Page 258 - Water Loss Control

Contr olling Appar ent Losses—Systematic Data Handling Err ors 231

Source meters: No lag time correction is made for source meters, because their
reading usually occurs on the days that the water audit period begins and ends. If
the last reading (December 31) was a day late (January 1), then the water supplied
for January 1 should be subtracted from the total water supply reading.
Customer meters: Because customer meter readings do not coincide neatly with
the study period, a correction must be made. The best way to account for changes
in the number of customers and in consumption patterns is to prorate water
consumption for the first and last billing periods within the water audit period.
The first billing period has only 10 days that actually occur in the water audit period.
Yet the billing information represents 31 days of consumption. If consumption for the
December 11 through January 10 period is 33.204 million gal, the amount applicable to
the water audit period is:

33.204 million gal × 10 days = 10 711 million gal
.
31 days
Thus, 10.711 million gal of the consumption read on January 10 applies to the water
audit period.
At the end of the water audit period, there are 21 days not included in the billing
data collected on December 10. Consumption for the last 21 days in December is
obtained from the following month’s billing. If sales for that month are 36.66 million
gal, the amount applicable to the water audit period is:
21 days
36.66 million gal × = 24 83 million gal
.
31 days
Thus, 24.83 million gal is added to the consumption read on December 10.
Adjusting for many meter routes. The preceding discussion describes the basic method
for correcting lag time in meter reading when all customers’ meters are read on the
same day. Unless fixed network AMR systems are used (see Chap. 13) that seldom
happens since most utilities have such large customer populations that it is
impossible to read all of the meters on a single day. Usually, meters are assigned to
different routes and read on different days. Therefore, a meter lag correction should
be used for each meter reading route, particularly if each customer’s meter is read
on the same date each month. Figure 14.1 gives an example of this.
A meter lag correction can involve a number of steps. In our example, County Water
Company has three meter routes, each with its own reading date. The water audit
period is one calendar year, and the consumption is prorated for each meter route or
book. Meters are read bimonthly: route A on the first of the month, route B on the tenth
of the month, and route C on the twentieth of the month.
The uncorrected total metered consumption is based upon bills issued during the
water audit period. But, because of the bimonthly billing schedule, these bills would not
include all water consumed during the year. Some water shown as used in the first billing
period (issued in February) actually occurred in the preceding December. The last set of
bills, issued in November and December, would not include water consumed in December.
Two corrections need to be made. First, water consumed in the month preceeding the
water audit period must be subtracted from consumption figures. Second, water con-
sumed in the final month of the water audit period must be added.