Instruments for measuring the water velocity of bodies of water such as rivers, lakes, and channels. There are three types of mechanical, electrical and ultrasonic testing. The mechanical type is mainly based on a rotor type, and has a propeller type and a rotary cup type flow meter. Electrometric type electromagnetic flow meter. Ultrasonic time difference method and Doppler flow meter.

The flow meter is generally suitable for measuring the average flow rate during a fixed period of time. The current flow rate can be measured by a magnetic current meter or a multi-turn rotor-type flow meter. It can also be used as a speed measurement by the ship method. The speed range of the instrument is generally 0.03 to 5.00 m/s, and the applicable water depth is generally 0.2 to 20 meters. In 1790, R. Waltmann made a rotor-type flow meter for flow rate measurement. The rotary cup type and propeller type flow meter manufactured and manufactured in China from the 1950s to the 1960s have the characteristics of good waterproof and sand-proof performance. The development direction of the flow meter is non-rotor electrical measurement technology, optical technology, ultrasonic technology and telemetry technology. (See color picture) Propeller-type flow meter is mainly composed of three parts: propeller, frame and tail. Rotating propellers are equipped with signal contacts, bearing shafts, etc. China's "25-1 type" propeller flow meter shaft system has a winding labyrinth structure, the interior is filled with light oil, has good water and sand performance, can be high Flow rate and use in sandy rivers. Propeller-type flowmeters, rotary cup-type flowmeters and rotary-blade-type flowmeters are all rotor-type flowmeters. The principle of operation is basically the same. They use water flow to force the rotor to rotate, and the flow velocity is derived based on the rotation speed. The formula is:

In the formula, υ is the flow velocity; N is the total number of revolutions in the time period; t is the time period duration; c is a constant, which reflects the friction resistance when the rotor rotates; K value depends on the pitch of the blade or the cup type. The K and c values ​​are verified in a special tank. At low speeds, due to the large influence of c-values, the velocity formula is a function of a curve, and the above formula is no longer used.

The number of revolutions is shown by a counter. The simplest counters are small lights, bells, and transistor sounders. For every 20 rotations of the propeller, the switch is turned on once, the light flashes or sounds, manual counting, stopwatch timing, and more complete electric counter, electronic counter, timing counter, and direct reading flow rate monitor, these counters can be used There are many signal flowmeters for every rotation of the propeller, which can improve the accuracy of speed measurement.

The principle of the electromagnetic flow meter is to use the water flow as a conductor to cut magnetic lines in a certain magnetic field, ie, to generate an electromotive force whose voltage is proportional to the flow velocity. The instrument has no rotor, has a smooth appearance, a small volume, and low power consumption. There are excitation coils in the body cavity, and a pair of electrodes are connected to the water body in a direction perpendicular to the magnetic force lines. When the water flows on the surface, a slight voltage signal is generated on the electrode, which is transmitted to the counter by the wire, and the flow rate can be directly displayed through circuit processing such as amplification and analog-to-digital conversion.

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