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 Section 2.
COMPOUND WATER METERS
1. INTRODUCTION. Compound meters are essentially two meters within a single
housing. They are normally used in situations that require accurate
measurement of cold water over a wide range of low to high flow rates.
Compound meters are often used for measuring water used at apartment or office
buildings, hotels, schools, hospitals, and industrial facilities.
1.1 Meter Designs. There are two types of compound meter design, parallel
and series. The parallel type has two registers and, if either unit fails,
the trouble can be detected by stoppage of its register. The series type has
only one register. The unit of measurement must be specified as either
gallons, liters, cubic feet, or cubic meters. Compound meters are available
in nominal sizes of 2 to 10 inches, with a maximum capacity of 2,300 gallons
per minute.
2. OPERATING PRINCIPLES. The main components of a compound meter are
maincase, main line measuring chamber (turbine type), bypass measuring chamber
(positive displacement type), compounding valve, and one or two registers.
Compound meters operate in two modes; at low flows, only the bypass meter
operate, as flow increases, the compounding valve opens, allowing the meter to
operate at a higher range. In a parallel meter (Figure 4-5), the main line
meter does not operate until the compounding valve opens. The bypass meter
may or may not continue operating when the main line meter starts up. In the
series meter, when the compounding valve is closed, water flows through the
bypass meter. When the pressure differential in the bypass meter is great
enough to cause the compounding valve to open, the main line meter is already
running. The register is driven by a pair of ratchet drives, so that the unit
that is producing q ore registration will drive the register. The main line
unit is not called upon to start from rest at the changeover point, and thus
loss of accuracy is avoided when the valve opens. Changeover usually begins
at approximately 5 or 6 percent of maximum rating of the meter. Operating
characteristics for compound meters are listed in Table 4-3.
3. LIMITATIONS. Rated maximum capacity for compound meters is shown in
Table 4-3. Normal flow for these meters should not exceed approximately
one-half of maximum capacity. Operating at maximum capacity should be limited
to short periods or peak loads occurring after long intervals. Maximum
pressure loss is 13.3 percent of maximum pressure for all sizes. Mechanical
drive and some magnetic drive meters have changeable gears in the geartrain.
Changing these gears allows the ratio between the motion of the positive
displacement or turbine measuring chamber and the register to be calibrated
for maximum accuracy of registration. The turndown ratio for these meters
4-8
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