Modeling Customer Meter Accuracy

Customer meters have been called the "cash register" of the utility and are responsible for ensuring an equitable distribution of water volume and income throughout various different customer types within a utility. It is therefore extremely important to assess the accuracy of the meters on a regular basis and make repairs or replace groups of meters to keep the customer meter population at an overall high level of accuracy. Accurate metered consumption data is also necessary for engineering functions such as hydraulic models, evaluation of water conservation programs, and sizing of infrastructure for water resources development. The reader should also refer to Chap. 16 which provides detailed information on meter performance, as well as procedures for meter accuracy testing.

It is necessary to model average weighted meter accuracy for the entire customer meter population and include it in the water audit. The water balance calculations are used to deduct the volumes of apparent losses from the total volume of losses in order to arrive at a top-down approximation of the annual volume of real losses.

In attempting to quantify the volume of apparent loss due to customer meter inaccuracy in the water audit, it is important to recognize that three primary occurrences cause a meter population to become inaccurate, namely

• Eventual decline of the inherent (mechanical) accuracy of a meter population through wear.

• The meter or the meter reading device may fail or "stop" altogether.

• Meters may not be of the proper size or type to accurately register the full range of water flows encountered in a given customer supply.

It is necessary to disaggregate, or separate, the activities of the water utility's meter management in these three occurrences in order to properly construct a representative picture of the annual volume of apparent loss attributed to customer meter inaccuracy and the reasons for each disaggregated volume. In this way, planning can be carried out to remedy the specific causes of meter inaccuracy in the most economic manner.

Loss of Accuracy Due to Mechanical Wear

Well-manufactured water meters can lose appreciable mechanical accuracy due to

• Aggressive water quality

• High rates of flow being measured

• Chemical or residual buildup

• Abrasive materials such as sand in suspension carried by the water

• Air running through the meter after a system outage

As the cumulative volume passed through the meter increases toward meter life cycle levels then the mechanical failures are compounded. Chapter 16 provides detailed information on the assessment of life cycle accuracy of customer meters and means to control losses that occur in this subcomponent of apparent losses.

Zero Consumption Billings from Stopped Meters or Vacant Properties

Meters or meter-reading devices can fail to register for various reasons. However, meters that show no registration might also reflect a customer property with no use, such as that which may occur at a vacant property. Large numbers of customer meters that mechanically fail to register any flow from billing cycle to billing cycle can account for large volumes of apparent losses and uncaptured revenue.

Many water utilities employ the use of an estimated consumption volume if they encounter periodic low or zero consumption volumes generated from meter reading. This practice can be effective if the zero reads are only periodic. However, when estimation is undertaken for many consecutive months, estimated volumes will likely deviate from the actual consumption volumes. If all values of consumption for a given account are based upon estimates for an entire audit year, then the volume assigned to that particular account for the water audit can be seriously in error. Water utilities should routinely review billing data and assess the occurrence of zero consumption bills, particularly those that register zero consumption for several consecutive months. It is worthwhile for the utility to dedicate personnel to physically inspect the meter site of a representative sample of customer accounts to determine the reason for the continuing zero consumption registrations. The findings of such inspections provide data that can be used to model the occurrence of apparent loss in the zero consumption population throughout the entire system.

By applying the above analysis, it is possible to model best case and worst case scenarios for customer meter losses occurring due to meters registering zero consumption. The best case reflects the overall accuracy of the entire meter population without including zero consumption meters, a scenario that would occur only in the ideal case of the water utility responding quickly to accounts registering zero consumption and correcting meter or meter-reading problems just after they occur. The worst case reflects customer meter population accuracy including the greatest potential extent of zero consumption meters, reflecting a water utility policy that ignores zero consumption registrations, allowing them to mount throughout the audit year. Calculate the apparent losses in both the best and worst case, then the average meter accuracy can be calculated for water balance purposes, representing the average inherent accuracy of the meter population, including the average response time to correct accounts that chronically register zero consumption.

For this type of analysis to be accurate, it is necessary that there is a large enough test sample of data from field inspections of zero consumption accounts in order to properly represent the total customer account population.

Improper Size or Type of Meter

Many brands of customer meters are known to become appreciably inaccurate when very high or very low flows (relative to the design range of the meter) are registered. If the size or type of meter in a given application results in the majority of flow occurring in these extreme ranges, then the meter will fail to register a large portion of the customer flow. Section 12.4 provides a detailed discussion on meter sizing impacts and the best practices to employ to ensure that losses due to poor sizing or typing are minimized. With direct-feed pressure systems as are typical in North American water utilities, customer meters need to be selected and sized to record a wide range of flow rates. Any underregistration of metered consumption is considered an apparent loss in the water audit, as the lost water is reaching the customer, but a portion of the consumption is not being registered or billed. A number of software models have been developed for this type of loss analysis.

Similar modeling techniques can be applied for the apparent loss components of data transfer error, systematic data handling error in customer billing systems, and unauthorized consumption. Detailed spreadsheet models for these components are not as common as those modeling customer meter inaccuracy; however, it is up to the water auditor to assess the occurrences of these losses and attempt to model their extent in their utility operations.

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