Say you make a U-shaped glass tube.
If you fill it with water you are able to measure gas pressure by looking at the height difference of the meniscuses.
In an inverted one, by turning it upside down and putting both ends in water (one end in reservoir, and one end in system under test), you should be able to measure the water pressure of the system under test.
How do you make the water reservoir end to exert a known pressure at its meniscus (i.e. calibrate the reservoir end of the U-tube to a known reference pressure, such as 1 atm)?