Page 157 - Instrumentation Reference Book 3E

Pressure transmitters 141

To YIC~II~X
circuit
Elemicri

(milliampere outeuf
Yerslons only1
Figure 9.43 Arrangementof helical Bourdon tubefor
resonant wire sensor. Courtesy, lnvensys Inc.

Figure 9.40 Arrangement of diaphragmassembly for
differential pressure measurements. Courtesy, Invensys Inc

Low $(de Ssnrar To osciiiaior High ride
backup plate body Circuit backJp plate

(a)
Maarur~ny cell lor differenrial The measuring cell for pressure
flow.
P~~SEUT~. and level and absolute pressure
1 Process flange
2 Iroiamydiaphraym
3 Ceramic chamber
4 Fined electrode
5 Sensing diaphragm
6 Fill fluid
7 Ring diaphragm
8 Hou~cng for mearuriny element
Figure 9.44 Arrangement of capacitance type
differential and absolute pressure sensors. Courtesy,
Siemens Ltd.
matching corrugations machined in the housing.
Figure 9.41 Arrangement of diaphragm assembly for The sensor comprises a hollow ceramic chamber
absolute pressure measurements. Courtesy, lnvensys Inc
divided into two by the sensing diaphragm. The
interiors of the chambers are filled with liquid
and sealed by a ring diaphragm. The interior
senrortioov
walls of both chambers are metallized and form
the fixed plates of the differential capacitor while
the sensing diaphra-gn forms the movable plates
of the capacitor.
When the measuring cell is subjected to a dif-
ferential pressure, the sensing diaphragm is dis-
placed slightly. causing a change in capacitance
which is converted by the associated measuring
circuit into a standard transmission signal such as
4 to 20 A d.c.
Figure 9.42 Arrangement of gauge pressure element for
resonant wire sensor Courtesy, lnvensys Inc For measuring absolute pressures, one side of
the measuring cell is evacuated to provide the
reference pressure, and for measuring gauge pres-
It is a flat cylindrical unit sealed at both ends sures. one side is vented to atmosphere.
by flexible corrugated diaphragms which provide Under these conditions the stiffness of the isola-
the interface between the process fluid and the ting diaphragm determines the range. For high
sensor. Under overload conditions, these seat on pressures the diaphragm is augmented by a spring.