How to use the Ideal-Gas Vessel Pressure Calculator
- Reference gas volume specifies the fixed gas amount at the reference absolute pressure and temperature. Use kelvin and the final vessel’s internal volume.
- Enter reference gas volume, final vessel volume, absolute reference pressure, both temperatures in kelvin and local atmospheric pressure.
- Select Calculate. Review the result and its working; if you change an input, calculate again before copying or sharing.
Ideal-Gas Vessel Pressure formula
For a fixed ideal-gas amount, absolute pressure varies inversely with volume and directly with absolute temperature. Gauge pressure subtracts local atmosphere.
P₂(abs) = P₁(abs) × V₁/V₂ × T₂/T₁; P₂(gauge) = P₂(abs) − atmosphere
What the terms mean
- Gas volume at reference conditions
- Entered in ft³.
- Final vessel volume
- Entered in ft³.
- Reference absolute pressure
- Entered in psia.
- Reference temperature
- Entered in K.
- Final temperature
- Entered in K.
- Local atmospheric pressure
- Entered in psia.
Ideal-Gas Vessel Pressure worked examples
Estimate vessel pressure at unchanged temperature
A fixed gas amount occupies 10 ft³ at 14.6959 psia and 293.15 K. Its final volume is 1 ft³ at the same temperature. Use 14.6959 psia for local atmosphere.
- P₂ = P₁ × (V₁ ÷ V₂) × (T₂ ÷ T₁)
- Gauge pressure = absolute pressure − atmospheric pressure
Answer: 132.2631 psig. Absolute pressure: 146.959 psia.
Compare a smaller fixed gas amount
Gas occupies 2 ft³ at 14.6959 psia and 293.15 K. Reduce its volume to 1 ft³ at the same temperature, with local atmosphere at 14.6959 psia.
- P₂ = P₁ × (V₁ ÷ V₂) × (T₂ ÷ T₁)
- Gauge pressure = absolute pressure − atmospheric pressure
Answer: 14.6959 psig. Absolute pressure: 29.3918 psia.
Ideal-Gas Vessel Pressure Calculator FAQs
What is the difference between psia and psig?
Psia is absolute pressure. Psig is pressure above local atmosphere. The calculator subtracts the entered atmospheric pressure from the absolute result.
How do I convert Celsius to kelvin?
Add 273.15. For example, 20 °C is 293.15 K. Never put Celsius directly into the temperature ratio.
Measurement guide for Cubic Feet to PSI
Use absolute reference pressure and temperatures in kelvin. Include the full fixed gas amount, including any gas already present in the final vessel.
Assumptions and limits
Cubic feet cannot directly convert to PSI. The model neglects real-gas compressibility; gas already in the vessel must be included in the total gas amount.
Ideal-gas estimate for a fixed gas amount, using absolute pressures and kelvin. Assumes no gas remains unaccounted for; real gas compressibility is excluded.
Check your result
Compare absolute and gauge pressure: gauge pressure is absolute pressure minus the local atmosphere. Use absolute temperature in the temperature ratio.