10
Section 2
Model 764 Differential Pressure Transmitter
when the unit is used in liquid installations, when installed in accordance with
standard practices.
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
. Individual bellows diaphragms are stamped from special order Type
316 ELC (Extra Low Carbon) stainless steel sheets. The diaphragms are as
-
sembled 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 material used. The number of springs
and their rate depends on the differential range desired.
Electronic Transmitter
The DPU senses the difference in pressure applied across the bellows unit as
-
sembly. The pressure causes a linear motion of the bellows which is mechani-
cally transmitted to the strain gages by the strain gage beam. Motion of the
end of the strain gage beam applies tension to one gage and compression on
the other. The gage in tension increases in resistance, while the one under
