Calibration is a check of the quality of the flow data, or more specifically, a check of the quality or correctness of the depth and velocity measurements.  Both depth and velocity are affected by specific site characteristics and the ability of a chosen measuring device (flow meter) to accurately record both depth and velocity over time (repeatability).  To provide quality flow data it is necessary to select a metering point where the flow depth and velocity exhibit a characteristic degree of uniformity permitting measurement by the selected metering device.  No degree of accuracy can be guaranteed when a flow site is selected whose surface turbulence or cross sectional velocities are so irregular that a true depth or velocity cannot be measured.  Since the continuity equation states that flow (Q) is the result of velocity (V) times area (A) then the quality of Q is totally dependent on providing true velocity and depth measurements.  Critical in this process is the choice of the metering device which must be capable of measuring and recording true depth and velocities at the chosen site.  Whichever technology is selected (pressure transducer, bubbler, or ultrasonic for depth and Doppler, radar, or electromagnetic for velocity) there must be verification that the specific instrument can accurately provide these measurements in the field.  Although field conditions can never be accurately reproduced in a controlled environment, manufacturers’ specifications can be verified under conditions controlled by the measurer prior to meter installation.  Both depth and velocity verification can be achieved by use of primary devices (buckets, flumes, and weirs) and secondary devices such as: rulers, portable velocity meters, and other flow metering devices using the same or different technologies as the selected flow meter.  These comparisons can and should be carried out in a controlled flow environment prior to flow meter field installation.  By definition, a controlled environment guarantees agreement of depth and velocity readings with the selected flow meter prior to field installation.  The importance to this calibration procedure cannot be overstated.  Where field measurements can be a subjective endeavor affected by unique site conditions and the ability/inability of the measurer to apply a second and third measurement to verify the first, a controlled “bench calibration check” eliminates that field subjectivity, and that dilemma as to which measurement, the meter or the measurers, is correct.  Thoroughly testing the applied flow meter prior to insertion in the field is the only guarantee that the meter and all of its components are measuring and recording depth and velocities accurately at the onset of any study.  The goal of the flow provider is then to maintain that calibration and accuracy with regular site visits for the purpose of cleaning, servicing, or verifying field depth and velocity conditions.