Page 216 - Water Loss Control

Contr olling Appar ent Losses—Customer Meter Inaccuracy 189

high side and a low side to capture the high and low flows, respectively. For compound
meters, it is important to know the level of the “crossover” flow, or the level where flow
switches from the high to low register, or vice versa. If the customer consumption rate
occurs frequently at flows in the crossover range, poor meter accuracy at this level could
result in a great loss of flow registration. Therefore, the crossover flow should be deter-
mined and the meter specifically tested at this rate in addition to the high and low
flows. The manufacturer’s accuracy curve is a proper information source as different
brands of compound meters offer variant crossover flow rates. This information is not
currently available in the AWWA M6 publication.
The test equipment and methods for determining the accuracy of small meters are
not applicable to accuracy tests on larger meters. The larger meters require specialized
test equipment which can handle a wide range of flow rates and provide accurate, valid
data. These devices may either be purchased as a manufactured assembly or fabricated
by the water utility.
The equipment for large meter testing is available as a portable test package,
installed on trailers, or mounted in a van or pickup truck. Regardless of the style, these
testers all contain certain basic elements, which are required to properly test turbine,
compound, and propeller meters. Because of the wide flow ranges involved, a tester
includes at least two, and sometimes three, calibrated test meters of varying capacities.
A shut-off valve is typically located downstream of each meter to control the flow rate
during the various tests. A pressure gage is required to check both the line pressure and
the residual pressure at the tester. Sometimes resettable registers and/or flow raters are
included to reduce the time required to conduct a complete test.
Flexible hoses are required to connect the test equipment to the test connection of
the meter being tested. Due to static pressures and hydraulic forces present, all hoses
must be in good condition and positioned as straight as possible between the two
meters. For the larger testers, it is important that the tester itself be anchored by means
of a vehicle, or similar restraining method, since significant hydraulic forces will impact
the meter tester during the test. The master meters used on the testers should be pro-
tected and handled with care. They should also be tested and recalibrated periodically
to ensure accurate measurement is being maintained.
Unfortunately large meters are often ignored as long as they continue to record
consumption. While large meters are usually relatively few in number in water utilities,
they account for a significant amount of revenue. If large meters mean so much to a
water system’s financial health, why are they not maintained to provide peak perfor-
mance? The explanation is multifold. Large meters are difficult to repair, spare parts are
expensive, assemblies are sometimes complex, and a relatively high skill level is neces-
sary for the service personnel to maintain them. The largest sized meters are very heavy
and difficult to handle and transport. Maintenance work is hindered by meter installa-
tions in crowded or cramped spaces and/or piping compromises have to be made.
Many times there is no bypass piping to continue supplying water to the customer dur-
ing a meter accuracy test, or it is difficult to dispose of the water discharged during the
test. Also, work space around the meter may be restricted and unsafe. Liability, safety
issues, and span of control of the testing personnel can sometimes be a concern to the
system’s management. Because many large meters are very important to the overall
billings, their operating condition must be monitored on a systematic and timely basis.
One common approach is testing large meters on-site by qualified test personnel.
Large customer meters may be tested on-site or at the water utility’s facilities. There
are certain advantages to testing large customer meters at the water utility test bench;