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Update FAQ.md

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Extend explanation,
mention reference state
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Matthis Thorade
2017-01-27 11:55:13 +01:00
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FAQ.md
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@@ -7,7 +7,15 @@ Usage
1. My enthalpy and entropy values are not the same as what are used in REFPROP, or EES, or ...
Enthalpy and entropy are only equivalent to within an additive constant, so you should always compare enthalpy and entropy *differences* rather than enthalpy and entropy values
Values for enthalpy, entropy and internal energy are calculated as *difference*
with respect to an arbitrarily chosen reference state.
As a consequence, absolute values may differ hugely between different tools,
but the difference between two states should be equal.
At least the following three states are frequently chosen as reference states:
IIR: saturated liquid (Q=0) at T=273.15 K
ASHRAE: saturated liquid (Q=0) at T=233.15 K
NBP: saturated liquid (Q=0) at p=101325.0 Pa
2. My calls to REFPROP fail with a mysterious `Segementation Fault`: