Calculator Input
Example Data Table
This table is generated from the current calculator inputs.
| Step | Head (m) | Head (m) | Discharge (m³/s) | Discharge (L/s) | Discharge (ft³/s) |
|---|---|---|---|---|---|
| 1 | 0.030000 | 0.030000 | 0.00004627 | 0.0463 | 0.001634 |
| 2 | 0.060000 | 0.060000 | 0.00026170 | 0.2617 | 0.009243 |
| 3 | 0.090000 | 0.090000 | 0.00072109 | 0.7211 | 0.025465 |
| 4 | 0.120000 | 0.120000 | 0.00140660 | 1.4066 | 0.049675 |
| 5 | 0.150000 | 0.150000 | 0.00231118 | 2.3112 | 0.081617 |
About This Calculator
A V-notch weir, also called a triangular weir, is widely used for open-channel flow measurement. It is especially helpful when low flow rates must be measured with better sensitivity than a rectangular opening. Because the notch narrows toward the crest, even small increases in water head can produce clear discharge changes.
This calculator is designed for engineering estimation and reporting. It lets you enter the notch angle, head above crest, discharge coefficient, gravity, and preferred display system. It then calculates ideal discharge, corrected discharge, theoretical velocity, and head sensitivity. Additional outputs convert the result into liters per second, cubic meters per hour, cubic feet per second, and gallons per minute.
The example table expands the current case into a small performance set, which is useful for design checks, lab exercises, field calibration notes, and hydraulic comparison work. The graph makes the nonlinear relationship visible. Since flow varies with head raised to the 2.5 power, the plotted curve steepens quickly as head increases.
This page is practical for irrigation checks, wastewater channels, stormwater studies, educational demonstrations, and preliminary hydraulic sizing. It should not replace a formal field calibration or a standard-specific certification workflow. Use measured site conditions, verified coefficients, and proper crest geometry when accuracy is important.
Formula Used
Actual discharge: Q = Cd × (8/15) × √(2g) × tan(θ/2) × h5/2
Ideal discharge: Qideal = (8/15) × √(2g) × tan(θ/2) × h5/2
Theoretical velocity: v = √(2gh)
Head sensitivity: dQ/dh = 2.5 × Q / h
Meaning of Terms
- Q = discharge
- Cd = discharge coefficient
- g = gravitational acceleration
- θ = notch angle
- h = head above the notch crest
- v = theoretical approach velocity from head
How to Use This Calculator
- Select whether your entered head is in meters or feet.
- Enter the V-notch angle in degrees.
- Provide the measured head above the crest.
- Enter the discharge coefficient that matches your setup.
- Keep gravity at the default value unless your analysis needs another value.
- Choose how many rows you want in the generated example table.
- Press Calculate to place the results above the form.
- Review the graph, example table, and download the CSV or PDF file.
Frequently Asked Questions
1. What does this calculator measure?
It estimates flow through a triangular weir using head, notch angle, gravity, and discharge coefficient. It also returns velocity, sensitivity, converted flow units, a graph, and a generated example table.
2. Why is the flow curve nonlinear?
The discharge term includes head raised to the 2.5 power. That means flow grows faster as head increases, so the graph bends upward instead of forming a straight line.
3. What is the discharge coefficient?
Cd corrects the ideal equation for real flow behavior. It captures contraction, viscosity, edge condition, and installation effects. Use a value that matches your crest geometry and measurement setup.
4. Can I use feet for head input?
Yes. Choose the imperial option, then enter head in feet. The calculator internally converts it for computation and still reports multiple output units for easier engineering review.
5. What is sensitivity dQ/dh used for?
It shows how strongly discharge changes when head changes. This is useful for instrumentation planning, uncertainty discussion, and understanding how small reading errors affect the calculated flow.
6. Is this suitable for final certified design?
It is best for estimation, comparison, education, and preliminary engineering checks. Final design and compliance work should use validated coefficients, field calibration, and the governing project standard.
7. Why include both ideal and actual discharge?
Ideal discharge shows the theoretical value with no correction losses. Actual discharge multiplies that value by Cd, giving a more realistic engineering estimate for the installed weir condition.
8. What does the example table represent?
It is a generated performance table based on the current notch angle, Cd, gravity, and a head range. It helps compare discharge values across several head points quickly.