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HPM-2002-OBEVacuum Gauge Page 25 of 39
4. Theory of Operation
The 2002s transducer tube is comprised of two very different sensors which provide a span of measurement
extending from 1000 Torr down to less than 1 × 10
-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 1 × 10
-4
Torr.
The two decade overlap in measurement range is convenient for smooth transition either descending or
ascending in pressure. Both sensors are small micro machined 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 the Figure 4.1.
4.1.1.
PCB
PIRANI
SENSOR
PIEZO
SENSOR
TO-8
HEADER
EEPROM
Figure 4.1