1. Introduction

Accurate application of water measuring devices generally depends upon standard designs or careful selection of devices, care of fabrication and installation, good calibration data and analyses, and proper user operation with sufficiently frequent inspection and maintenance procedures. In operations, accuracy requires continual verification that the measuring system, including the operators, is functioning properly. Thus, good training and supervision is required to attain measurements within prescribed accuracy bounds. Accuracy is the degree of conformance of a measurement to a standard or true value. The standards are set by users, providers, governments, or compacts between these entities. Accuracy is usually stated in terms of deviation of discharge discussed subsequently. All parts of a measuring system, including the user, need to be considered in accessing the system's total accuracy.

A measurement system usually consists of a primary element, which is that part of the system that creates what is sensed, and is measured by a secondary element. For example, weirs and flumes are primary elements. A staff gage is the secondary element.

Purchasers and users of water measurement devices generally depend upon standard designs and manufacturers to provide calibrations and assurances of accuracy. A few irrigation water users or providers have the facilities to check the condition and accuracy of flow measuring devices. These facilities have comparison flowmeters and/or volumetric tanks for checking their flowmeters. These test systems are used to check devices for compliance with specification and to determine maintenance needs. However, maintaining facilities such as these is not generally practical.

One purpose of this chapter is to define terms used by manufacturers and sales representatives related to measuring device specifications, calibration, and error analyses. Various disciplines and organizations do not fully agree on some of these definitions. Therefore, one should ask for clarification of these definitions when others use these terms. Another purpose is to provide example analyses which can help in managing the accuracy of irrigation water delivery.