**ENTROPY**

ENTROPY [J/kg-K, J/kmol-K, kJ/kg-K, kJ/kmol-K, Btu/lbm-R, Btu/lbmol-R] returns the specific entropy of a specified substance. The value and units of the returned value depends on the Unit System setting. For all pure substances, the entropy function always requires two arguments, in addition to the substance name.

For AirH2O, three arguments are required. One of these arguments must be total pressure (P). The remaining two can be any of the following: temperature (T), enthalpy (H), internal energy (U), relative humidity (R), humidity ratio (W), wetbulb (B), or dewpoint (D). Note also that for substance AirH2O (psychrometrics), the specific entropy returned by this function is the entropy of the air and water vapor mixture per unit mass of dry air.

The specific entropy of incompressible substances is a function of only temperature. Temperature must be the only argument, in addition to the substance name.

The reference state upon which the value of enthalpy is based varies with the substance. However, all ideal gas substances (which have a chemical symbol name, e.g., N2, CO2, CH4) have enthalpy values corresponding to JANAF table references. The JANAF table reference for entropy is based on the Third Law of Thermodynamics which references the entropy of all pure crystalline substances to zero at absolute zero temperature.

The list of property identifiers needed in the calling arguments and instructions are available in the Thermophysical Function help.

Examples:

$UnitSystem SI C kPa mass

s_air=entropy(Air,T=25 [C],P=200 [kPa])

s_stm=entropy(Steam,T=100 [C],P=50 [kPa])

s_sat=entropy(Steam,T=100 [C],x=1)

s_copper=entropy(Copper,T=25)

~~{Solution:~~

~~s_air=5.5 [kJ/kg-K]~~

~~s_copper=0.5271 [kJ/kg-K]~~

~~h_sat=7.354 [kJ/kg-K]~~

~~s_stm=7.694 [kJ/kg-K]}~~