Calculator Inputs
Use absolute pressures whenever possible. Gauge mode adds one standard atmosphere before computing the choked flow condition.
Plotly Graph
This graph shows how the critical pressure ratio changes with the specific heat ratio γ. Your selected gas point is highlighted after calculation.
Formula Used
Critical pressure ratio
(P₂ / P₁)critical = (2 / (γ + 1))γ / (γ - 1)
Critical downstream pressure
P₂,critical = P₁ × (P₂ / P₁)critical
Choked condition test
Flow is choked when P₂ / P₁ ≤ (P₂ / P₁)critical
These relations come from one-dimensional, isentropic compressible flow theory for gases. They are commonly used for nozzles, orifices, regulators, and valves when determining whether velocity has reached the sonic limit at the throat.
The critical ratio depends on γ only. Temperature affects mass flow and sonic speed, but it does not change the ideal critical pressure ratio itself.
How to Use This Calculator
- Select a gas preset or choose Custom.
- Enter the specific heat ratio γ if using Custom.
- Choose your pressure unit and whether values are absolute or gauge.
- Enter upstream pressure P₁ and downstream pressure P₂.
- Pick the number of decimal places you want.
- Click the calculate button.
- Read the result block above the form for choking status, critical ratio, and pressure limits.
- Use the CSV or PDF buttons to export the result summary.
Example Data Table
The sample below assumes an upstream pressure of 600 kPa absolute and uses typical γ values for each gas.
| Gas | γ | Upstream Pressure P₁ (kPa abs) | Critical Ratio P₂/P₁ | Critical Downstream Pressure P₂ (kPa abs) |
|---|---|---|---|---|
| Air | 1.4000 | 600.00 | 0.5283 | 316.97 |
| Steam | 1.3300 | 600.00 | 0.5404 | 324.22 |
| Carbon Dioxide | 1.2890 | 600.00 | 0.5477 | 328.63 |
| Helium | 1.6600 | 600.00 | 0.4881 | 292.85 |
Frequently Asked Questions
1. What does choked flow mean?
Choked flow means the gas reaches sonic velocity at the controlling section, usually a throat or orifice. Lowering downstream pressure further does not increase local velocity there.
2. Why does γ affect the critical pressure ratio?
γ changes how pressure, density, and temperature interact during isentropic expansion. Different gases therefore reach the sonic condition at different downstream-to-upstream pressure ratios.
3. Can I use gauge pressure inputs?
Yes. This calculator can accept gauge values. It converts them to absolute pressure using one standard atmosphere before evaluating the choking condition.
4. Does temperature change the critical pressure ratio?
For the ideal isentropic relation used here, the critical pressure ratio depends only on γ. Temperature still matters for mass flow rate, density, and sonic velocity.
5. Is this calculator valid for liquids?
No. This tool is for compressible gas flow. Liquid choking and cavitation require different models based on vapor pressure, density, and two-phase effects.
6. Why might the critical gauge pressure be negative?
If the critical downstream absolute pressure falls below atmospheric pressure, its equivalent gauge value becomes negative. That is physically possible in vacuum-related systems.
7. What if downstream pressure is higher than upstream pressure?
Then the actual ratio exceeds one, the flow is not choked, and the pressure difference does not support normal forward expansion through the restriction.
8. When should I use the pressure margin output?
Use the pressure margin to see how far the current downstream pressure is from the critical limit. It helps during design reviews, troubleshooting, and operating envelope checks.