Fisher-Rosemount 3095FT, Manual

Page: 120 / 184

Rosemount Model 3095FT HART Flow Transmitter
6-2
MULTIVARIABLE SENSOR
MODULE The advanced sensor module of the Model 3095FT (see Figure 6-2)
measures three process variables simultaneously. The multivariable
module incorporates a high-accuracy capacitance sensor for differential
pressure, a high-accuracy piezoresistive silicon sensor for absolute
pressure, and a four-wire RTD input for process temperature
measurement. In addition, the sensor electronics convert the process
variables directly into digital format for further correction and
compensation within the sensor module.
Capacitive Differential
Pressure Sensor In the differential pressure sensor, process pressure is transmitted
through the isolating diaphragm and fill fluid to the sensing diaphragm
in the center of the capacitance cell. Capacitor plates on both sides of
the sensing diaphragm detect its position. The differential capacitance
between the sensing diaphragm and the capacitor plates is directly
proportional to process pressure.
Piezoresistive Absolute
Pressure Sensor
The absolute pressure silicon sensor is fabricated utilizing a processing
method called chemical vapor deposition (CVD). This technique, which
is superior to other technologies that are vulnerable to drift over time,
isolates the sensing element from the silicon substrate to achieve high
accuracy and repeatability.
The absolute sensor consists of a Wheatstone bridge circuit made from
polysilicon resistors deposited on a silicon substrate. The absolute
pressure sensor is hydraulically connected to the high pressure side of
the transmitter. Process pressure is transmitted through the fill fluid to
the sensing element, creating a very small deflection of the silicon
substrate. The resulting strain on the substrate changes the bridge
resistance in proportion to the pressure applied.
Both the absolute and differential sensors are laser welded and isolated
mechanically, electrically, and thermally from the process medium and
the external environment. Mechanical and thermal isolation is
achieved by moving these sensors away from the process flange to a
position in the neck of the electronics housing. This design relieves
mechanical stress on the cell, thereby improving static pressure
performance and removing the sensors from direct process heat.
Glass-sealed pressure transport tubes and insulated cell mountings
provide electrical isolation, thus improving the flexibility, performance,
and transient protection of the electronic circuitry.
Process Temperature Process temperature is measured using an input connection on the
sensor module for a standard resistance temperature device (RTD).
Rosemount Inc. offers a special shielded cable with connector for
connecting the RTD input to the Model 3095FT (see Page 7-5).
The Model 3095FT can accept a signal from any 100-ohm platinum
RTD that conforms to IEC-751 Class B. The Model 3095FT Flow
Transmitter can be supplied with an optional Rosemount Series 68 or
78 RTD temperature sensor. For further information on Rosemount
temperature sensors and accessory hardware, refer to Product Data
Sheet PDS 2654.