Developing the Customer Meter Accuracy Testing Program

In order to assess and maintain good physical accuracy of the customer meter population, many water utilities operate their own meter test facility and equipment, and perform ongoing accuracy testing of meters that have been rotated out of service. For these operations, testing of targeted groups of meters can be readily accommodated. Water utilities that do not have their own facilities can outsource their testing to specialty companies.

Total customer consumption meter error includes meter errors from all meter sizes, including residential, industrial, commercial, agriculture, and others. In general, meter error can be assessed for small meters (5/8 in and 3/4 in), which are typically employed for residential use, and all other (larger) meters which include industrial, commercial, agricultural, and meters for other applications. Testing can serve both the general purpose of providing information to the water audit on the system-wide level of apparent loss due to customer meter inaccuracy, and to identify the accuracy of individual meters, thereby allowing meter improvements to be implemented where needed.

AWWA's guidance manuals on meters give excellent instruction on meter accuracy testing. These include the M22 publication and the M6 publication, Water Meters— Selection, Installation, Testing, and Maintenance, the latter of which provides comprehensive information on the basics of customer meter management.9 Generally, accuracy tests should be conducted at low, medium, and high flow rates. For small, residential meters sample groups of meters can be tested. A randomly selected sample of several dozen to several hundred meters (depending on the size of the meter population) can be selected and tested. A separate sample of meters with high cumulative consumption should also be tested. Results of the latter testing can help to develop a long-term meter change-out strategy based upon the level of cumulative consumption at which accuracy begins to decline.

Because there are hundreds or thousands of customer meters in a drinking water utility, it is impractical to inspect and test every one each year. Instead, the water utility manager can identify sample numbers of customer meters of various sizes and types for inspection and testing. The results of such sample tests give a reasonable indication of the status of the entire customer meter population.

Residential (small) meter testing: Many utilities operate meter testing and rotation programs. Particularly for small meters, it has become more cost-effective to replace meters than to repair them. Random or specific testing to determine the accuracy of installed customer meters can be conducted to monitor the wear of meters. A representative sample of newly purchased residential meters should also be tested to confirm the acceptability of the newly delivered meters. All of this test data represents a good source of information to infer the overall degree of inaccuracy existing in the customer meter population. In this way the level of apparent loss in the system can be quantified for the water audit. Test a random sample of residential meters, 50 to 100 is a sufficient number, but the optimal number to be tested depends upon the size of the customer meter population, the degree of confidence required in the test results, and the variance in the actual test results observed. Residential meters may be tested on a test bench or sent to the factory or a testing service contractor for testing.

Tables 12.3 to 12.5 give an example of calculations using small meter accuracy test data to determine the level of apparent loss from small meter inaccuracy included in the water audit for County Water Company (AWWA 2008).1 Weighting factors for small meter flow rates are given in Table 12.3. The weighting factors reflect common percentages of time that flows are found in the low, medium, and high flow ranges, respectively, with flows existing most often in the medium range for most properly sized meters.10 In the example 50 randomly selected residential meters are tested for low, medium, and high flows with summary test results shown in Table 12.4. These results, shown as a percentage of accuracy, are used to calculate the total meter error at average flow rates. Table 12.5 demonstrates how to use existing meter test data to calculate total residential meter error. The resulting residential (small) meter error for County Water Company is given at the bottom of Table 12.5 as a value of 134.33 million gal for calendar year 2006.

Industrial/commercial (large) meter testing: Large industrial, commercial, and agricultural meters register a much greater portion of consumption and produce a much larger share of revenue per account than do residential meters. For many water utilities over

Percent of Time

Range, gpm

Average, gpm

Percent Volume*

15

Low 0.50-1.0

0.75

2.0

70

Medium 1-10

5.00

63.8

15

High 10-15

12.50

34.2

* Percent volume refers to the proportion of water consumed at the specified flow rate, as compared to the total volume consumed at all rates. In this example, only 2.0 % of the total water consumed occurs at the low-flow range of approximately 0.5-1.0 gpm.

Instead of using the percentage of volumes shown here, you may compute your own percentage volume data. Using special dual-meter yokes and recording meters, you can determine the actual flow rates for your water meters.

Source: Ref.1

Table 12.3 Weighting Factors for Flow Rates Related to Volume Percentages for 5/8- and 3/4-in Water Meters

Test Flow Rates

Mean Registration, percent

Low flow (0.25 gpm)

88.8

Medium flow (2.0 gpm)

95.0

High flow (15.0 gpm)

94.0

Table 12.4 Mean Meter Testing Data from a Random Sample of 50 Meters for County Water Company

Source: Ref. 1

Table 12.4 Mean Meter Testing Data from a Random Sample of 50 Meters for County Water Company

Total

Volume at

Meter

Sales

Flow Rate

Meter

Error

Percent

Volume*

(Vf)

Registration

Meter Error (ME)

(ME)

Volume*

(Vt)

(%V ë Vt)

(R)t

ME = Vf/(0.01R) - Vf

milion

(%V)

milion gal

milion gal

percent

milion gal

gal

2.0

2,318.8

46.38

88.8

[(46.38/0.888) - 46.38]

5.85

63.8

2,318.8

1,479.39

95.0

[(1,479.39/0.95) - 1,479.39]

77.86

34.2

2,318.8

793.03

94.0

[(793.03/0.94) - 793.03]

50.62

Total Residential meter error (line 8)

134.33

f Based on residential water sales data in Table 12.2. t From Table 12.4. Source: Ref. 1

Table 12.5 Calculation of Residential Water Meter Error

50% of revenue is received from less than 20% of customers accounts with large meters. Therefore it is critical that these accounts are systematically reviewed to ensure that they are being metered and billed correctly. Large meters should be inspected for proper selection and sizing before installation. Additionally, large meters should be tested for accuracy before they are used, since not all new meters are sufficiently accurate. In the United States, meters sized 1 in and larger are typically considered to be large meters, although the specific size convention can vary from one utility to another.

All water utilities, regardless of their number of customer accounts, should strive to regularly inspect, test, and confirm appropriate sizing for the relatively small number of meters serving the largest of water consumers. These meters provide the basis for the largest billings in the water utility and every effort must be made to keep them accurate. Inspecting and testing the top 10 largest users in the system on an annual basis will help ensure that optimal customer billings are occurring. Ideally, a representative segment of the large meter population should be tested each year, including 1- , 1%- , and 2-in meters, a mid-range that sometimes is overlooked by utilities.

Tables 12.6 to 12.8 illustrate the use of meter test data to calculate total large meter error.1 The mean registration data in Table 12.6 are used to calculate the meter error for large meters. The actual test results are shown in Table 12.7 and the resulting large meter error for County Water Company is shown in Table 12.8 as 29.97 million gal. The results of the individual large meter tests can be used to estimate the amount of revenue to be gained by improving the function of large meters by applying the appropriate cost factor.

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Responses

  • napoleone
    What percent of newly purchased water meters should be tested before put into service?
    2 years ago

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