Residual pressure
Barometric corrections

The part of the barometer tube above the upper meniscus is assumed to be perfectly evacuated. In fact, this space is at least filled with mercury vapour, at a pressure equal to the saturation vapour pressure p_s . This, expressed in pascals, is given by the following empirical formula [8]:$\text{lg}{p}_s=\mathrm{10,162}1-\frac{3204}{T}$

where T is the thermodynamic temperature expressed in kelvin. For temperatures close to room temperature (20°C), this pressure is of the order of a few tenths of a pascal, or a few thousandths of a millibar, and is therefore completely negligible for our purposes.