Hydraulic laboratory testing of Sontek-IQ Plus

The SonTek-IQ Plus (IQ Plus) is a bottom-mounted Doppler instrument used for the measurement of water depth and velocity. Evaluation testing of the IQ Plus was performed to assess the accuracy of water depth, discharge, and velocity measurements. The IQ Plus met the manufacturer’s specifications and the U.S. Geological Survey (USGS) standard for depth accuracy measurement when the unit was installed, according to the manufacturer’s instructions, at 0 degrees pitch and roll. However, because of the limited depth testing conducted, the depth measurement is not recommended as a primary stage measurement. The IQ Plus was tested in a large indoor tilting flume in a 5-foot (ft) wide, approximately 2.3-ft deep section with mean velocities of 0.5, 1, 2, and 3 ft per second. Four IQ Plus instruments using firmware 1.52 tested for water-discharge accuracy using SonTek’s “theoretical” discharge method had a negative bias of -2.4 to -11.6 percent when compared with discharge measured with a SonTek FlowTracker and the midsection discharge method. The IQ Pluses with firmware 1.52 did not meet the manufacturer’s specification of +/-1 percent for measuring velocity. Three IQ Pluses using firmware 1.60 and SonTek’s “theoretical” method had a difference of -1.6 to -7.9 percent when compared with discharge measured with a SonTek FlowTracker and the midsection method. Mean-velocity measurements with firmware 1.60 met the manufacturer’s specification and Price Type AA meter accuracy requirements when compared with FlowTracker measurements. Because of the instrument’s velocity accuracy, the SonTek-IQ Plus with firmware 1.60 is considered acceptable for use as an index velocity instrument for the USGS. The discharge computed by the SonTek-IQ Plus during the tests had a substantial negative bias and will not be as accurate as a discharge computed with the index velocity method. The USGS does not recommend the use of undocumented computation methods, such as SonTek’s “theoretical” method for computing discharge.