Differential Pressure And Flow; Flanged Level - Siemens 7MF4333 User Manual

UMSITRAPDS3-1

8.3 DIFFERENTIAL PRESSURE AND FLOW

Differential pressure is applied to the silicon pressure sensor (4), a seal diaphragm (7) on either side of the
measuring cell body, and the fill fluid (8). See Figure 8-3. When measuring limits are exceeded, the
overload diaphragm (5) deflects until one of the seal diaphragms (7) comes into contact with the
measuring cell body (4) to protect the silicon pressure sensor (3).
The silicon pressure sensor consists of four piezo-resistors in a bridge circuit. The bridge is located on the
measuring diaphragm. Bridge resistance changes in response to a change in input pressure. The change in
resistance causes a bridge output voltage that is proportional to differential pressure.
1 Input pressure P+
2 Process Flange (qty. 2)
3 O-ring (qty. 2)
4 Measuring cell body
5 Silicon pressure sensor
6 Overload diaphragm
7 Seal diaphragm (qty. 2)
8 Fill fluid
9 Input pressure P-
FIGURE 8-3 Differential Pressure and Flow Measuring Cell

8.4 FLANGED LEVEL

Input pressure (hydrostatic pressure) acts on the high pressure (+) side of the measuring cell through the
seal diaphragm (10) at the mounting flange, the fluid filled capillary tube (8), the fluid filled cavity in the
process flange, a seal diaphragm, and the fill fluid in the measuring cell body (4). See Figure 8-4. A
differential pressure is applied to the low pressure (-) side of the measuring cell via the seal diaphragm (6)
and fill fluid (7) to the silicon pressure sensor (3).
When measuring limits are exceeded, the overload diaphragm (5) deflects until one of the seal
diaphragms (6) comes into contact with the body of the measuring cell (4) to protect the silicon pressure
sensor (3).
The measuring diaphragm is flexed by the differential pressure. The silicon pressure sensor consists of
four piezo-resistors in a bridge circuit that is located on the measuring diaphragm. Bridge resistance
varies with pressure changes causing the bridge output voltage to vary in proportion to the change in
applied differential pressure.
November 2010
Circuit Description
8-3