Blue Oil Manometer

This Blue Oil-Filled U-Tube Manometer Will Lie to You When Used with a Mechanical Ventilation System

It will tell you that radon is low even when your radon exposure  is at maximum. and it will lie to you for years and do it with a straight face.

This is a Blue Oil-Filled U-tube manometer designed to advise of Sub Slab Depressurization System negative pressure in the manifold. It will advise of a PRESSURE DIFFERENTIAL between the air  in the radon system manifold and the air in the breathing zone. For this purpose, they are good, but not without fault. However, they are not designed to measure airflow and they do not do so without being connected to a device called a pitot tube.

The home in which the U-Tube manometer above is installed appears to be radon-protected.

But it is not. Recent testing revealed that the actual radon level is near ten picoCuries per liter. WHY? Because a homemade filter has not been serviced every 60-90 days. It was not serviced for nearly four years. Why was it not serviced?

  1. Because the owners/occupants did not know that the homemade filter needed to be serviced to keep the radon system working
  2. Because no one reminded them periodically that the filter needed servicing
  3. Because the filter was about 13 feet off the ground, was unmarked and revealed no clue that it contained a filter that needed frequent service in order to preserve proper operation of the radon mitigation  system


This is the air intake protected by a short-life homemade filter. It is about 13 feet off the ground. Photo taken with a telephoto lens.

Owner advised that she called the installer who told her that they did not repair systems that they installed, but that she needed to change the air filter.

Blue or Red Oil-Filled U-tube manometers are calibrated to show inches of water column air pressure differentials. When installed on a sub slab depressurization system they reveal the negative air pressure that is being generated inside the duct/pipe into which they are attached with respect to the air pressure in the breathing zone.

In many cases, it is assumed that the radon fan will generate about 2 inches of water column suction. So long as the oil column is displaced to the 2″ marker on the scale, it is assumed that the fan  is sucking radon out of the soil, and that the soil gas pressure is lower than the air pressure in the breathing zone.

This is a homemade air intake filter frequently installed on a Mechanical Ventilation Radon Mitigation System

Conclusion? If you have a homemade air intake filter you should service it every 60 days and watch out for mold issues from indoor relative humidity increases