CHAPTER 3 - MEASUREMENT ACCURACY

4. Comparison Standards

Water providers may want or be required to have well developed measurement programs that are highly managed and standardized. If so, irrigation managers may wish to consult International Organization for Standardization (1983), American Society for Testing Materials standards (1988), American Society of Mechanical Engineers Test Codes (1992), and the National Handbook of Recommended Methods for Water Data Acquisition (1980).

Research laboratories, organizations, and manufacturers that certify measurement devices may need to trace accuracy of measurement through the entire hierarchy of increasingly rigid standards.

The lowest standards in the entire hierarchy of physical comparison standards are called working standards, which are shop or field standards used to control quality of production and measurement. These standards might be gage blocks or rules used to assure proper dimensions of flumes during manufacture or devices carried by water providers and users to check the condition of water measurement devices and the quality of their output. Other possible working standards are weights, volume containers, and stop watches. More complicated devices are used, such as surveyor's levels, to check weir staff gage zeros. Dead weight testers and electronic standards are needed to check and maintain more sophisticated and complicated measuring devices, such as acoustic flowmeters and devices that use pressure cells to measure head.

For further measurement assurance and periodic checking, water users and organizations may keep secondary standards. Secondary standards are used to maintain integrity and performance of working standards. These secondary standards can be sent to government laboratories, one of which is the National Bureau of Standards in Washington, DC, to be periodically certified after calibration or comparison with very accurate replicas of primary standards. Primary standards are defined by international agreement and maintained at the International Bureau of Weights and Measurements, Paris, France.

Depending upon accuracy needs, each organization should trace their measurement performance up to and through the appropriate level of standards. For example, turbine acceptance testing combined with severe contractual performance penalties might require tracing to the primary standards level.