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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