Impeller current meter
Anniina Kittilä, ETH Zürich, Institute of Geophysics, Geothermische Energie u. Geofluide. Sonneggstrasse 5, 8092 Zürich, Switzerland e-mail: anniina.kittila(at)erdw.ethz.ch
Impeller current meters measure the velocity of the flowing water, and the flow rate is obtained when the velocity is multiplied by the cross-section area of the stream. There are several different types of instruments and techniques for measuring the flow rate with velocity – cross-section methods, from which impeller current measurements is probably the most common. The method is based on rotating impeller that is driven by the fluid flow, and the rotational velocity of the impeller is proportional to the fluid flow velocity (Mustonen 1986, Hughes 1993).
Description and performance of the technology
The measurements with impeller current meters are usually done in one of the five ways:
- from an overhead structure, such as a bridge
- from a boat
- with a cable track that allows a current meter attached to it being moved horizontally and vertically over the cross-section, especially in deeper channels
- wading in shallow channels
- from holes in the ice.
Before measuring the flow velocity, the cross-section perpendicular to the channel must be marked. To determine the cross-sectional area the channel width and depth must be known. Width measurements are rather straightforward when wading across the channel is possible, and can be done, for example, with tape measure. In non-wading conditions, laser range finder can be used. The width should be taken from the edges of the water and the bank. The depth, on the other hand, is measured from sub-sections along the cross-section. These sub-sections do not need to be of the same size (width), but as the depth is measured from the centre of each of them, they should be representable to have adequately accurate cross-sectional measurements for the whole channel. (Mustonen 1986, Shaw et al. 2011, Box et al. 2012)
After the cross-sectional measurements, the flow velocity can be measured with current meter. Average velocity in a channel occurs at 0.6D depth of the channel, where D is the total depth. Particularly in deeper channels and partial sections the flow velocity should be measured from several depths, but in shallower places one measurement from the 0.6D depths should be adequate. In deeper sections the average velocity can be measured by taking two readings, from 0.2D and 0.8D depths, and taking average from those two readings (Shaw et al. 2011, Box et al. 2012). The duration for taking the necessary measurements along one cross-section might take approximately one hour, but preparatory actions, especially setting a cable track, might increase the duration of the whole procedure significantly (Mustonen 1986). If taken by wading, the flow velocity measurements should be taken so that one disturbs the flow the least, i.e. standing on the downstream side facing upstream behind the chosen cross-section. If taken from a bridge, upstream side is recommended (Box et al. 2012). The flow rate is first calculated separately for each sub-section by multiplying the area of the sub-section by the flow velocity. The total flow rate is obtained by summing the result of each of the sub-sections (Mustonen 1986, Shaw et al. 2011, Box et al. 2012).
The conditions are the best for measuring flow rate with impeller current meter when a cross-section with certain requirements fulfilled is chosen (Mustonen 1986, Box et al. 2012):
- factors affecting the flow rate (water level, flow velocity and velocity distribution) should not change significantly during the measurements
- the threads of velocity are as perpendicular to the cross-section as possible and the flow does not have much turbulence
- the profiles of the cross-section and the channel right above it are as regular as possible; that way also the velocity distribution is uniform
- the channel walls are relatively even; for example, abundant vegetation disturbs the measurements.
Natural conditions rarely satisfy all of the requirements fully, but the cross-section best meeting the criteria should be chosen. In some cases rocks and other debris within a chosen cross-section and upstream can be removed to improve the flow conditions and the measurements (Box et al. 2012).
As the impeller current meters come in many sizes and configurations, there are many different types of instruments to choose from in order to have the one that best fits the desired purpose and site. The most important factors to consider in choosing the right one are i) method of measuring, for example, wading or from overhead structures, and ii) the approximate water speed, as not all of the meters are able to measure very slow flow, fast flow velocity changes, or very fast flow. So the operator of the current meter must be aware of both the lower and upper operating limits (Fulford et al. 1994).
Current meters must be calibrated in order to maintain their high accuracy. As mechanical devices their performance depends on the inertia of the rotor, friction in the bearings, and the ease with which the water turns the rotor. The meters are usually also battery powered to register the rotation, so maintenance to check the batteries is needed every now and then. The assemblage or different meters varies; some are relatively easy to maintain and clean, for example from sediment and fibre, but others require more complex disassembly for cleaning and oiling, others use water as lubricant (Fulford et al. 1994).
The resources and time needed for measuring the flow rate with impeller current meter depends strongly on the size of the channel. In smaller channels more modest instruments are necessary, and the measurements usually do not require more than two operators. However, because of the large variety in different available instruments, impeller current meter method is suitable in variety of sites and conditions and has been used for over a century already, so good amount of reference literature is available too. The advantages of the method are also rather easy maintenance, durability, and in most cases fast preparatory work (Mustonen 1986). Additionally, the impeller current meters are mostly capable of measuring unsteady flows, and they can measure a wide range of flow speeds (Hughes 1993). However, Fulford et al. (1994) noticed that the repeatability and responses were poorest at very low flow velocities. Other major disadvantages are long measuring times across whole cross-section, which causes errors in large channels that have unsteady flow, and poor accuracy in more difficult conditions, such as abundant vegetation or strong turbulence (Mustonen 1986, Hughes 1993, Shaw et al. 2011).
Box, S., Deatrick, J. & Lewis, B. 2012. Hydrological studies. SESD Operating Procedure, SESDPROC-501-R3, U.S. Environmental Protection Agency, 25 p.
Fulford, J.M., Thibodeaux, K.G. & Kaehrle, W.R. 1994. Comparison of current meters used for stream gaging. In: Clifford, A.P. (ed.) Fundamentals and Advancements in Hydraulic Measurements and Experimentation. American Society of Civil Engineers, New York, 376-385.
Hughes, S.A. 1993. Physical models and laboratory techniques in coastal engineering. Advanced Series on Ocean Engineering – Volume 7, World Scientific, Singapore, 588 p.
Mustonen, S. 1986. Sovellettu hydrologia. Vesiyhdistys r.y., Helsinki, 436 p. (In Finnish)
Shaw, E.M., Beven, K.J., Chappell, N.A. & Lamb, R. 2011. Hydrology in Practice. 4th edition, Spon Press, New York, 546 p.