(f) Resize venturi if necessary, based on flow of previous year.
(g) Test dp transmitter every 6 months with dead weight pressure tester
and rescale if necessary.
4.2 Annual Maintenance.
(a) Clean and paint exterior of meter.
(b) Dismantle and check for corrosion.
(c) Clean and restore smoothness to internal surfaces.
(d) Check flange gasket and replace if it is leading or intruding into
5. ACCURACY AND RELIABILITY. Accuracy is up to %O.75 percent of full
scale. Accuracy is diminished at lowest turndown. Venturis are more reliable
than orifice plate meters because they are less susceptible to wear.
6. DIFFERENTIAL PRESSURE DEVICES. Dp devices are used to provide a
quantitative display of the differential pressure; they are also called
delta P and ∆ P devices. The four most common dp devices are: manometer,
diaphragm, bellows and electronic. All new or replacement installations
should be electronic.
6.1 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.
6.2 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.
6.3 Bellows. A bellows device is similar to the diaphragm device in that the
indicator pointer is attached to a component that is subject to movement
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.