Teledyne 4250 Installation And Operation Manual page 34

4250 Flow Meter
Section 2 Programming
2-6
channel, and the velocity of the flow stream. Occasionally, the
4250 may be used with a primary measuring device. In such
cases, the area-velocity sensor will function as a level-only mea-
suring device, with velocity as a secondary indicator.
A primary measuring device is a structure placed across a flow
stream through which the entire stream must flow. These devices
are made in a number of styles and sizes, but they all have one
thing in common: For any type of primary measuring device
there is a known relationship between the level in the flow
stream ahead of the device and flow rate through the device.
Consequently, after you measure level with the flow meter, it can
calculate flow rate and total flow from the measured level, by
consulting built-in look-up tables.
Detailed information about many commonly-used primary mea-
suring devices is provided in the Isco Open Channel Flow Mea-
surement Handbook. This book provides formulas, flow rates at
various levels, and values for maximum head, as well as much
interesting descriptive material, and is available from Teledyne
Isco. If your installation uses a nonstandard primary device, you
should consult the manufacturer of the device for flow rates at
given levels. The flow meter will then calculate a flow conversion
for such a device on the basis of the manufacturers' data you
enter as data points or an equation. In some instances, a non-
standard primary device could be supplied with a flow equation;
you can enter that equation into the flow meter and the flow
meter will calculate the flow rate from that equation.
Area-velocity flow conversion – is the customary flow con-
version used with the 4250. The area-velocity sensor measures
both the level of the flow stream and the velocity at which the
liquid is moving. It uses ultrasonic reflection to measure the
velocity and a pressure transducer to measure the level. you
provide the third dimension of the flow cross-section by entering
the width or diameter of the pipe or channel. The 4250 uses this
combination of measured level, velocity, and the dimension of the
channel to calculate flow. This method is unique because it is the
only flow conversion that can measure all of these different types
of flow: submerged, full pipe, surcharged, and reverse.
Standard (non-area-velocity) Flow Conversion – The 4250
normally calculates flow based on the velocity of the flow stream,
the cross-sectional shape of the channel, and the measured level.
However, you can also configure the 4250 for level-to-flow con-
version rather than area-velocity. The conversion types are:
Weir/Flume, Manning, Data Points, and Equation.
You use Weir/Flume flow conversion when your primary mea-
suring device is a weir or a flume. A weir is a wall or dam across
the flow stream. Water must rise to the point where it flows over
the top of the wall. The measured level upstream behind the wall
is used to calculate the flow rate. Flumes differ from weirs in that
there is no wall or barrier, but instead a restriction, typically a
sharp narrowing or change in the slope of the channel that
restricts the flow. Again, the measured level of the stream at
some point ahead of the restriction is used by the flow meter to