Enter Venturi Parameters
Formula Used
This calculator uses the standard Venturi meter relation for volumetric flow rate:
- Cd accounts for real-flow losses and calibration.
- Y is the expansibility factor (useful for gases).
- ΔP is the measured pressure drop between taps.
- ρ is the fluid density at flowing conditions.
How to Use This Calculator
- Enter the upstream diameter d₁ and throat diameter d₂.
- Provide the measured differential pressure ΔP and select units.
- Enter fluid density ρ and a realistic discharge coefficient Cd.
- If the fluid is a gas, enable Y and set a value ≤ 1.
- Optionally enable viscosity to estimate throat Reynolds number.
- Press Calculate to view results above the form.
- Use Download CSV or Download PDF to export.
Example Data Table
| Case | d₁ | d₂ | ΔP | ρ | Cd | Approx. Q |
|---|---|---|---|---|---|---|
| Water test | 50 mm | 25 mm | 10 kPa | 998 kg/m³ | 0.98 | ≈ 3.7 L/s |
| Higher ΔP | 50 mm | 25 mm | 20 kPa | 998 kg/m³ | 0.98 | ≈ 5.2 L/s |
| Larger throat | 80 mm | 40 mm | 12 kPa | 1000 kg/m³ | 0.98 | ≈ 12.0 L/s |
Venturi Flow Rate Guide
1) Why Venturi meters are used
A Venturi meter turns a pressure measurement into a reliable flow estimate with low permanent pressure loss. The converging section accelerates the fluid, the throat fixes the minimum area, and the diffuser recovers pressure. This makes Venturi devices popular in water systems, HVAC hydronics, and process lines where long‑term stability matters.
2) What differential pressure tells you
The measured differential pressure (ΔP) between the upstream section and the throat reflects the change in velocity head predicted by Bernoulli’s principle. Because ΔP scales approximately with the square of flow, doubling the flow roughly quadruples ΔP. In many practical installations, ΔP values from a few kPa up to a few tens of kPa provide a good signal while limiting losses.
3) The role of diameter ratio β
The diameter ratio β = d₂/d₁ controls sensitivity. A smaller throat (lower β) creates a larger pressure drop for the same flow, improving measurement resolution but increasing permanent loss and noise sensitivity. The term (1 − β⁴) appears in the formula, so β changes can strongly affect Q. Common designs often use β between about 0.30 and 0.75 depending on allowable ΔP.
4) Discharge coefficient and calibration
The discharge coefficient Cd corrects the ideal equation for real effects such as boundary layers and slight non‑uniform velocity profiles. Venturi meters typically have Cd around 0.97–0.99, but it can shift with manufacturing tolerances, tap geometry, and surface condition. For high accuracy work, use a calibrated Cd or a standard‑based value for your meter type.
5) Choosing density and temperature data
Density ρ should match operating conditions. For water, ρ is about 998 kg/m³ near 20°C and drops toward roughly 971 kg/m³ near 80°C, which can noticeably change mass flow results. For oils, brines, or refrigerants, use datasheet or process values. When reporting mass flow, density is the dominant fluid‑property input.
6) Compressibility and expansibility factor Y
Liquids are commonly treated as incompressible, so Y ≈ 1. Gases expand through the throat, so the effective flow is slightly lower than an incompressible prediction. The expansibility factor Y (often around 0.95–1.00 for modest ΔP) accounts for this. If you have a standard or manufacturer method for Y, enter it here to improve gas‑flow estimates.
7) Reynolds number and viscosity checks
Viscosity affects the flow profile and can influence Cd, especially at low Reynolds number. This calculator can estimate throat Reynolds number using μ so you can sanity‑check conditions. Many Venturi applications operate at high Re (often > 2×105), where Cd is more stable. If Re is low, consider calibration or a different meter.
8) Practical installation and uncertainty
Installation strongly impacts repeatability. Use adequate straight pipe runs (commonly 5–10 diameters upstream and several diameters downstream), avoid nearby elbows or pumps, and ensure pressure taps are clean and correctly located. With good practice and a known Cd, Venturi systems can achieve around 1–2% flow uncertainty; poor installation can be significantly worse.
FAQs
1) What discharge coefficient should I use?
Use a meter‑specific calibrated value when possible. For many Venturi meters, Cd is commonly around 0.97–0.99. If you are unsure, start near 0.98 and refine using reference measurements.
2) Can I use this calculator for gas flow?
Yes. Enable the expansibility factor (Y) and enter an appropriate value (≤ 1). For modest pressure drops, Y is often close to 1, but it can be lower when ΔP is large or the gas is at low pressure.
3) Why must the throat diameter be smaller than upstream?
The Venturi principle relies on a reduced area that increases velocity and creates a measurable pressure drop. If d₂ is not smaller than d₁, the device does not create the required acceleration and the equation becomes invalid.
4) Does viscosity change the flow rate result?
Viscosity does not appear directly in the ideal equation, but it can influence Cd at low Reynolds number. The viscosity option helps you estimate Reynolds number so you can judge whether a constant Cd assumption is reasonable.
5) What if my differential pressure is in water column units?
Select mmH₂O or inH₂O in the ΔP unit list and enter your reading directly. The calculator converts it to pascals internally before computing Q, so your result remains consistent across unit choices.
6) Why is the flow very sensitive to ΔP changes?
Flow depends on the square root of ΔP. Small ΔP changes can still matter, especially at low pressure drops where sensor resolution and noise become significant. Use stable instrumentation and ensure taps and impulse lines are free of blockage.
7) How do I improve accuracy in the field?
Use a calibrated Cd, verify density at operating temperature, keep taps clean, and provide sufficient straight pipe runs upstream. If possible, compare against a reference flow measurement to confirm performance after installation.