Overall Operation; Pressure - Siemens 7MF4033 series User Manual

Circuit Description

8.1 OVERALL OPERATION

The measuring cell sensor (1) detects the applied process pressure. The sensor's analog output signal is
amplified by an instrument amplifier (2) and converted into a digital signal in an analog-digital converter
(3). This data is evaluated by a microcontroller (4) that corrects the signal and temperature characteristics.
Microcontroller output is converted to a 4-20 mA output current by the digital-analog converter (5).
A diode circuit (10) provides reverse polarity protection in the event of reversed loop connections. An
external digital milliammeter can be connected across the diode. The diode and the electronics discussed
in the previous paragraph are housed in an assembly named the electronics module.
Measuring cell-specific data (e.g. measuring range, measuring cell material, oil fill) are stored in a non-
volatile memory (6) located in the measuring cell assembly. Transmitter configuration data (e.g. turn-
down, electric damping) are stored in a second non-volatile memory (6) in the electronics module.
The transmitter can be configured locally using three built-in magnetic pushbuttons (8) or remotely, via
the HART modem (7), using the HART Communicator. The digital display (9) shows input pressure,
output current, error messages, modes of operation, or other data as determined by the configuration and
by user commands via the magnetic pushbuttons or HART Communicator.
In the following sections, individual measuring cell types will be explained.

8.2 PRESSURE

As shown in Figure 8-2, process pressure p
connection (3). This pressure is applied to the seal diaphragm which flexes to transfer the pressure to the
fill fluid (5) which in turn transfers the pressure to the silicon pressure sensor (6).
The silicon pressure sensor consists of four piezo-resistors in a bridge circuit grown on the measuring
diaphragm. The applied pressure causes a change in the bridge resistance that is proportion to the input
pressure.
A transmitter with a measuring span of up to 925 psi will measure the input pressure compared to
atmospheric pressure. A transmitter with a measuring span of 2320 psi compares input pressure to a
vacuum.
1 Reference pressure
2 Measuring cell
3 Process connection
4 Seal diaphragm
5 Filling fluid
6 Silicon pressure sensor
p Input variable pressure
e
8-2
is applied to the measuring cell (2) through the process
e
FIGURE 8-2 Pressure Measuring Cell
UMSITRPDS3-1
November 2005