Residual pressure

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...