Compute entropy of air using thermodynamic relationships. Input temperature, pressure, humidity to analyze moist air. Designed for HVAC, combustion, atmospheric science, and teaching professionals.
The calculator treats moist air as an ideal mixture of dry air and water vapor. The main relationships are:
pws = 0.61078 × exp(17.27 T / (T + 237.3)) (kPa) – saturation vapor pressure with temperature T in °C.pv = RH/100 × pws – water vapor partial pressure.ω = 0.62198 × pv / (P - $p_v) – humidity ratio.s = cpa ln(T/Tref) - Rda ln(pda/pda,ref)
+ ω [cpv ln(T/Tref) - Rv ln(pv/pv,ref)]
Here, T is absolute temperature in kelvin, pda is dry-air partial pressure,
and the reference state uses standard atmospheric pressure and a moderate humidity assumption.
| Temperature (°C) | Relative humidity (%) | Pressure (kPa) | Entropy (kJ/(kg·K)) | Humidity ratio (kg/kg) |
|---|---|---|---|---|
| 25 | 50 | 101.325 | 0.0859 | 0.0097 |
| 35 | 60 | 101.325 | 0.1112 | 0.0211 |
| 5 | 40 | 101.325 | 0.0183 | 0.0030 |
Entropy describes how energy spreads within a system and indicates its disorder. For atmospheric air, entropy links temperature, pressure, and moisture content, revealing how far real states deviate from ideal reference conditions. Interpreting these values helps engineers understand whether a process is thermodynamically efficient or highly irreversible.
Real atmospheric air is a mixture of dry air and water vapor. Treating it as an ideal gas mixture allows entropy to be estimated from temperature, pressure, and humidity ratio for many engineering applications. This simplification is accurate for HVAC work, combustion air studies, and moderate-pressure process calculations.
The calculator uses dry-bulb temperature, relative humidity, and total pressure as primary inputs. You may also choose between SI and imperial units, enabling easy comparison of laboratory data with industrial plant measurements. Sensible defaults, such as standard atmospheric pressure, help you start quickly while still allowing advanced customization. Multiple runs with varied inputs let you explore sensitivity, checking how strongly entropy reacts to small changes in humidity or operating pressure across realistic environmental conditions.
The calculator models moist air as a combination of two ideal gases: dry air and water vapor. It applies specific heat capacities for both components, gas constants, and logarithmic relationships between reference and actual states to estimate specific entropy per kilogram of dry air. This framework mirrors psychrometric relationships used in professional engineering handbooks. While simplified, the method captures the dominant trends and remains transparent, so advanced users can trace each assumption or even adapt constants for specialized research cases.
Relative humidity is first converted to water vapor partial pressure using a saturation pressure correlation based on temperature. From this value, the humidity ratio is found, then inserted into the entropy expression to capture the contribution of water vapor to total entropy. The result highlights how even modest moisture changes significantly affect thermodynamic behavior.
For deeper gas analysis, combine this tool with the Molar Mass of Gas Calculator when you need accurate composition data. When compressibility is important, the gas compressibility factor calculator helps refine gas property estimates in high-pressure systems, complementing entropy calculations for rigorous design or academic study.
Entropy of air calculations support HVAC sizing, combustion air analysis, and energy audits. Comparing inlet and outlet entropy shows losses in heat exchangers, dryers, or environmental test chambers, guiding improvements in process control and equipment design. Students can also use the calculator to visualize textbook concepts with realistic engineering numbers. By comparing scenarios, you can rank design options, quantify expected performance gains, and justify equipment upgrades or control changes using clear thermodynamic reasoning rather than intuition alone.
Entropy of air measures the degree of energy spreading and disorder in atmospheric or process air. It combines temperature, pressure, and moisture effects into one thermodynamic state property, usually expressed per kilogram of dry air.
The calculator supports SI units using degrees Celsius, kilopascals, and kilojoules per kilogram-kelvin. It also works in imperial units with degrees Fahrenheit, pounds per square inch, and Btu per pound-Rankine for engineers using North American design data.
The method assumes ideal-gas behavior for dry air and water vapor and uses standard correlations for saturation pressure. This is sufficiently accurate for HVAC, combustion air, and most laboratory conditions but may deviate at very high pressures.
Yes, the calculator is suitable for homework, lab reports, and introductory research projects. Always cite your assumptions and note that an ideal-gas mixture model is used when comparing against experimental measurements or advanced equation-of-state software.
Water vapor adds additional molecular freedom, increasing the number of accessible microstates. As humidity rises, the vapor contribution to entropy becomes larger, so two air samples at the same temperature and pressure can have noticeably different entropy values.
This tool currently requires dry-bulb temperature, relative humidity, and total pressure as inputs. You can convert dew point and dry-bulb readings to relative humidity using a separate psychrometric calculator, then return here and compute the corresponding entropy.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.