**WinSteam**, **@Air** and **@Gas** allow the user to select one of seven unit
sets when using the add-in functions in any of the supported
applications or when calling the functions from programming languages.
Each unit set defines the units for each of the property values whether
used for input or output. You select the unit set by entering
either the unit set name or number as the last argument to a function. While the units within each unit set are
fixed, the user may selects different unit sets for each function used.
Within the DeskTop Calculators such as *DeskTop Steam*, each property unit
may be selected independently for more flexibility and variety of units.

The following table shows the units for each property value in each unit set. Not all properties are available in all of the products.

Unit Set |
English |
SI Customary |
English Gauge |
SI
Formal |
SI kPa | Metric | Metric Formal |

Unit Set Number | 0 | 1 | 2 | 3 | 4 | 5 | 6 |

Unit Set Name | "ENG" | "SI" | "ENGG" | "SIF" | "SIK" | "MET" | "METF" |

Pressure | psia | bar | psig | MPa | kPa | bar | kg/cm^{2} |

Temperature | °F | °C | °F | °K | °C | °C | °C |

Specific Volume | ft^{3}/lbm |
m^{3}/kg |
ft^{3}/lbm |
m^{3}/kg |
m^{3}/kg |
m^{3}/kg |
m^{3}/kg |

Enthalpy | Btu/lbm | kJ/kg | Btu/lbm | kJ/kg | kJ/kg | kcal/kg | kcal/kg |

Entropy | Btu/lbm/°F | kJ/kg/°C | Btu/lbm/°F | kJ/kg/°K | kJ/kg/°C | kcal/kg/°C | kcal/kg/°C |

Specific Heat | Btu/lbm/°F | kJ/kg/°C | Btu/lbm/°F | kJ/kg/°K | kJ/kg/°C | kcal/kg/°C | kcal/kg/°C |

Dynamic Viscosity |
lbm/ft/hr | Centipoise | lbm/ft/hr | Pa-sec | Centipoise | Centipoise | Centipoise |

Thermal Conductivity |
Btu/hr/ft/°F | Watt/m/°C | Btu/hr/ft/°F | Watt/m/°K | Watt/m/°C | Watt/m/°C | kcal/m/h/°C |

Humidity Ratio | lb water/ lb gas | kg water/ kg gas | lb water/ lb gas | kg water/ kg gas | kg water/ kg gas | kg water/ kg gas | kg water/ kg gas |

Molecular Weight | lb/lb mole | kg/kg mole | lb/lb mole | kg/kg mole | kg/kg mole | kg/kg mole | kg/kg mole |

Sonic Velocity | ft/sec | m/sec | ft/sec | m/sec | m/sec | m/sec | m/sec |

In addition to the basic unit sets, each product has specific modifiers that can be added to any unit set to change specific aspects. This is discussed in detail in the respective users manual but a brief summary of these modifiers is presented as follows:

- WinSteam normally uses the IF97 formulations to calculate steam properties. If you wish to use the old 1967 formulations, add an "O" to the end of the unit set name or else add 8 to the unit set number.
- When using the IF97 formulations, you can sacrifice some accuracy and increase the speed of the inverse functions by adding an "S to the unit set name or add 16 to the unit set number.

- The Reference temperature for enthalpy and entropy defaults to 0°F when using English units or 0°C when using SI or Metric units. You can change the reference temperature to 0°K in any unit set by adding an "A" to the unit set name or 8 to the unit set number.
- All mass related properties such as specific volume, enthalpy, entropy specific heat viscosity and conductivity are expressed per mass of dry gas as a default. You can change it to a wet basis by adding a "W to the unit set name or 16 to the unit set number.
- In @Gas, the gas composition is input as mole fractions on a dry basis and the moisture argument is input as a humidity ratio by default. In @Air, the moisture argument is input also as a humidity ratio by default. You can choose to input moisture as a mole fraction (and all other composition components as mole fractions on a wet basis in @Gas) by adding a "C" to the unit set name or 32 to the unit set number.
- You may substitute mass fractions wherever mole fractions are required by adding an "M" to the unit set name or 64 to the unit set number.