Nuclear detection techniques
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The nuclear water detection method employs radioisotopic thermalization to emit high-velocity neutrons and measure backscatter. Although this method is directly detecting the presence of hydrogen, and therefore water, in the roof assembly, it requires operation by licensed personnel due to its complexity (e.g. unit angle sensitivity and baseline reading requirements).
Radioisotopic thermalization (TAS 126-95 and American National Standards Institute/Single Ply Roofing Industry [ANSI/SPRI]/RCI NT-1-2017, Detection And Location Of Latent Moisture in Building Roofing Systems by Nuclear Radioisotopic Thermalization) involves a process where a nuclear moisture meter emits high-velocity neutrons and measures backscattered ‘slow’ neutrons that have lost much of their energy in collisions with hydrogen atoms. Thus, higher levels of slowed neutrons are recorded at wet areas, as water contains a significant amount of hydrogen atoms.
Nuclear leak detection is also able to identify the accumulation of moisture in insulation, but cannot pinpoint the source of the leak(s). Core samples may be taken of dry and wet locations to determine moisture content.
This technique is applicable to all conventional roof assemblies except metal roofs. Nonconventional PMR (upside down) roofs are incompatible with nuclear watertightness evaluations. Roofs with overburden may be tested only if the ballast or pavers are removed from the test area. In most cases, the practicality of using this time-consuming method of watertightness detection beneath vegetative roof assemblies would be of questionable value to specifiers.
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As with other methods of watertightness detection outlined here, there is the potential for false positives.
Typically, a baseline reading to calibrate the nuclear meter must be taken in a known dry area of the roof. The nuclear meter samples about 0.6 m2 (6.5 sf) at each grid point in a 0.9 x 0.9-m (3 x 3-ft), 1.8 x 1.8-m (6 x 6-ft), or 3 x 3-m (10 x 10-ft) pattern. The equipment has a depth limitation of 1.8 to 2.4 m (6 to 8 ft). However, unlike several other methods of watertightness evaluation that are employed, nuclear scanning is not dependent on climatic conditions.
It is important to note areas of ponding water on the roof surface will result in increased readings from the nuclear meter. Readings can also be affected by inconsistencies seen among roofing components, such as joints where the different elements meet one another.
Further, technicians may need to comply with Canadian Nuclear Safety Commission (CNSC) regulations.
Electrical impedance evaluations
The electrical impedance method uses a device to create an alternating electrical field for penetrating the roofing material. Since wet insulation provides less resistance to electrical current than dry, the current can be correlated to the presence of water. The alternating current flowing through the field is inversely proportional to the impedance of the moisture-absorbing materials. On the downside, the electrical impedance unit contains a scanner sensitive to interply and surface moisture and inconsistencies in the roof system.
Like the nuclear method, this technique is applicable to most conventional roof assemblies except for black EPDM membranes or assemblies treated with aluminized protective coatings. PMR roofs are incompatible with electrical impedance watertightness evaluations, and any roofs with overburden may be tested only if the ballast or pavers are removed from the test area. It is critical for the membrane to be free of surface moisture to obtain accurate readings.
Similar to the nuclear and infrared methods, the impedance method is also able to identify moisture accumulation in insulation, but cannot pinpoint the source of the leak(s). Again, core samples may be taken of dry and wet locations to determine the moisture content.
As always, the operator’s experience is essential for interpreting results accurately. Other potential issues reducing instrument sensitivity include aggregate-ballasted and/or aggregate-surfaced membranes with variable size and weight. As mentioned, the scanners are also more sensitive to interply moisture and water closer to the scanner electrodes, which can make readings further below the membrane difficult. Roof patches dissimilar to the system under testing may also result in erroneous readings.
While the electrical impedance watertightness method is easy to use and less complex than other evaluation techniques, the presence of dew, rain, snow, and ice significantly affects the readings.
