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Parshall Flume Accuracy

Under controlled laboratory conditions, Parshall flumes have been observed to be accurate to within +/-2%. However, practical considerations such as approach flow, installation, and dimensional tolerances make it free-flow discharge precision to be +/-5% per ASTM D1941.  

Parshall flume installations where the upstream / downstream / installation conditions are less than optimal or where the flume is out of dimension will exhibit accuracies less than above and may require field calibration.

For Parshall flume that have settled (or been installed at a slope) corrections have been developed and can be applied.   

The above holds true for Parshall flumes experiencing free-flow.  For installations where downstream conditions restrict the flow out of the flume, submergence may become a factor.  Submerged flumes should not use free-flow discharge equations, as they will overindicate the flow rate.  

Parshall Flow Equations

For free-flow conditions, the level-to-flow equation for the Parshall flume can be expressed as:

Free flow discharge equation for Parshall flume

Per ASTM D1941:

Throat Width

K (Imperial)

K (SI)

n

1”

0.338

0.0479

1.55

2”

0.676

0.0959

1.55

3”

0.992

0.141

1.55

6”

2.06

0.264

1.58

9”

3.07

0.393

1.53

1’

4

0.624

1.522

1’-6”

6

0.887

1.538

2’

8

1.135

1.55

3’

12

1.612

1.566

4’

16

2.062

1.578

5’

20

2.5

1.587

6’

24

2.919

1.595

7’

28

3.337

1.601

8’

32

3.736

1.607

10’

39.38

4.709

1.6

12’

46.75

5.590

1.6

19’

57.81

6.912

1.6

20’

76.25

9.117

1.6

25’

94.69

11.32

1.6

30’

113.13

13.53

1.6

40’

150

17.94

1.6

50’

186.88

22.35

1.6

Parshall Flume Flow Tables 

Free-flow discharge tables for Parshall flumes can be found in the Discharge Tables section.