The u-tube (manometer) is a gauge that measures negative air pressure inside the radon system pipe. It is usually installed on the main suction point in the basement, but may be located in a garage or crawl space. The u-tube has red or blue non-toxic liquid in it. Viewing the u-tube should be done at least once a month to confirm system operation. During the initial installation of the radon system, the u-tube was marked , identifying the initial pressure levels of your system. (Not all systems will have the same pressure readings.)
The levels on the u-tube may fluctuate or change slightly over time. However, if the levels on the u-tube become considerably further apart, this might indicate that there is ground water collecting under the floor, thereby restricting air-flow to the radon system. This is a strong indicator that the sump pump has failed. The sump pump should be repaired or replaced immediately. If you do not have a ground water sump pump and it has rained, the water under the floor will subside within a couple days to a week and then the negative pressure inside the system will return to normal levels.