What is CVM™?

The principle behind Comparative Vacuum Monitoring (CVM™) is uniquely simple; a vacuum contained in a small volume is extremely sensitive to any leakages.

The CVM™ principle relies on placing a sensor onto the surface of a component where damage is expected to occur. The sensor contains fine channels in parallel which are open to the surface. Once the sensor has been installed on the surface, the channels form closed “galleries” to which a vacuum can be applied. It is important to note that the surface of the component forms part of the sensor system, with the crack itself providing the leakage path for air into the vacuum galleries (Figures 1, 2, 3).

The sensor is connected to a vacuum source with an accurate flow meter. Figures 4, 5 and 6 show a snippet of the equipment used in laboratory trials which allows continuous monitoring of a sensor, or several sensors connected in series or parallel (not shown). At the beginning of the test, a continuity test is completed to ensure galleries do not contain a blockage (Figure 4). If there is no damage on the component, then the vacuum in the sensor will be approximately the same as the vacuum source (Figure 5). If however a crack develops, a leakage path will exist and the vacuum level will be reduced in the sensor manifold (Figure 6.)

The PM200 being a portable stable vacuum source with an accurate flow meter allows continuous monitoring of a sensor, or several sensors connected in series (Figure 7).

It is permissible to install CVM™ sensors in series to monitor multiple expected flaw locations from a single interrogation point. This reduces the time to inspect sensors by grouping together each test the PM200 completes. Total number of sensors that can be joined is proportional to sensor gallery and tube length. The PM200 records the location of sensors and detected value at interrogation completion (Figure 8).