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SimuPipe

Gas Properties Table

Molar mass, specific gas constant, specific-heat ratio, specific gravity, density at normal (0 °C) and standard (15 °C) conditions, viscosity, and speed of sound for 23 gases — plus the critical constants behind real-gas (Peng-Robinson) calculations. The same data SimuPipe's compressible solvers use.

Gas properties (ideal-gas densities at 1 atm)
GasFormulaM (g/mol)R (J/kg·K)γ (Cₚ/Cᵥ)SG (air = 1)ρ at 0 °C (kg/m³)ρ at 15 °C (kg/m³)μ at 20 °C (µPa·s)a at 20 °C (m/s)
Airmixture28.96287.11.4021.0001.2921.22518.21343.5
NitrogenN₂28.01296.81.4010.9671.251.18517.58349.2
OxygenO₂32259.81.3971.1051.4281.35320.28326.2
HydrogenH₂2.01641241.4060.0700.089940.085268.7981304
HeliumHe4.00320771.6670.1380.17860.169319.621007
ArgonAr39.95208.11.6701.3791.7821.6922.31319.2
Carbon DioxideCO₂44.01188.91.2971.5191.9641.86114.68268
Carbon MonoxideCO28.01296.81.4010.9671.251.18517.53349.2
MethaneCH₄16.04518.31.3080.5540.71580.678511.04445.8
Natural Gasmixture17.38478.41.3000.6000.77540.73511427
EthaneC₂H₆30.07276.51.1971.0381.3421.2729.208311.5
PropaneC₃H₈44.1188.61.1391.5221.9671.8658.011250.9
ButaneC₄H₁₀58.12143.11.1082.0072.5932.4587.279215.5
AmmoniaNH₃17.03488.21.3190.5880.75980.72039.91434.4
Hydrogen SulfideH₂S34.082441.3321.1771.5211.44111.87308.7
Sulfur DioxideSO₂64.06129.81.2842.2122.8582.70912.7221
EthyleneC₂H₄28.05296.41.2500.9691.2521.18610.3329.5
NeonNe20.184121.6670.6970.90030.853431.21448.7
SteamH₂O18.02461.51.3370.622——12.42*479.8*
R-134aC₂H₂F₄10281.491.1223.5234.5524.31511.62163.7
R-32CH₂F₂52.02159.81.2561.7962.3212.213.14242.6
R-125C₂HF₅12069.281.1054.1445.3555.07612.75149.8
R-23CHF₃70.01118.81.2042.4173.1242.96114.67204.7

Densities are ideal-gas values ρ = PM/RᵤT at 101.325 kPa absolute — "normal" (Nm³ basis) at 0 °C, "standard" (Sm³ basis) at 15 °C. Real-gas deviation is under ~1% at these conditions except near condensation. * Steam is quoted at 100 °C (it condenses at ambient temperature, so the 0/15 °C columns do not apply). Natural gas is a fixed pipeline-quality pseudo-composition (SG 0.60), not a variable mixture. Derived columns (R, SG, ρ, a) are computed from unrounded values, so recomputing them from the rounded M or γ shown may differ in the last digit. 1 µPa·s = 10⁻⁶ Pa·s.

Critical constants (Peng-Robinson EOS inputs)
GasTc (K)Tc (°C)Pc (bar abs)Acentric factor ω
Carbon Dioxide304.1330.9873.770.2239
Methane190.56-82.5945.990.0114
Ethane305.3232.1748.720.0995
Propane369.8996.7442.510.1523
Butane425.13151.9837.960.2002
Ammonia405.40132.25113.330.2560
Hydrogen Sulfide373.1099.9590.000.0942
Sulfur Dioxide430.64157.4978.840.2560
Ethylene282.359.2050.420.0862
Steam647.10373.95220.640.3443
R-134a374.21101.0640.590.3268
R-32351.2678.1157.820.2769
R-125339.1766.0236.180.3052
R-23299.2926.1448.320.2634

Above its critical temperature a gas cannot be liquefied by pressure alone. These constants feed the Peng-Robinson equation of state, from which SimuPipe computes the compressibility factor Z for non-ideal gases. Near-ideal gases (air, N₂, O₂, H₂, He, Ar, Ne, CO) are treated with Z = 1 and are not listed.

Definitions & Key Relations

Gas density follows from the ideal-gas law in mass terms:

ρ=P MRu T=PR T\rho = \frac{P\,M}{R_u\,T} = \frac{P}{R\,T}
  • ρ\rho — density (kg/m³), PP — absolute pressure (Pa)
  • MM — molar mass (kg/kmol = g/mol), TT — absolute temperature (K)
  • Ru=8,314.5R_u = 8{,}314.5 J/(kmol·K) — universal gas constant; R=Ru/MR = R_u/M — specific gas constant (J/kg·K)

Specific gravity relative to air and the speed of sound:

SG=MMaira=γ R TSG = \frac{M}{M_{air}} \qquad a = \sqrt{\gamma\,R\,T}
  • Mair=28.965M_{air} = 28.965 g/mol, γ=Cp/Cv\gamma = C_p/C_v — specific-heat ratio
  • aa — sonic velocity (m/s), the choked-flow limit for gas in a pipe or valve throat

For a real gas, density becomes ρ=PM/(ZRuT)\rho = PM/(Z R_u T) with the compressibility factor Z from an equation of state. SimuPipe's compressible solvers carry Z (Peng-Robinson) through the flow equations, warn as flow approaches Mach 1, and cap it at the choke limit — see the isothermal vs adiabatic gas flow guide.

Data source and basis
  • Molar mass and specific-heat ratio are from CoolProp 7.2.0 (Helmholtz-energy equations of state); γ is evaluated at 20 °C and 1 atm. Natural gas is a pipeline-quality pseudo-gas (SG 0.60); steam uses IAPWS-IF97.
  • Viscosities are evaluated from the same temperature correlations the solver uses — CoolProp-fitted polynomial fits for 18 gases, Sutherland-form handbook correlations for CO, SO₂, C₂H₄ and Ne.
  • Densities are ideal-gas values at the stated reference conditions, computed from M — not measured values — so they are exactly consistent with the Nm³/Sm³ flow-conversion bases. R, SG, and sonic velocity are derived via the relations above.
  • Critical constants (Tc, Pc, ω) are the Peng-Robinson inputs the simulator uses for real-gas density; refrigerant blends (R-410A class) carry ASHRAE pseudo-critical values in the application, and the four pure refrigerants listed here use CoolProp values.
  • This table is a view of the application's fluid data — the same 23 gas presets the editor offers — not a separate transcription, so published values and simulated values cannot drift apart.

Frequently Asked Questions

What is the difference between normal (Nm³) and standard (Sm³) conditions?
Both are reference states for quoting gas volumes, and they differ: "normal" conditions (Nm³) are 0 °C and 1 atm (101.325 kPa); the most common "standard" conditions (Sm³) are 15 °C and 1 atm, while US gas practice often uses 60 °F (15.56 °C). The same mass of gas occupies about 5.5% more volume at 15 °C than at 0 °C, so mixing up the bases is a real 5% error. This table quotes ideal-gas density at both. To convert flows between bases, use the gas flow converter.
What is the specific gas constant R?
The specific gas constant is the universal gas constant divided by the molar mass: R = Rᵤ/M with Rᵤ = 8,314.5 J/(kmol·K). It appears in the ideal-gas law in mass terms, P = ρRT. Air is 287 J/(kg·K); hydrogen, with the smallest molar mass, has the largest R at 4,124 J/(kg·K); heavy refrigerants sit below 100 J/(kg·K). Note that gas density at fixed pressure and temperature is inversely proportional to R — light gases are the least dense.
What is the specific-heat ratio γ used for?
γ = Cp/Cv governs compressible-flow behaviour: it sets the speed of sound a = √(γRT), the choked-flow (critical) pressure ratio (2/(γ+1))^(γ/(γ−1)) — about 0.528 for air — and the temperature change of a gas expanding or compressing adiabatically. Monatomic gases (helium, argon, neon) are near 1.67, diatomic gases (air, N₂, O₂) near 1.40, and larger molecules progressively lower — heavy refrigerants approach 1.1.
What is the specific gravity of a gas?
Gas specific gravity SG is the ratio of the gas's molar mass to air's (28.965 g/mol), which equals its density ratio at the same temperature and pressure for ideal gases. Methane's SG of 0.554 is why natural gas rises; propane's 1.52 is why LPG pools in low points — a key ventilation-safety distinction. SG is also the gas property used in Weymouth/Panhandle pipeline equations and in gas flow converter calculations.
When does the ideal-gas assumption break down?
The ideal-gas law is accurate within about 1% for air, nitrogen and methane at ambient conditions, but real-gas deviation (compressibility factor Z ≠ 1) grows with pressure and with proximity to the critical point. CO₂ at 0 °C is already 0.7% denser than ideal; at 50 bar the deviation reaches tens of percent for many gases. SimuPipe computes Z from the Peng-Robinson equation of state — using the critical constants Tc, Pc and acentric factor ω in the second table — for CO₂, ammonia, hydrocarbons, refrigerants and other non-ideal gases.
Why is gas viscosity so much lower than liquid viscosity?
Gas viscosities are of order 10 µPa·s — roughly a hundred times below water's 1,000 µPa·s (1 mPa·s) — because momentum transfer in a gas happens by molecular collisions rather than intermolecular cohesion. Unlike liquids, gas viscosity rises with temperature (roughly as T^0.7). Despite the low absolute values, viscosity still matters: it sets the Reynolds number and hence the friction factor of every gas pipeline calculation.

Model compressible gas flow

Pick a gas, set pressure and temperature, and SimuPipe solves the network with real-gas density, temperature-dependent viscosity, and sonic-choking checks.

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