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Model 764 Differential Pressure Transmitter
Section 2
Section 2—Theory of Operation
Basic Components
Differential Pressure Unit (DPU)
HP Housing
Valve Stem
HP Bellows
LP Housing
LP Bellows
Figure 2.1—DPU
The differential pressure range of the dual-bellows type DPU is determined by
the force required to move the bellows through their normal range of travel.
To provide for various ranges, range springs are incorporated into the Bellows
Unit Assembly (BUA). The range springs, which are available in various
factory assemblies, accurately balance the differential pressure applied to the
DPU.
In operation, the two bellows (which are connected by the valve stem shown
in Figure 2.1) move in proportion to the difference in pressure applied across
the BUA. The linear motion of the bellows is picked up by the tip of the sili
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cone strain gage beam, which is actuated directly by the valve stem connect
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ing the two bellows. If the bellows are subjected to a pressure greater than the
differential pressure range of the DPU, they will move through their normal
range of travel, plus a small additional amount of "overtravel," until the valve
on the stem shaft seals against its valve seat. As the valve closes on the seat, it
"traps" the fill liquid in the bellows, protecting the unit from damage or shift
in calibration.
Since the fill fluid is essentially non-compressible, the bellows are fully sup
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ported and cannot rupture regardless of the over-pressure (up to the full rated
pressure of the instrument) applied to the unit. Furthermore, since the unit
contains opposed valves, protection against "overrange" in either direction is
provided.
Draining or Venting.
The high and low pressure housings of the DPU are
provided with both top and bottom pressure connections which provide a
draining feature when the unit is used in gas installations, or a venting feature
