4.4.3.7 Measuring cell for level
① Inlet pressure P
+
⑦ Overload diaphragm
② Seal diaphragm on the measuring cell ⑧ Inlet pressure P
-
③ Pressure cap ⑨ Filling liquid of the measuring cell
④ O-ring ⑩ Capillary tube with the fill fluid of the mounting
flange
⑤ Differential pressure sensor ⑪ Flange with a tube
⑥ Measuring cell body ⑫ Seal diaphragm on the mounting flange
Figure 4-13 Function chart of the measuring cell for level
● The inlet pressure (hydrostatic pressure) ① works hydraulically on the measuring cell
through the seal diaphragm on the mounting flange ⑫.
● The differential pressure at the measuring cell is transmitted to the differential pressure
sensor ⑤ through the seal diaphragms ② and the filling liquid ⑨.
● When measuring limits are exceeded, the overload diaphragm ⑦ is displaced until one of
the seal diaphragms ② or ⑫ rests on the measuring cell body ⑥. The seal diaphragms
② thus protect the differential pressure sensor ⑤ from overload.
● The seal diaphragm ② is displaced by the differential pressure. The displacement changes
the resistance of the four doped piezoresistors in the bridge circuit.
● The change in the resistance causes a bridge output voltage proportional to the differential
pressure.
Description
4.4 Functional principle
SITRANS P320/P420 (mA/HART)
36 Operating Instructions, 06/2018, A5E44852162-AA
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