Absolute Pressure (Differential Construction); Absolute Pressure (Gauge Construction) - Siemens 7MF4033 series 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 grown on the measuring diaphragm change their resistances 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 on which four piezo-resistors in a bridge circuit have
been grown. 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 2005