2002-055-RES ACCEPT PROPOSAL OF CARTER & BURGESS FOR PROFESSIONAL ENGINEERING SERVICES TO WATER LOST AND UNACCOUNTED FOR STUDY IN CITY'S WATER AND ACCOUNTING AND OPERATING SYSTEM

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I ,. I I I I I I I I I I r I I Proposal Comprehensive Water Distribution System Audit * Allowing air to enter the process flow, resulting in metering error. * Failure to properly ground the instruments, resulting in induced error in the electronic components. * Installing a meter that is over-sized for the flow to be measured. * Installing the meter in a location that makes future maintenance impossible. Meter Accuracy The accuracy of flow meters is a common problem encountered in the water industry. The science that deals with accuracy in measurements is metrology, and there have been numerous books written on the issues associated with obtaining measurements that are both accurate and precise. In the United States, the National Institute of Science and Technology (NISD, which used to be known as the National Bureau of Standards, is the organization that is charged with supporting metrology at the national scale. Therefore, in a purest sensei all measurements should be traceable to NIST. Error in flow metering systems is introduced by each element used in the flow measuring process. Consider a venturi tube system that consists of the venturi tube, the differential pressure transmitter, and the indicator/recorder. Each of the three devices has some internal error that contributes to the overall error of the measured flow. It is important to realize that these errors are multiplicative, not additive. In Situ Meter Calibration Several techniques can be used to perform in situ meter calibration, or to check the accuracy of meters that are currently in service. The techniques that are most applicoble to the City of Paris' project are discussed in the follOWing paragraphs. Carter ::Burgess Volumetric Calibration The volumetric calibration method consists of timing the increase or decrease in water volume in a tank or basin of known geometry. The technique requires that the geometry of the volumetric tank be well-known, that the change in water surface elevation can be determined relatively accurately, and that the time between different elevations can be measured with reasonable accuracy. When performed carrectly, volumetric calibration is among the most accurate methods for calibrating flow meters. The method is ohen referred to as the "bucket and stop watch" procedure, which is descriptive terminology, but does not indicate the relative complexity of the procedure. The procedure involves the measurement of three separate parameters: the volume ofthe tank, the change in water surface elevation, and the time. For the procedure to be accurate, all three measurements must be preformed with reasonable accuracy. Tracer Dilution Method The tracer dilution method of in situ calibration consists of adding a known amount of the tracer to the process flow and determining the concentration in the process flow downstream of the meter to be calibrated. Figure 4 shows a typical tracer dilution set up/ and the equipment requirements are minimal. A calibrated container for the tracer is required to confirm the rate at which the metering pump is adding the tracer to the process flow. There must be two taps in place, one through which to add the tracer, and the other from which to collect the downstream sample. The flow in process is calculated from the following simple equation: q X C1 = (Q + q) x (2 where q is the flow rate of the tracer, (I is the concentration of the tracer in the tracer solution container, Q is the process flow rate, and C2 is the concentration of the tracer in the downstream sample. 17 -.I