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Given:

Atmospheric pressure=76cm of Hg

Saturation vapor pressure=0.80 cm of Hg

When the water is introduced in barometer, water evaporates.

Thus, it exerts its vapor pressure over the mercury meniscus.

As more and more water evaporates, the vapor pressure increases that forces down the mercury level further down.

Finally, when the volume is saturated with the vapor at atmospheric temperature, the highest vapor pressure, i.e. saturation vapor pressure is observed, and the fall of mercury level reaches its minimum.

Thus,

Net pressure acting on the column = atmospheric pressure -saturation vapor pressure

= 76-0.80 =75.2 cm of Hg.

The height of the mercury column when it reaches its minimum value is 75.2cm.