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DISCHARGE MEASUREMENTS AT GAGING STATIONS 7 The care and rating of vertical-axis meters is described below and by Smoot and Novak (1968). Horizontal-axis current meters The types of horizontal-axis meters in use are the Ott, Neyrpic, Haskell, and Hoff. The Ott meter is made in Germany, the Neyrpic meter in France, and both are used extensively in Europe. The Haskell and Hoff meters were developed in the United States where they are lIsed to a limited extent. The Ott meter is a precision instrument but is not used extensively in this country because it is not as durable as the Price meter under extreme conditions. (See fig. 7.) The makers of the Ott meter have developed a component propeller which in oblique currents auto- matically registers the velocity projection at right angles to the measuring section for angles as much as 450 and velocities as much as 8 fps. For example, if this component propeller were held in the position AB in figure 8 it would Figure 7.-0tt CUffent meter. v cos a ~ ".. A\ s Figure S.-Velocity components measured by Ou and Price current meters. register V cos a rather than V, which the Price meter would register. The N eyrpic meter is used rarely in this country because it has the same disadvantages as the Ott meter. The Haskell meter has been used by the U.S. Lake Survey, Corps of Engineers, in streams that are deep, swift, and clear. By using propellers with a variety of screw pitches, a considerable range of velocity can be measured. The meter is durable, but has most of the other disadvantages of horizontal-axis meters. The Hoff meter is used by the Geological Survey, the Department of Agriculture, and others, especia.lly for measuring pipe flow. (See fig. 9.) The lightweight propeller has three or four vanes of hard rubher. The meter is suited to measurement of low velocities, but not for rugged use. Optical current meter The Geological Survey, in cooperation with the California Department of Water Resources, has developed an optical current meter. (See fig. 10.) This meter is a stroboscopic device designed to measure surface velocities in open channels without immersing equipment in the stream. The optical current meter will find its principa.l use in measurements of surface velocity during floods when it is impossible to use conventiona.l stream-gaging equipment be- cause of extremely high velocities and a high debris content in the stream. Care of the vertical~a)(is CUlTet\' meter The calibration and maintenance of vertical- axis type current meters is presented in detail Figure 9.-Hoff current meter.