FEATURES
Electromagnetic meters
Mag meters are technically a velocity-type water
meter, except that they use electromagnetic properties
to determine the water flow velocity, rather than the
mechanical means used by jet- and turbine meters.
Mag meters use the physics principle of Faraday’s law
of induction for measurement, and require AC or DC
electricity from a power line or battery to operate the
electromagnets. Since mag meters have no mechanical
measuring element, they normally have the advantage of
being able to measure flow in either direction and they
use electronics for measuring and totalising the flow. Mag
meters can also be useful for measuring raw (untreated/
unfiltered) water and wastewater, since there is no
mechanical measuring element to get clogged or damaged
by debris flowing through the meter. Strainers are not
required with mag meters, since there is no measuring
element in the stream of flow that could be damaged.
Since stray electrical energy flowing through the flow
tube can cause inaccurate readings, most mag meters
are installed with either grounding rings or grounding
electrodes to divert stray electricity away from the
electrodes used to measure the flow inside the flow tube.
Ultrasonic meters
Ultrasonic water meters use one or more ultrasonic
transducer to send ultrasonic sound waves through the
fluid to determine the velocity of the water. Since the
cross-sectional area of the meter body is a fixed and
known value, when the velocity of water is detected,
the volume of water passing through the meter can
be calculated with very high accuracy. Because water
density changes with temperature, most ultrasonic
water meters also measure the water temperature as a
component of the volume calculation.
Two primary ultrasonic measurement technologies are
used in water metering:
• Doppler effect meters, which use the Doppler
effect to determine the velocity of water passing
through the meter; and
• Transit time meters, which measure the amount
of time required for the ultrasonic signal to pass
between two or more fixed points inside the meter.
Ultrasonic meters may either be of flow-through or
‘clamp-on’ design. Flow-through designs are those
where the water passes directly through the meter
and are typically found in residential or commercial
applications. Clamp-on designs are generally used for
larger diameters where the sensors are mounted to the
exterior of pipes, and so forth.
Ultrasonic water meters are typically very accurate,
with residential meters capable of measuring down to
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0.01 gallons or 0.001 cubic feet.
In addition, they have wide flow
measurement ranges, require
little maintenance, and have long
lifespans due to the lack of internal
mechanical components to wear
out. While relatively new to the
American water utility market,
ultrasonic meters have been used
in commercial applications for many
years and are becoming widely
accepted due to their advantages
over traditional mechanical designs.
Prepaid water meters
Meters can be prepaid or post-paid,
depending on the payment method.
Most mechanical type water meters
are of the post-paid type, as are electromagnetic and
ultrasonic meters. With prepaid water meters, the user
purchases and prepays for a given amount of water
from a vending station. The amount of water credited is
entered on media such as an IC- or RF-type card. The
main difference is whether the card needs a contact
with the processing part of the prepaid water meter. In
some areas, a prepaid water meter uses a keypad as the
interface for inputting the water credit.
Domestic water meter.
SABS standards
SANS 10400-W:2011 (Ed. 3.00)
The application of the National Building Regulations
Part W: Fire installation.
SANS 1529-1:2006 (Ed. 2.03)
Water meters for cold potable water Part 1:
Metrological characteristics of mechanical water
meters of nominal bore not exceeding 100 mm.
SANS 1529-3:2006 (Ed. 1.01)
Water meters for cold potable water Part 3: Physical
dimensions.
SANS 1529-4:2004 (Ed. 1.01)
Water meters for cold potable water Part 4:
Mechanical meters of nominal bore exceeding
100mm but not exceeding 800mm.
SANS 1529-9:2008 (Ed. 2.01)
Water meters for cold potable water Part 9:
Requirements for electronic indicators used with
mechanical water meters, electronic water meters and
electronic prepayment water measuring systems. PA
June 2017 Volume 23 I Number 4