Measuring Flow
The Most Accurate Method for Measuring Flow
A Weir is perhaps the most accurate way to
measure small and medium sized streams. All the water is
directed through an area that is exactly rectangular, making it
very easy to measure the height and width of the water to
compute FLOW.
A Weir is a temporary dam with a
rectangular slot, or Gate. The bottom of the Gate should
be exactly level, and the width of the Gate should allow all the
water to pass through without spilling over the top of the dam.
A narrower Gate will increase the depth of the water as it
passes through, making it easier to measure.
It’s important to note that your depth
measurement is not taken at the gate itself because the water
depth distorts as it moves through the gate. Instead,
insert a stake well upstream of the Weir gate and make the top
of the stake exactly level with the bottom of the Wier gate.
Measure the depth of the water from the top of the stake.
Once the width and depth of the water are
known, a Weir Table is used to compute the flow. The Weir
Table shown below is based on a Gate one inch wide; you simply
multiply the table amount by the width (in inches) of your Gate.
For example, let’s assume your Weir Gate is 6” wide, and the
depth of the water passing over it is 7-1/2 inches. On the
left side of the table, find “7” and move across the row until
you find the column for “+1/2”. The table shows 8.21 cfm
flow for a one-inch Gate with 7-1/2” of water flowing through
it. Since your gate is 6” wide, simply multiply the 8.21
by 6 to get 49.26 cfm.
Inches
|
|
+0/8
|
+1/8
|
+1/4
|
+3/8
|
+1/2
|
+5/8
|
+3/4
|
+7/8
|
0
|
0.00
|
0.01
|
0.05
|
0.09
|
0.14
|
0.19
|
0.26
|
0.32
|
1
|
0.40
|
0.47
|
0.55
|
0.64
|
0.73
|
0.82
|
0.92
|
1.02
|
2
|
1.13
|
1.23
|
1.35
|
1.46
|
1.58
|
1.70
|
1.82
|
1.95
|
3
|
2.07
|
2.21
|
2.34
|
2.48
|
2.61
|
2.76
|
2.90
|
3.05
|
4
|
3.20
|
3.35
|
3.50
|
3.66
|
3.81
|
3.97
|
4.14
|
4.30
|
5
|
4.47
|
4.64
|
4.81
|
4.98
|
5.15
|
5.33
|
5.51
|
5.69
|
6
|
5.87
|
6.06
|
6.25
|
6.44
|
6.62
|
6.82
|
7.01
|
7.21
|
7
|
7.40
|
7.60
|
7.80
|
8.01
|
8.21
|
8.42
|
8.63
|
8.83
|
8
|
9.05
|
9.26
|
9.47
|
9.69
|
9.91
|
10.13
|
10.35
|
10.57
|
9
|
10.80
|
11.02
|
11.25
|
11.48
|
11.71
|
11.94
|
12.17
|
12.41
|
10
|
12.64
|
12.88
|
13.12
|
13.36
|
13.6
|
13.85
|
14.09
|
14.34
|
11
|
14.59
|
14.84
|
15.09
|
15.34
|
15.59
|
15.85
|
16.11
|
16.36
|
12
|
16.62
|
16.88
|
17.15
|
17.41
|
17.67
|
17.94
|
18.21
|
18.47
|
13
|
18.74
|
19.01
|
19.29
|
19.56
|
19.84
|
20.11
|
20.39
|
20.67
|
14
|
20.95
|
21.23
|
21.51
|
21.80
|
22.08
|
22.37
|
22.65
|
22.94
|
15
|
23.23
|
23.52
|
23.82
|
24.11
|
24.40
|
24.70
|
25.00
|
25.30
|
16
|
25.60
|
25.90
|
26.20
|
26.50
|
26.80
|
27.11
|
27.42
|
27.72
|
17
|
28.03
|
28.34
|
28.65
|
28.97
|
29.28
|
29.59
|
29.91
|
30.22
|
18
|
30.54
|
30.86
|
31.18
|
31.50
|
31.82
|
32.15
|
32.47
|
32.80
|
19
|
33.12
|
33.45
|
33.78
|
34.11
|
34.44
|
34.77
|
35.10
|
35.44
|
20
|
35.77
|
36.11
|
36.45
|
36.78
|
37.12
|
37.46
|
37.80
|
38.15
|
A Weir is especially effective for
measuring FLOW during different times of the year. Once
the Weir is in place, it is easy to quickly measure the depth of
the water and chart FLOW at various points in time.
Even though your Flow may be very high
after exceptionally rainy periods, it probably won’t be cost
effective to design your turbine system to handle all that water
for just a few days of the year. Instead, it makes sense
to build a system that uses Flow you can count on for much of
the year. This is called Design Flow, and it is the
maximum Flow your hydro system is designed to accommodate.
Design Flow, along with Net Head,
determines everything about your hydro system, from pipeline
size to power output.
< BACK |
NEXT >