Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 73

− Measuring the stream velocity. The flow rate is defined as the integral of the
stream velocity across the cross-section of the stream. If this cross-section is
known, a horizontal arranged propeller driven by the current measures the flow
speed. The measured values allow calculating the through-flow. Also magnetic
induction gauges or acoustic Doppler can be used for the same purpose. If this
is done at different flow levels, the location-specific water level-flow ratio can
be derived as well.
− Measuring the water level. If the relation between the water level and the flow
(e.g. from measurements, hydraulic calculations or sample experiments) is
available, measuring the water level with slat, floating or other level gauges is
sufficient. The respective flow can then be derived from the flow curve. Be-
cause of its simplicity, this method of measuring the water level is most suit-
able for the automatic recording of level data and flows over many years.
− Measuring tracer concentration. For this method, salts or dyes are added to the
river upstream. Downstream, the respective concentration of the added sub-
stances is measured. Assuming that the concentration of the salts or the dye
remains almost constant throughout the entire cross-section of the stream or
river and that the flow is stationary, the flow can be calculated on the basis of a
salt or dye balance.
Flow measurement. Measuring the through-flow in pipelines is mainly done with
a built-in gauge. The method is normally much simpler compared to measuring
the flow of running waters. The following methods are used.
− The pressure difference before and behind a cross-section change within a
pressure pipeline can be measured relatively easily thanks to technical facili-
ties. It correlates directly with a change of the flow speed. If the diameter of the
pipe is known, the through-flow can be calculated (Venturi pipe).
− The flow resistance of a moving body can also be measured relatively easily. If
a body is included in a vertical pipeline transporting e.g. water, a balance be-
tween the static weight and the sum of static and dynamic buoyancy will set up.
Thus if the weight of the body is known, the through-flow can be identified. As
this measurement method leads to hydraulic losses in the pipeline, it is hardly
applied nowadays.
− When moving an electrical conductor vertically to the power lines of a mag-
netic field, voltage is induced in the conductor that can be measured. This
change in voltage is proportional to the movement of the electrical conductor.
Ions in the water function as such conductors. If the pipe diameter and the
magnetic inductively are known, the through-flow is calculated on the basis of
the measured voltage.
− The flow through a pipeline can also be measured, with or against the flow
direction, using ultrasound. This is due to the varying sound speeds of water in
relation to the pipeline.