CO2 Equilibrium Pressure Calculator at Selected Temperatures
Calculate saturation pressure of carbon dioxide for one or many temperatures using a high-accuracy vapor pressure correlation near the liquid-vapor region.
Model validity for liquid-vapor equilibrium: 216.592 K to 304.128 K (about -56.56°C to 30.98°C). Outside this range, this specific model is not valid.
Expert Guide: How to Calculate the Equilibrium Pressure of CO2 at Different Temperatures
Calculating the equilibrium pressure of carbon dioxide at a specified temperature is essential in refrigeration, beverage packaging, supercritical extraction, fire suppression systems, and carbon capture transport design. The concept is straightforward, but the engineering details matter. At equilibrium, pressure and temperature are linked by thermodynamics, so if you know one in a closed system that contains both liquid and vapor CO2, the other is fixed.
This page gives you a practical calculator and a technical framework so you can produce defensible numbers quickly. You can enter one temperature or a list of temperatures and generate pressure values and a trend chart. This is especially useful for pressure vessel checks, compressor suction and discharge planning, cylinder logistics, and process safety studies where temperature swings change pressure dramatically.
What “equilibrium pressure” means for carbon dioxide
For a pure substance like CO2, equilibrium pressure is usually the saturation pressure at a given temperature for phase coexistence. In simple terms, it is the pressure at which liquid CO2 and CO2 vapor can exist together without net evaporation or condensation. If temperature increases, equilibrium pressure increases strongly. This is why a CO2 cylinder left in a warm environment can see much higher internal pressure.
Two landmarks are very important:
- Triple point: 216.592 K and about 5.18 bar, where solid, liquid, and vapor can coexist.
- Critical point: 304.128 K and about 73.77 bar, above which no distinct liquid and vapor boundary exists.
The calculator here targets the liquid-vapor saturation domain between those points. If you go below the triple point, you need sublimation pressure relations for solid-vapor equilibrium, which are different.
Why temperature has such a strong effect on CO2 pressure
CO2 has a relatively low critical temperature near 31°C. That means common ambient conditions can be close to the critical region, where pressure rises quickly with temperature. In engineering practice, this creates design and operational constraints:
- Storage and transport pressure can vary widely with weather.
- Pressure relief devices must be selected with realistic hot-day scenarios.
- Piping class and seal selection must account for peak saturation pressure, not just average conditions.
- Instrumentation ranges should avoid clipping near expected maximum pressure.
A rough memory rule is useful: between about -20°C and +30°C, the saturation pressure of CO2 more than triples. That is large enough to affect almost every practical design decision.
Reference equilibrium pressure values for CO2
The table below shows representative saturation pressures for carbon dioxide in the liquid-vapor region. Values are rounded and consistent with common high-accuracy correlations and standard references used in thermophysical property databases.
| Temperature (°C) | Temperature (K) | Equilibrium Pressure (bar) | Equilibrium Pressure (psi) |
|---|---|---|---|
| -56.6 | 216.6 | 5.18 | 75.1 |
| -40 | 233.15 | 10.0 | 145.0 |
| -20 | 253.15 | 19.7 | 285.7 |
| 0 | 273.15 | 34.9 | 506.2 |
| 10 | 283.15 | 45.0 | 652.7 |
| 20 | 293.15 | 57.3 | 831.1 |
| 25 | 298.15 | 64.4 | 933.9 |
| 30 | 303.15 | 72.9 | 1057.3 |
Pressure growth comparison across typical operating windows
The next table helps compare how quickly pressure changes across common ranges. This is useful for selecting pressure ratings and setting alarm thresholds.
| Range | Start Pressure (bar) | End Pressure (bar) | Absolute Increase (bar) | Relative Increase (%) |
|---|---|---|---|---|
| -40°C to -20°C | 10.0 | 19.7 | 9.7 | 97% |
| -20°C to 0°C | 19.7 | 34.9 | 15.2 | 77% |
| 0°C to 20°C | 34.9 | 57.3 | 22.4 | 64% |
| 20°C to 30°C | 57.3 | 72.9 | 15.6 | 27% |
Even without complex thermodynamics, these numbers show why a fixed pressure assumption can be risky. A system that is acceptable at 0°C can approach much higher pressure near 30°C, especially in sun-exposed or poorly ventilated areas.
Calculation method used in the calculator
The calculator uses a Wagner type reduced-pressure equation for CO2 saturation pressure in the liquid-vapor region:
ln(P/Pc) = (Tc/T) x [a1τ + a2τ^1.5 + a3τ^2 + a4τ^4], where τ = 1 – T/Tc.
With CO2 constants:
- Tc = 304.1282 K
- Pc = 7.3773 MPa
- a1 = -7.0602087
- a2 = 1.9391218
- a3 = -1.6463597
- a4 = -3.2995634
This form is widely used in engineering thermodynamics for accurate saturation pressure estimation in the valid temperature interval. The script converts user input to Kelvin, computes pressure in MPa, then converts to your selected output unit.
Step by step workflow
- Enter one temperature or multiple temperatures separated by commas or new lines.
- Choose the temperature unit.
- Select output pressure unit (bar, MPa, kPa, or psi).
- Click Calculate Equilibrium Pressure.
- Review the numeric table and plotted curve.
- Check warning labels for out-of-range temperatures.
If you are evaluating many cases, list all expected operating temperatures in one run. This creates a quick pressure envelope for design review.
Where to find authoritative property sources
For audited engineering work, always cross-check with recognized references. Recommended sources include:
- NIST Chemistry WebBook phase data for carbon dioxide
- NIST REFPROP program information
- U.S. Department of Energy carbon management resources
These sources support better traceability than ad hoc online charts and provide technical confidence for design, procurement, and safety documentation.
Common mistakes and how to avoid them
- Mixing gauge and absolute pressure: thermodynamic equations use absolute pressure.
- Using the wrong phase relation: below the triple point, sublimation equations are needed.
- Applying one equation outside its validity range: always verify temperature domain.
- Ignoring unit conversions: bar, MPa, kPa, and psi can lead to large reporting errors.
- Assuming linear behavior: CO2 pressure response to temperature is nonlinear.
Design and safety implications
Equilibrium pressure calculations directly affect pressure vessel code compliance, relief valve sizing, compressor map selection, and maintenance intervals. For cylinders and bulk tanks, fill level and thermal exposure can increase risk if pressure margin is too small. For process plants, a conservative pressure envelope based on highest credible temperature is a practical safety baseline.
In operations, it is good practice to pair pressure trending with local temperature logging so abnormal behavior can be interpreted quickly. If pressure is high but consistent with thermal conditions, action differs from pressure rise caused by blocked flow, contamination, or control malfunction.
Advanced practical notes for engineers
Near the critical point, fluid properties become very sensitive and small temperature uncertainties can create larger pressure uncertainty. If your operating window approaches 31°C and 73.8 bar, use high-accuracy EOS tools and property packages in process simulators. For pipelines and dense phase transport, include composition effects if impurities are present because pure CO2 correlations may underpredict or overpredict actual behavior.
If you work in food and beverage systems, keep in mind that dissolved CO2 in liquid products and headspace dynamics involve additional equilibrium relationships beyond pure-component saturation pressure. The current calculator is for pure CO2 saturation pressure, which is the right starting point for storage and mechanical design but not the full carbonation mass-transfer model.
Bottom line
To calculate equilibrium pressure of CO2 at specific temperatures, use a validated thermodynamic correlation, stay inside its valid range, keep units consistent, and review pressure trends across your full thermal envelope. The calculator above gives a fast and technically sound estimate for liquid-vapor CO2 saturation pressure between the triple and critical points, with tabular and visual output that can support both quick checks and formal engineering workflows.