C.4.3 Horizontal Axis Current Meters
Horizontal axis meters are capable of very accurate flow measurement in areas of local turbulence. The component effect of the rotors compensates for angular flow in both the horizontal and vertical planes, and the orientation of the rotor provides for a balanced translation of the linear motion when measuring near the vertical faces at either edge of a channel. All models use the magnetic reed switch to generate the rotational pulse count, thus avoiding the variable frictional component inherent in `make and break' systems.
A small horizontal axis current meter continues to be the principle instrument used by Provincial agencies in British Columbia to measure small shallow streams, while the larger models are the preferred for use with bank-controlled cableways and bridge rods.
Note: The criteria for surface and bottom observations using horizontal axis meters is that the axis of the current meter should not be situated at a distance less than 1½ times the rotor height from the water surface, nor should it be situated at a distance less than 3 times the rotor height from the streambed. The rotor height (diameter) of the meters vary from a minimum of 30 mm to a maximum of 125 mm.
This miniature current flow meter is supplied in kit form, the accessories include a sturdy 1.5-m, 2 section wading rod graduated in 1-cm divisions, a control unit and 3-m connecting cable with quick release connectors. An optional cable suspension kit includes a 30-m suspension/signal cable, suspension bar and current meter tailfin. Longer sectional rods can be supplied as an option.
The Braystoke current meter, manufactured by Valeport Developments Ltd. UK, is designed for the measurement of flow velocities in fresh water or salt water, and is not affected by water quality. The meter will operate in shallow streams of only 6 cm or suspended to any depth and covers the entire range of velocities from 0.03 to 5.0 m/s with the 50-mm-diameter x 0.10-pitch impeller. The manufacturer does not specify the extent of oblique flow up to which the propeller measures the true velocity value; however, field and laboratory tests conducted by others, indicate the impeller follows the rule of cosine in oblique flows of up to 10° with an accuracy of ±2%.
The impeller shaft and bearings are protected by the design of the impeller hub; when the chamber between the shaft and the impeller fill with water, no silt or weeds will enter the bearings. The neutrally buoyant impeller, fixed on a stainless steel shaft by a washer and two nuts, is fitted with plastic bearings lubricated by the water in which the meter operates. The reed switch contacts housed in the quick release adapter fitted to the end of the cable are energized on each revolution of the impeller by two magnets mounted in the impeller hub. Revolution pulses are registered in the upper LCD display; see Section C.4. for a description.