Page 202 - Water Loss Control

176 Cha pte r T w e l v e

brand of meter can suffer a loss of accuracy due to a variety of reasons. Some of the
common causes of loss of meter accuracy include:
• Incorrect installation, particularly meters installed vertically or askew
• Build-up of scale or deposits due to aggressive water quality
• Debris in the water
• Air entrained in the piping/meter
• Excessively high or low velocity of the flow through the meter
• Manufacturing defects
• Extreme environment: high or low temperature, humidity, vibration, and
the like
• Vandalism or destruction
Properly installing appropriate meters and maintaining them by testing and rotation
should ensure a high level of accuracy of the customer meter population.
Even under the best of conditions meters wear from long-term flow registration and
eventually reach a threshold beyond which they will appreciably lose accuracy, some
meter types deteriorating more quickly than others. Therefore meters must be tested,
repaired, or replaced with new or refurbished meters (meter rotation) on a structured
basis.
Historically, AWWA guidelines recommended that water meters be rotated on a set
time schedule based upon meter size with small 5/8-in meters every 20 years and the
largest of meters rotated as often as every 4 years. This approach has merits in terms of
planning for mass deployment of meter rotation personnel and commensurate budget-
ing, planning, and so on. However, water meters experience different consumption
patterns and, after 20 years of service, some may have lost appreciable accuracy, while
others can offer many more years of reliable service. Rotating customer meters based
upon fixed time intervals may have significant economic drawbacks, particularly in the
large meter classes since these meters are expensive and require much more effort to
rotate than small meters.
The current thought on meter rotation strategy bases meter rotation scheduling on
the cumulative water volume that has passed through the meter, rather than a fixed
time interval. Cumulative flow registered by a meter is the most important factor in
long-term accuracy of the meter. Targeting meter rotations based upon cumulative
measured volume is similar to automobile maintenance, where the 3,000 mi oil and
filter change occurs not at any set time, but only when the 3,000 mi odometer reading
is reached. This approach can be more efficient since heavily used meters will see a
timely rotation that will ensure accuracy is maintained, while lightly used meters will
not waste resources by rotating the meters too soon. Decisions regarding meter rota-
tion based upon cumulative consumption should be formulated in conjunction with
crew deployment scheduling realities, since it may be advantageous to have crews
rotate multiple meters in a given area all at the same time, even if some of the meters
have not yet reached their cumulative volume target. Small meter rotation scheduling
may be best guided by a combination of cumulative volume target and geographic
proximity, while large meter rotations are perhaps better formulated around cumula-
tive volume targets and the characteristics of the individual meters and consumption
profiles.