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4.4.2.5
Measuring cell for absolute pressure from the differential pressure series
S
H
①
Pressure cap
⑥
Overload diaphragm
②
Seal diaphragm on the measuring cell
⑦
Measuring cell filling liquid
③
O-ring
⑧
Reference pressure
④
Measuring cell body
p
e
Pressure input variable
⑤
Absolute pressure sensor
Figure 4-11
Function chart of measuring cell for absolute pressure
● Absolute pressure is transmitted to the absolute pressure sensor
⑤
through the seal
diaphragm
②
and the filling liquid
⑦
.
● When measuring limits are exceeded, the overload diaphragm
⑥
is displaced until the seal
diaphragm
②
rests on the measuring cell body
④
. The seal diaphragm thus protects the
absolute pressure sensor
⑤
from overload.
● The difference between the inlet pressure (p
e
) and the reference pressure
⑧
on the
negative side of the measuring cell displaces the seal diaphragm
②
. The displacement
changes the resistance of the four piezoresistors (bridge circuit) of the absolute pressure
sensor.
● The change in the resistance causes a bridge output voltage proportional to the absolute
pressure.
Description
4.4 Functional principle
SITRANS P320/P420 with 4 to 20 mA/HART
36
Operating Instructions, 11/2019, A5E44852162-AC