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Model 2002 page 5
General Information
SECTION 1
The Model 2002 is a wide range vacuum measuring instrument consisting of a digital microproces-
sor-based display unit, a miniature, rugged, thin film based sensing element that measures from
below 1x10
-4
up to 100 Torr, a piezoresistive sensor that measures from 1 to 1000 Torr,
EEPROM (Electrically Erasable/Programmable Read Only Memory), and an interconnecting
cable. Together, these components provide accurate vacuum measurement over 7 decades of
vacuum from 1x10
-4
to 1000 Torr. This instrument is a new design approach based on over 50
years of experience by Hastings Instruments with well-known and widely used thermal conduc-
tion vacuum measurement techniques. The Model 2002 is designed for quick, easy installation
and will provide the user with long lasting, trouble free, accurate vacuum measurement.
1.1 Features
• Wide dynamic range: 1x10
-4
to 1000 Torr
• Compact rugged design
• Attitude insensitive
• Fast dynamic response
• Withstands 150 psig positive pressure
• Linear per decade
• Bakeable to 250
0
C
• TTL output process control
• Interchangeable transducer
• Corrosion resistant
1.2 Model 2002 Sensors
The Model 2002 transducer is comprised of an ion implanted, piezoresistive, direct force sensor
and a thin film Pirani type sensor. The Pirani sensing element is a Pt thin film serpentine element
(see Figure 5.3a) deposited on a 1 micron thick Si
3
N
4
membrane. The membrane is peripherally
supported by a Si box shaped die and is covered by a thick Si lid parallel to the membrane and
open on two ends (see Figure 5.3b). The piezoresistive unit is an ion implanted Wheatstone
bridge in a 50 micron thick Si membrane peripherally supported by a Si box shaped die which
has been anodically bonded to a Pyrex substrate.
The dual sensor assembly is encased in a corrosion resistant 316 stainless steel tube shell. The
durable tube design withstands high pressure (150 psig/10.2 bar) and high pressure surges. Since
the Pirani sensor is miniaturized and employs a Pt thin film on a Si
3
N
4
membrane (instead of a
conventional long fragile wire), the transducer can withstand high levels of mechanical shock.
The Model 2002 is designed for fast response. The micromachined sensing elements have a very
small mass and operate in a constant temperature (Pirani) and a constant current (piezo) feed-
back mode. This makes response time very fast as compared to other commercially available
sensors which have to change the temperature of a significant mass to reflect pressure changes
and have a large internal volume which must equalize in pressure with the system before the
sensor can reach its final value. The transducer’s small internal volume (<1.5cc) permits rapid
pneumatic response to system pressure changes. Further, the small geometry of the transducer
prevents thermal convection currents which allows the sensor to be mounted in any orientation
without calibration shifts.