Capillary depression
Barometric corrections

Due to capillary effects, the position of the meniscus in the manometer tube is lower than it would be for a very large-diameter tube. This effect is particularly noticeable with small-diameter tubes. Capillary depression can of course be calculated at [9] , but in practice it depends on the angle of contact of the mercury surface with the glass wall of the tube. This angle itself varies considerably as the mercury rises or falls in the tube, and also over time, due to the state of purity of the mercury. Indeed, given mercury's high density, the slightest impurities are carried to the surface and gradually deposited on the tube walls. It is therefore very difficult to accurately estimate capillary depression, especially in the case of a narrow tube.

In the case of a U-tube barometer, in principle...