Selecting the Right Water Meter
Choosing the appropriate meter is key to high quality water treatment.
Two-thirds of the world is covered with water. However, only
approximately 3 percent of that water is fresh water for consumption. The
population of the world steadily is increasing causing that 3 percent finite
resource to become stretched, making conservation and water quality very
significant in today’s marketplace.
Water meters have for many years made the public more aware
of consumption. Now, with all the new water quality products that are becoming
available, water metering is more meaningful every day.
There are many ways to measure water, but there are three
types of design that have been in the mainstream of metering for many years.
They are volumetric (positive displacement), inferential (turbine) and
electronic (magnetic) metering. The bulk of meters utilized for water quality applications is smaller meters, generally in sizes including 1⁄2-, 3⁄4- and 1-inch positive displacement, but 11⁄2- and 2-inch sizes also
are available. Inferential (turbine) meters are used in sizes 11⁄2 to 4
inches with sizes up to 12 inches available. Electronic cold water meters are
available in sizes from 1⁄2
to 24 inches. They cover a wide
range of applications such as potable water, deionization or reverse osmosis
water, desalination, water vending and various types of water treatment either
by filtration, chemicals or ozone.
Positive displacement cold water meters are manufactured in
three basic types of materials: modified acetal copolymer (plastic), Waterworks
bronze or “low lead” EnviroBrass. These meters are appropriate to
applications requiring direct reading of the total number of U.S. gallons,
cubic feet or cubic meters of water that have passed through the meter. The
principle of operation of the positive displacement cold water meter is an
oscillating piston style, each piston revolution being the equivalent to a
known volume of water and an appropriate number of revolutions equal to one
gallon, cubic foot or cubic meter of water. The piston movement is transferred
by a magnetic drive to a direct read register or to an appropriate pulser. Some
meters are NSF Standard 061 certified for sizes 5⁄8 ¥ 1⁄2,
5⁄8 ¥ 3⁄4 and 3⁄4 ¥ 3⁄4. Positive displacement
meters can be installed into any inclination except upside down.
Low speed pulsers are utilized for connection to remote
counters, batch controllers, dataloggers or computers. High-speed pulsers often
are connected to batch controllers, rate indicators/rate transmitters with
4–20mA ouput, dataloggers or computers for higher accuracy applications.
There also are pulsers utilized in applications for water vending where a coin
or push button actuates the vending sequence and the machine fills either a
one-gallon or one-liter bottle.
Inferential or horizontal Woltman-type (turbine) cold water
meters are available in sizes 11⁄2 inches through 12 inches and are
manufactured with a bronze or cast iron body with round flat faced flanged
ends. This turbine-style meter operates by passing water through the meter
without a change in flow direction, driving a helix rotor in direct proportion
to the quantity of water passing through the meter. These meters are utilized
where larger volumes of water are required to be accurately measured. These
inferential meters are appropriate for applications for direct read in U.S.
gallon, cubic feet or cubic meters. Pulsers are available for low resolution or
high resolution outputs for use with remote counters, batch controllers, rate
of flow devices, dataloggers or computers. The American Water Works Association
(AWWA) and ISO Installation Procedures recommend these meters have 10 pipe
diameters of straight pipe the same size as the meter ahead of the meter and
five pipe diameters after the meter to ensure high quality flow characteristics
and minimal turbulence.
The electronic water meters (magnetic) have no moving parts
and operate according to Faraday’s principle of magnetic induction. A
conductor (water) moving through a magnetic field (produced by the electronic
water meter) will induce an electric current proportional to the velocity of
the conductor (water). These electronic meters are available in sizes ranging
from 2 through 24 inches and have many advantages such as rangeability (more
than 1,000 to 1 turn down), accuracy to .25 percent AC powered or 0.5 percent
battery powered, low head loss, long-term stable calibration, no noise and no
moving parts. An electronic water meter can be calibrated in U.S. gallon, cubic
feet or cubic meters and features total and rate of flow indication at the
display. These meters also will interface with via pulse output to rate of flow
transmitters with 4–20mA output, dataloggers or DCS systems for
management. These meters require approximately three pipe diameters ahead of
the meter and two pipe diameters after the meter of straight pipe the same size
as the meter. (Note: These meters are not appropriate to applications for
reverse osmosis or deionization water.)
The mechanical cold-water meters should fully comply with
AWWA’s Standard for their respective type of meter with regard to
performance and design.