
In the following sections, the process variable to be measured is generally called inlet pressure.
WARNING
Destruction of the seal diaphragm
Danger of injury or damage to device
If the seal membrane is destroyed, the sensor may also be destroyed. If the seal membrane
is destroyed, no reliable measured values can be output.
Hot, toxic and corrosive process media can be released.
● Ensure that the material of the device parts wetted by the process medium is suitable for
the medium. Refer to the information in section Technical data (Page 199).
● Make sure that the device is suitable for the maximum operating pressure of your system.
Refer to the information on the nameplate and/or in Technical data (Page 199).
● Define maintenance intervals for regular inspections in line with device use and empirical
values. The maintenance intervals will vary from site to site depending on corrosion
resistance.
4.4.2.1
Measuring cell for gauge pressure
S
H
①
Reference pressure opening
⑤
Filling liquid
②
Measuring cell
⑥
Gauge pressure sensor
③
Process connection
p
e
Inlet pressure
④
Seal diaphragm
Figure 4-7
Function chart of measuring cell for gauge pressure
The inlet pressure (p
e
) is transferred to the gauge pressure sensor
⑥
via the seal diaphragm
④
and the fill fluid
⑤
, displacing its measuring diaphragm. The displacement changes the
resistance of the four piezoresistors (bridge circuit) of the gauge pressure sensor. The change
in the resistance causes a bridge output voltage proportional to the inlet pressure.
Pressure transmitters with measuring span ≤ 63 bar measure the inlet pressure against
atmosphere, those with measuring spans ≥ 160 bar the inlet pressure against vacuum.
Description
4.4 Functional principle
SITRANS P320/P420 with 4 to 20 mA/HART
Operating Instructions, 11/2019, A5E44852162-AC
33