Absolute Pressure (Differential Construction); Absolute Pressure (Gauge Construction) - Siemens 7MF4333 User Manual

Circuit Description
1 Process flange (qty. 2)
2 O-ring (qty. 2)
3 Silicon pressure sensor
4 Measuring cell body
5 Overload diaphragm
6 Seal diaphragm at the
measuring cell (qty. 2)
7 Fill fluid
8 Capillary tube with fill
fluid
9 Mounting flange with
capillary tube
10 Seal diaphragm at the
mounting flange

8.5 ABSOLUTE PRESSURE (DIFFERENTIAL CONSTRUCTION)

Absolute pressure p is transferred via the seal diaphragm (2) and the fill fluid (7) to the silicon pressure
e
sensor (5). The pressure difference between the input pressure (p
low pressure side of the measuring cell flexes the measuring diaphragm. Four piezo-resistors in a bridge
circuit are located on the measuring diaphragm. The bridge changes resistance in response to the change
in applied pressure. The change in resistance causes a bridge output voltage change that is proportional to
the absolute pressure.
If measuring limits are exceeded, the overload diaphragm (5) will deflect until the seal diaphragm comes
into contact with the body of the measuring cell (4) protecting the silicon pressure sensor (5) against the
overload.
1 Process flange (qty. 2)
2 Seal diaphragm at the
measuring cell (qty. 2)
3 O-ring (qty. 2)
4 Measuring cell body
5 Silicon pressure sensor
6 Overload diaphragm
7 Fill fluid
8 Reference pressure
p Input variable pressure
e
FIGURE 8-5 Absolute Pressure Measuring Cell, Differential Construction

8.6 ABSOLUTE PRESSURE (GAUGE CONSTRUCTION)

Pressure p is applied via the seal diaphragm (3) and full fluid (4) to the absolute pressure sensor (5). See
e
Figure 8-6. This flexes the measuring diaphragm which has four piezo-resistors in a bridge circuit. Bridge
resistance will vary with the applied pressure to generate a bridge output voltage that is proportional to
the input pressure.
8-4
FIGURE 8-4 Flanged Level Measuring Cell
) and the reference pressure (8) on the
e
UMSITRPDS3-1
November 2010