Xylem 6-Series Multiparameter - Manual (Page 281)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-43
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5.18 FLOW
Flow is a calculated value. Whenever there is a one to one relationship between the level of water in an
open channel and the flow of water through it, then flow can be calculated from a level measurement. YSI
sondes that are equipped with shallow vented level can calculate flow based on several different methods.
Flow is only available in the Sensors menu on those sondes that have shallow vented level. The Flow
Setup menu only appears when Flow is enabled in the Sensors menu.
Note: In this manual we describe how to use our sondes to calculate flow from vented level. While weirs,
flumes and the Manning equation are described briefly in this manual, it is not a complete treatment of the
subject. We make no claims on the accuracy or appropriateness of any of these techniques for any
particular application.
WEIR AND FLUME
Many devices have been designed for the determination of flow in an open channel. For example, a weir is
a dam of specific geometry that restricts the flow of water while giving a very repeatable and accurate
relationship between level and flow. There are several varieties of weirs; each designed for a specific
application. Similarly, a flume also restricts flow producing a repeatable and accurate flow/level curve by
forcing the flow not over a dam, but through a narrower portion of the channel. The flow is gradually
narrowed, passed through a throat in the channel, and then gradually expanded back to the original channel
width. As with weirs, there are several varieties of flumes, each designed for a specific application.
The weir or flume is referred to as the primary measuring device, and the level meter is referred to as the
secondary measurement device. There are commonly 3 types of weirs and 7 types of flumes. Most of
these are available in a number of sizes. Flow/level curves for common types and sizes are already
programmed in the sonde so that it is only necessary to describe the primary measuring device to get flow
readings. If you have a primary measuring device that is not already programmed, you have the option of
entering either an equation or a table that defines the flow/level curve for your device. The table can also
be used to calculate the flow of water in stream for which the flow/level data is available.
MANNING EQUATION
YSI sondes with shallow vented level can be used with the Manning equation. In an open channel without
any restriction built explicitly for measuring flow, the Manning Equation can sometimes be used to
calculate flow. Under the right conditions the channel itself is the primary measurement device and flow
can be calculated from the level of water in the channel. Careful use of the Manning equation under ideal
conditions can be accurate to 10%. Less careful use under worse conditions can give errors of 50% or
more. The formula is:
Where: Q = Flow rate
A = Cross sectional area of flow
R = Hydraulic radius
S = slope
n = Manning coefficient of roughness
K = constant dependent on units
The Manning roughness coefficient n is an index of the frictional resistance to flow on the surface of the
channel. Values of n are published for different materials. However, in the field, determination of n is
perhaps the largest source of error. For example, n for a concrete channel can vary from 0.011 to 0.020
depending upon how the surface of the concrete was finished during construction. Occasional debris or
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S • R • A • K = Q