Siemens SITRANS P - Manual (Page 66)

SITRANS P measuring instruments for pressure
Transmitters for gage, absolute and differential pressure, flow and level
DS III, DS III PA and DS III FF series
Technical description
2/66 Siemens FI 01 · 2009
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Mode of operation of the FOUNDATION Fieldbus electronics
Function diagram of the electronics
The bridge output voltage created by the sensor (1, Figure
"Function diagram of the electronics") is amplified by the instru-
ment amplifier (2) and digitized in the analog-to-digital converter
(3). The digital information is evaluated in the microcontroller, its
linearity and temperature response corrected, and provided on
the FOUNDATION Fieldbus through an electrically isolated
FOUNDATION Fieldbus Interface (7).
The data specific to the measuring cell, the electronics data, and
the parameter data are stored in the two non-volatile memories
(6). The one memory is coupled to the measuring cell, the other
to the electronics. As the result of this modular design, the elec-
tronics and the measuring cell can be replaced separately from
each other.
Using the three input keys (8) you can parameterize the pressure
transmitter directly at the point of measurement. The input keys
can also be used to control the view of the results, the error mes-
sages and the operating modes on the digital display (9).
The results with status values and diagnostic values are trans-
ferred by cyclic data transmission on the FOUNDATION
Fieldbus. Parameterization data and error messages are trans-
ferred by acyclic data transmission. Special software such as
National Instruments Configurator is required for this.
Mode of operation of the measuring cells
Measuring cell for gage pressure
Measuring cell for gage pressure, function diagram
The pressure pe is applied through the process connection (2,
Figure "Measuring cell for gage pressure, function diagram) to
the measuring cell (1). This pressure is subsequently transmitted
further through the seal diaphragm (3) and the filling liquid (4) to
the silicon pressure sensor (5) whose measuring diaphragm is
then flexed. This changes the resistance of the four piezo-resis-
tors fitted in the diaphragm in a bridge circuit. This change in re-
sistance results in a bridge output voltage proportional to the in-
put pressure.
Measuring cell for gage pressure, with front-flush diaphragm for
paper industry
Measuring cell for gage pressure, with front-flush diaphragm for paper in-
dustry, function diagram
The pressure pe is applied through the process connection (2,
Figure "Measuring cell for gage pressure, with front-flush dia-
phragm for paper industry, function diagram") to the measuring
cell (1). This pressure is subsequently transmitted further
through the seal diaphragm (3) and the filling liquid (4) to the sil-
icon pressure sensor (5) whose measuring diaphragm is then
flexed. This changes the resistance of the four piezo-resistors fit-
ted in the diaphragm in a bridge circuit. This change in resis-
tance results in a bridge output voltage proportional to the input
pressure.
6
8
pe
1
2
C m
M
0 0 00 0
0 0
EEPROM
EEPROM
9
6
3 4 5 7
10
1 Measuring cell sensor
2 Instrument amplifier
3 Analog-to-digital converter
4 Microcontroller
5 Electrical isolation
6 One non-volatile memory
each in the measuring cell
and electronics
7 FF interface
Sensor
Electronics
Measuring cell
FF
interface
Power
supply
unit
Coup-
ler
Power supply
8 Three input keys
(local operation)
9 Digital display
10 Power supply
p
e
Input variable
p
1
4
3
2
e
5
1 Measuring cell
2 Process connection
3 Seal diaphragm
4 Filling liquid
5 Silicon pressure sensor
p
e
Pressure as input variable
p
1
4
3
2
e
5
1 Measuring cell
2 Process connection
3 Seal diaphragm
4 Filling liquid
5 Silicon pressure sensor
p
e
Pressure as input variable
© Siemens AG 2008