The reading should be taken at the center of the curved surface of the meniscus, regardless whether the manometer is water-filled or mercury-filled.

Water does not wet the glass; therefore, the reading should be taken at the top of the meniscus on water-filled manometers.

The reading should be taken at the edges of glass contact of the curved surface of the meniscus, regardless whether the manometer is water-filled or mercury-filled.

Mercury wets the glass; therefore, the reading should be taken at the top of the meniscus of mercury-filled manometers.