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Measuring spillways

Measuring spillways are used when measuring the instantaneous flow on the basis of the Q/h curve - consumption equation (dependence between the instantaneous flow and the level before the specific overflow).

Measuring spillways with a triangular cross-section

Sharp-edged triangular measuring spillways are often used in practice for their simplicity and low acquisition costs. They are usually made of stainless steel and their edge should not be wider than 1-2 mm in order to clearly define the location of the measuring spillways beam.

Measuring spillways of this type are used mainly in localities with a large range of flows. With a linearly increasing spillway height, the flow area increases quadratically. The measuring spillway is therefore increasingly sensitive to the correct setting of the measured level. The setting error is manifested when calculating the flow with the power n = 2.5. When accurately measuring spillway level, the triangular measuring spillway is one of the most accurate flow meters.

Consumption equations

The consumption equation of the triangular measuring spillway has the form:

Q=A1*h^A2[m³/s; m]

The equation expresses the functional dependence between the instantaneous flow and the level before the measuring spillway.

Consumptive equation is valid provided that: 0,05 < h < 0,55 m; S > 3h; B > 8h

Other triangular measuring spillways

Measuring spillways with an angle of less than 90 ° are used for accurate measurement of smaller flows

	MTP90	MTP60	MTP45	MTP30	MTP20
Qmax [l/s]	240	139	100	65	42,6
Angle α	90°	60°	45°	30°	20°
A1 [-]	1,3310	0,7701	0,5533	0,3585	0,2363
A2 [-]	2,47	2,47	2,47	2,47	2,47

Legend to the picture and to the table:

US1 Ultrasonic level sensor entering the ACH: Level channel

Qmax Maximum instantaneous flow for H = 0,5 m [l/s]

h Measured level height from the top of the triangle [mm]

A1,A2 Preset coefficients of the consumption equation [-]

Triangular measuring spillways with a different angle

Consumption equation for a triangular measuring spillway with any apex angle α in the range from 20 ° to 120 ° (according to Grave):

Q= 1.331 * (tg α/2)^0.996 * h^2.47[m³/s; m]

In FIEDLER AMS flowmeters, the coefficients A1 and A2 are already predefined for the Thomson measuring spillways with a 90 ° apex angle. For non-standard vertex angles it is necessary to set the coefficients A1 and A2.

Trapezoidal Cipoletti measuring spillway

One of the frequently used sharp-edged spillway for monitoring flow in open streams of smaller streams and river tributaries is the trapezoidal Cipoletti specific spillway with 1: 4 side walls.