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Section 2
Model 752 and 752A Differential Pressure Transmitters
Temperature Compensation
. The high pressure side of the DPU has extra
bellows convolutions to provide for expansion and contraction of the fill
liquid caused by ambient temperature changes. These extra convolutions are
connected to the measuring bellows by a passageway to permit the fill liquid
to change volume without materially affecting the internal pressure or the
physical relationship of the measuring bellows.
Bellows
. The bellows used in the DPU were specifically developed for use
in sensing and measuring instruments. They are designed to provide exacting
linearity characteristics as well as long life, and to be free of the effects of
work hardening. Individual bellows diaphragms are stamped from special or-
der Type 316 ELC (Extra Low Carbon) stainless steel sheets. The diaphragms
are assembled and seam welded to form the bellows.
Strain Gage Assembly.
The strain gage assembly (Figure 2.2) consists of
a strain gage beam and a glass-to-metal seal feed-through assembly. Strain
gages are bonded to opposite sides of the strain gage beam. The end of the
strain gage beam is installed directly into a cutout in the valve stem con-
necting the two bellows of the DPU. Any movement of the bellows in either
direction causes a corresponding linear movement of the strain gage beam
which acts upon the strain gages. Any action of the strain gages is monitored
by the electronic transmitter circuit.
Tension Strain Gage
Compression Strain Gage
Beam & Strain Gage Assembly
Figure 2.2—Strain gage assembly
Range Springs
. The range springs act with the bellows to balance the differ-
ential pressure applied to the unit. The springs are fabricated of a material that
is compatible with the specific bellows fill fluid used. The number of springs
and their rate depends on the differential range desired.
