No Nitrogen available : AUTO mode

Another alternative is to assume that you know the water vapor content, either from a previous measurement and assuming the weather is stable, or from theoretical curves as a function of pressure, temperature and relative humidity. With the knowledge of the atmospheric parameters, the model predicts $T_{sky}$ and $Tau$. In this mode, instead of $Trec$ above, you must supply $Water$ the amount of precipitable water vapor in millimeter. $Trec$ is then given by (from Eq. 7)

  $$Trec = \frac{T_{load} * Mean\_atm - T_{emi} * Mean\_load} {Mean\_load - Mean\_atm}$$ (11)

This is what happens in OBS when used in AUTO mode (specified by OBS$\backslash$SET CAL AUTO) and when you do a CAL command without the argument COLD.