	p = 100 kpa , T = 20 °C	p = 10,000 kpa , T = 100 °C
$\begin{array}{l}c_g\end{array}$	10,000 GPa^-1	100 GPa^-1

It is related to gas compressibility factor $\begin{array}{l}Z\end{array}$ as:

(1)	$\begin{array}{l}\displaystyle c_g = \frac{1}{p} - \frac{1}{Z} \frac{dZ}{dp}\end{array}$

Derivation

(2)	$\begin{array}{l}\displaystyle c_g = - \frac{1}{V} \frac{dV}{dp} = - \frac{d}{dp} \left( \ln V \right)\end{array}$

Substituting $\begin{array}{l}V\end{array}$ from (Compressibility factor:1) into (2) one arrives to:

(3)	$\begin{array}{l}\displaystyle c_g = - \frac{d}{dp} \left( \ln \left( \frac{Z}{p} + \ln (\nu R T) \right) \right) = - \frac{p}{Z} \frac{d}{dp} \left( \frac{Z}{p} \right) = - \frac{p}{Z} \left( \frac{1}{p} \frac{dZ}{dp} - \frac{Z}{p^2} \right) = \frac{1}{p} - \frac{1}{Z} \frac{dZ}{dp}\end{array}$

Reference

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