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Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-44
vegetation in the channel also affects the value of n, and in most applications the value changes depending
upon the depth of water in the channel.
Other uncertainties can also make the measurement inaccurate. Best results are obtained at the end of a
straight channel 1000 feet (300 meters) long. However, in reality it is difficult to find channels that are
very long, and very straight, and with constant slope, and with uniform roughness.
Even with all these uncertainties, the Manning equation can yield useful results as long as the user is aware
of its limitations.
EQUATION
The YSI sondes that are equipped with shallow vented level can also calculate flow based on an equation.
The equation must be of the form:
This allows the use of primary measuring devices other than the standard ones already programmed. Users
may enter values for K
1
, P
1
, K
2
, and P
2
and the sonde will calculate flow. Note: if you do not need the
second term in the equation, simply enter 0 for K
2
.
TABLE
YSI sondes that are equipped with shallow vented level can also calculate flow based on a table. Users can
enter up to 50 pairs of (level, flow) data. The sonde will then calculate flow from the resulting table,
linearly interpolating between points when necessary. This feature can be used with non-standard primary
measuring devices for which there is a table rather than an equation. This data is often available for rivers
and streams so that flow can be calculated from the level in a river at the proper location.
See Appendix F, Flow for additional practical information on the calculation of flow with a sonde.
2 1
P
2
P
1
H • K + H • K = Q