Inputs
Use absolute or gauge as selected above.
Water at 20 °C ≈ 2.339 kPa.
Results
Dynamic pressure q = ½ ρ V²
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[Pa]
Cavitation index σ
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You entered gauge pressure. The calculator adds ambient pressure to convert to absolute.
Example data
Use a row to populate the inputs and recompute.
| Case | Fluid | ρ [kg/m³] | pinlet,abs [kPa] | pvap [kPa] | V [m/s] | σ |
|---|
Formula used
Definition: The cavitation index σ quantifies proximity to cavitation onset in flowing liquids:
σ = (pinlet,abs − pvap) / (½ ρ V²)
- pinlet,abs — absolute static pressure at the point of interest.
- pvap — fluid vapor pressure at operating temperature.
- ρ — fluid density; V — local flow velocity.
- ½ ρ V² is the dynamic pressure q.
Interpreting σ (rule‑of‑thumb):
- σ < 1 — high cavitation likelihood.
- 1 ≤ σ < 2 — moderate risk; assess carefully.
- 2 ≤ σ < 3 — lower risk for many valves/orifices.
- σ ≥ 3 — typically safe margin; verify for your hardware.
How to use
- Select your unit system and whether pressure is absolute or gauge.
- Enter inlet pressure, vapor pressure, density, and flow velocity.
- If gauge pressure is used, adjust ambient pressure as needed.
- Click Calculate to compute dynamic pressure and σ.
- Use Export CSV or Export PDF to save results.
- Click Share URL to copy a link with your inputs.
- Try the Load Example button or a case from the table.
FAQs
It is a dimensionless ratio comparing the margin between local static pressure and vapor pressure against dynamic pressure. Lower values indicate a greater tendency for cavitation to occur.
The formula requires absolute pressure. If you have gauge pressure, add ambient pressure to convert it to absolute before calculating.
Use reliable data for your fluid at the operating temperature. For water at 20 °C, vapor pressure is about 2.339 kPa. Many fluids have strong temperature dependence.
It depends on equipment. A common guideline is σ ≥ 2–3 reduces cavitation risk for many throttling devices, but pumps/turbines may require different criteria. Always follow vendor guidance.
Higher temperature generally increases vapor pressure and decreases density, both of which lower σ and increase cavitation risk for the same flow conditions.
Yes. The calculator converts lbm/ft³ (or slug/ft³) to SI internally before computing dynamic pressure and σ.
They are related but not identical. Thoma’s coefficient is widely used for turbines and uses head terms; this calculator uses the dynamic pressure form common in general fluid mechanics.
For most liquids at modest speeds, compressibility is negligible. At very high speeds or with flashing, advanced models beyond this simple index can be necessary.
For pumps, higher NPSHa relative to NPSHr increases the pressure margin to vapor pressure, effectively increasing σ and reducing cavitation tendency.
The index is a screening tool. Real cavitation depends on geometry, turbulence, dissolved gases, transients, and surface roughness. Validate against experiments or manufacturer limits.
Disclaimer: Engineering calculators are for educational and preliminary sizing checks. Verify results using standards, detailed analysis, and vendor data sheets.