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 9.2.1 Manometer. The manometer is a rather simple device. One end of the
manometer is attached to the high-pressure tap and the other end to the
low-pressure tap of the orifice plate installation. As the dp created by the
orifice plate is sensed by the manometer, a column of fluid in the manometer
allows the dp to be read directly on a scale (Figure 5-7). Refer to the
manufacturer's instructions before installing a manometer.
9.2.2 Diaphragm. The diaphragm device includes a hermetically sealed
diaphragm that is in an enclosure with one side open to the high-pressure tap
and the other side to the low-pressure tap (Figure 5-8). The diaphragm moves
as the dp created by the orifice meter is transmitted to the diaphragm
chamber. A pointer attached to the diaphragm pivots about a fulcrum in the
wall of the chamber and mechanically indicates the dp directly on a scale.
Refer to the manufacturer's instructions before installing a diaphragm.
9.2.3 Bellows. A bellows device is similar to the diaphragm device in that
the indicator pointer is attached to a component that is subject to q ovement
caused by the dp. In a bellows device, a partition is hermetically sealed
between two bellows in a confined compartment with an opening on one side to
the high-pressure tap and another to the low-pressure tap (Figure 5-9). The
input ends of the bellows are fixed to the compartment walls. As the dp
forces the partition to move, compressing and expanding the respective
bellows, a lever system causes a pointer to directly indicate the dp on a
scale. Refer to the manufacturer's instructions before installing a bellows.
9.2.4 Electronic. Electronic devices are also known as capacitance devices.
In an electronic device, the dp is transmitted through an isolating diaphragm
to a hermetically sealed sensing diaphragm in the center of the device
(Figure 5-10). The sensing diaphragm is surrounded by silicone oil contained
between capacitors. As the sensing diaphragm deflects in proportion to the
dp, the position of the diaphragm is detected by capacitor plates on each side
of the diaphragm. The differential capacitance between the plates and the
diaphragm is converted electronically to a 2-wire, 4-20 mA, or 0-10 volt data
transmission signal. Electronic devices are available in both square root and
linear function models. Solid state, plug-in components simplify
maintenance/repairs.
9.2.5 Calibration. Static calibration should be performed on all dp devices
at least every 6 months.
9.3 Data Transmission. Readings from various dp devices must often be
transmitted to remote data collection and recording sites. his is because
the dp device may be too remote to warrant onsite reading. Data transmission
may also be necessary because there may be many widely dispersed devices to be
read and it would be uneconomical to have each one read onsite; or a central
data management point has been set up to collect, record, plot, reduce, and
analyze all flow data. All the taps for dp will accommodate remote
transmission fittings along with the various dp devices.
5-11
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