The vapor pressure of a liquid is measured by determining the rate of effusion of the gas in equilibrium with it. To do this, the weight loss w in the interior due to the escaping molecules is measured.

\begin{equation} \frac{dw}{dt}=m\frac{dN}{dt}=-\frac{A_{or}P_vm}{(2\pi mkT)}^{1/2}=- P_vA_{or}\left(\frac{m}{2\pi kT}\right)^{1/2} \end{equation}

In a finite time interval $\Delta t$, the following loss of weight:

\begin{equation} \Delta w=-P_vA_{or}\left(\frac{m}{2\pi kT}\right)^{1/2}\Delta t \end{equation}

Weighing the container at the beginning and at the end of the experiment we determine $\Delta w$ and using the previous equation the vapor pressure.

\begin{equation} P_v=\frac{\Delta w}{A_{or}\Delta t}\left(\frac{2\pi kT}{m}\right)^{1/2} \end{equation}