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Model 2002 page 21
Theory of Operation
SECTION 5
The Model 2002 transducer is comprised of two very different sensors which provide a span of
measurement extending from 1000 Torr down to less than 1x10
-4
Torr. The piezoresistive device is
a direct force sensor which provides pressure indication from 1000 Torr down to less than 1 Torr.
The thin film Pirani device is a thermal conductivity sensor that provides pressure indication
from 100 Torr down to less than 1x10
-4
Torr. The two decade overlap in measurement range is
convenient for smooth transition either descending or ascending in pressure. Both sensors are
small micromachined die that are bonded to a Au coated Al
2
O
3
preform (stress isolation) which in
turn is bonded to a TO-8 header. The header is resistance welded into a 316 stainless steel
envelope as shown in Figure 5.1.
5.1 Piezoresistive Sensor
Electronics board
(contains EEPROM)
Pirani sensor
Piezoresistive
sensor
TO-8
HEADER
Top cap
Cable connector
Tube shell
Figure 5.1
Dual Sensor Embodiment
Preform