from atmosphere
all the way down to 1 or 0.1mTorr (0.001 to 0.0001mbar)
depending on the controller. In addition to its superior
performance, it features a modern locking bayonet type
connector rather than the old industry standard octal type
which relies on friction of the electrical contacts to stay on.
In load lock applications where sensitivity, repeatability, and
fast response are requirements, a convector gauge can
replace a much more expensive capacitance manometer at a
fraction of the cost. The gauge can be used to control a two-
stage pump down in which initial pumping from atmosphere
is done slowly to minimize particulate circulation, and then
when the pressure reaches a predetermined level a valve is
opened to allow full pumping speed. The same gauge can
then be used to signal the time to open the door to the main
chamber. Upon venting, the gauge is used to signal back to
atmosphere condition.
In large systems, the excellent sensitivity to pressure changes
near atmosphere can be used to indicate potential leaks or
other pumping problems by measuring the pressure at
specific points in time.
High and Ultra-High Vacuum Gauges
These gauges comprise hot cathode and cold cathode types
and measure pressure by ionization of the gas. Hot cathode
type gauges employ a filament for the electron source, while
the cold cathode types rely on field emission of electrons
under a high electric field. There are advantages and
disadvantages of each type. Hot cathode gauges of the Bayard-Alpert (B-A) design,
the only ones Varian supplies, are preferred over cold
cathode gauges for their better accuracy and repeatability
over time. They are offered in glass, metal, and nude
versions. Because of low initial cost, a glass B-A gauge can
be the most cost effective gauge for measuring high
vacuum. However, the danger of glass breakage may make
a totally metal encased version, such as the MBA series,
a better choice.
The filament provides a ready source of electrons and
therefore operation is assured over the entire high vacuum
range. However, the filament is hot, and though more
rugged than that of a light bulb, it is subject to degradation
over time and can be damaged by shock and vibration.
The filament is also a source of gas and excess electrons.
These electrons generate x-rays when they hit the chamber
walls and other surfaces causing an offset current,
called the x-ray limit, that effectively limits the lower
range of the gauge.
Hot filament gauges are available with either tungsten or
thoriated-iridium filaments. Tungsten is the material of
choice for low cost and stable operation or when operating
with hydrogen or halogen gases. However, it runs very hot
and oxidizes readily if accidentally exposed to air.
Thoriated-iridium filaments operate at lower temperatures
and can withstand extended exposure to air while
operating before failure. The lower temperature contributes
to less outgassing when used in a UHV gauge. Performance
over time of these gauges is not as stable as tungsten,
and the thoria coating is a potential source of particles. The cold cathode gauges are available in magnetron and
inverted magnetron versions. These gauges operate by the
application of a high voltage (2-3KV) to the cathode. Field
emission causes an electron to leave the cathode surface
and is captured in a magnetic field. It collides with a gas
molecule and starts a current flow to the anode. The
magnitude of this current is proportional to gas density
and pressure. Advantages of these gauges over hot
filament types are that there is no filament, and hence
they are very rugged, that they generate little gas, and that
there is no x-ray current since there are no excess electrons
to cause x-rays.
The magnetron gauge is a very low cost gauge characterized
by high sensitivity but limited to operation above 1 x 10
-8
Torr
(mbar). Varians 525 is a gauge of this type. Varian offers two
inverted magnetron gauges, the IMG-100 for high vacuum,
and the UHV-IMG for UHV. Both of these gauges offer
excellent starting times at low pressures, in contrast to the
cold cathode types, and fast response times, which are ideal
for interlock systems.
Please refer to the Table of Vacuum Transducers on the next
page when selecting your high or ultra high vacuum gauge. 268
Other Considerations
All of Varians gauges (except for the 531 TC and the Glass
B-A Gauges) are constructed of 304L or 304ESR SST and
employ either glass compression or ceramic to metal seals.
Each gauge is individually tested and leak checked to assure
reliable operation. No o-rings are used in the construction
of the gauge.
When operating below the 10
-8
T (mbar) range, ConFlat
flanges are recommended. Below 1 x 10
-9
Torr,
they are required.
Controllers
Varian offers both traditional rack mount controllers as well
as active type gauges. Your choice depends upon your
system needs.
In general, superior measurement performance is obtained
with rack mount systems. If there is a need for many gauges
in a system, a Multi-Gauge controller can be the most cost
effective solution as it can operate up to three high vacuum
gauges and eight thermocouples simultaneously. Sentorr is
ideal for laboratory use or for small experimental or general-
purpose systems.
The EyeSys family of active gauges is ideal for embedding in
systems where a control computer o