The purpose of this section is to review several commonly used current meter types and models available in Canada and to describe the care, maintenance and testing of the current meter.
All mechanical-current meters measure velocity by translating linear motion into angular motion.
An ideal current meter should respond instantly and consistently to any changes in water velocity, and should accurately register the desired velocity component. Also, the meter should be durable, easily maintained, and simple to use under a variety of environmental conditions. Due to the fundamental difference in their axial alignment, vertical- and horizontal-axis meters differ in their maintenance and performance. Meter performance depends on the inertia of the rotor, water movement, and friction in the bearings.
Two common types of current meters are used by the various agencies engaged in the quantification of streamflow in British Columbia: the vertical axis meter and the horizontal axis meter. With either meter, the rate of rotation of the rotor or propeller is used to determine the velocity of the water at the point where the current meter is set. Before the current meter is placed in service, the relationship between the rate of rotation and the velocity of the water is established in a towing tank. The rating procedure is explained later in this section. In its streamflow measurement program, Water Survey of Canada uses the Price current meter almost exclusively.
A third type, an electromagnetic current meter measures velocity using Faraday's Law, which states that a conductor (water) moving in a magnetic field (generated by the current meter probe) produces a voltage that varies linearly with the flow velocity. Electrodes in the probe detect the voltages generated by the flowing water. As these meters have no moving parts they are not subject to many of the operational and maintenance problems associated with mechanical current meters. However the earlier models were equipped with a counter that displayed an instant velocity value and failed to produce the time averaged velocities required for the measurement of discharge. Generally their use has been confined to fish habitat and water quality studies.
Table C-2 lists the current meters have been used for the collection of streamflow data by BC government survey staff. All current meters are periodically calibrated by the National Calibration Service at the National Water Research Institute, Burlington, Ontario.
Hydrologists of the U.S. Geological Survey have studied both the laboratory and field results for meter precision, linearity, and response to oblique flow angles. The current meter types tested included most of the meters shown below and compare the performance of vertical and horizontal axis mechanical meters. The electromagnetic meter was also tested.
Percent standard error for all meters tested was less than 2% with the vertical axis meters providing the most consistent response.
Both horizontal and vertical-axis meters tested had good precision (percent standard errors < 0.75 % for velocities >24.4 cm/s) and a similar linearity of response (Root Mean Square < 2.01 cm/s).
Table C-2. Commonly used current meters.
Vertical Axis Current Meters |
Horizontal Axis Current Meters |
Price 622AA |
Valprot BFN 002 (Braystoke) |
Price 622AA Magnetic |
OTT 5 (Arkansas), 2 impellers (replaced by C31) |
Price 622AA Photo-Fibre Optic |
OTT, C31, 3 impellers |
Price Winter Model AA |
OSS, B1, 2 impellers (identical to C31) |
Price Pygmy |
OTT, C1, 3 impellers (replaced by C2 & OSS, PC1) |
Price Pygmy Photo-Fibre-Optic |
OSS, PC1, 2 impellers (identical to C2) |
Swoffer 2100, 1 impeller |
The magnitude of error for horizontal-axis meters is usually smaller than those for vertical-axis meters in oblique flows Two horizontal-axis meters, the Ott C-31 meter with A and R impellers had the smallest error in oblique flows. At angles between ±100, errors ranged from -7.87% to 8.92% for the vertical-axis meters and from 2.02% to 3.77% for the horizontal-axis meters.
Unfortunately neither the Ott C2 nor the OSS PC1 were included in the above study. (The Ott and OSS meters are identical - components are interchangeable). However, the impellers supplied with these meters accurately compensate for angular flow to the limits specified by the manufacturers (Table C-5). These meters have, together with the earlier model Ott C1, served as the preferred meter for the measurement of flow in watercourses less than 0.3 m deep.
The Swoffer retrofitted Price type AA and the Price Pygmy do not have relatively large chambers on the upper section of yoke and therefore present a more symmetrical shape to the current. The presence of the contact chamber is one of the factors listed as a possible cause of the rather large errors encountered in the oblique flow tests on standard Price meters It is therefore likely that the absence of these chambers on the retrofit models will produce more accurate results in oblique flows.
The electromagnetic current meter has smaller errors than the vertical axis meters for most angles tested.