Mariotte's bottle

[2] As shown in the diagram, a stoppered reservoir is supplied with an air inlet and a siphon.

The drawback of the design is that it is sensitive for gas inlet leakage and that during operation liquid cannot be added, since it would change the pressure control.

Constant head is important in simplifying constraint when measuring the movement of water in soil.

Several measurement techniques employ the Mariotte's bottle to provide constant head.

The Guelph Permeameter measures unsaturated hydraulic conductivity in the field and uses this principle to create a constant head.