CHAPTER 5 - INSPECTION OF WATER MEASUREMENT SYSTEMS
2. Standard Devices Versus Nonstandard Devices
The use of standard devices usually results in lower total costs over the lifetime of a measurement structure. Their long general use has generated more backup data and experience, making them potentially more reliable. A truly standard device has been fully described, accurately calibrated, correctly made or installed, and sufficiently maintained to fulfill the original requirements. Standard discharge equations and tables or curves may then be relied upon to provide accurate water measurements. Maintaining a standard device involves only a visual check and measurement of a few specified items or dimensions to ensure that the measuring device has not departed from the standard.
Even though a standard device might have been selected for a particular measurement situation, water providers and users frequently find themselves unexpectedly stuck with nonstandard and, at times, unusable devices. This situation can occur when a device is installed improperly, is poorly maintained, is operated above or below the prescribed discharge limits, or has poor approach or downstream submergence conditions.
Accurate discharges from nonstandard structures can be obtained only from specially prepared curves or tables based on calibration tests, such as multiple current-meter ratings. The accuracy of a nonstandard device cannot be determined by visual inspection. Accuracy can only be ensured by recalibration, which is costly when properly performed. Ratings must be made at close discharge intervals over the complete operating range. Then, curves and/or tables must be prepared. Installation and proper inspection and maintenance of standard devices are not difficult and are less costly in the long run. Standard discharge tables may then be used with full confidence.