Three novel capacitance techniques for making moisture measurements in insulated roof systems are being studied at the Oak Ridge National Laboratory (ORNL). These are designated as the Pin Probe, the Thermocouple Probe, and the Planar Probe. The Pin Probe, developed at the Massachusetts Institute of Technology (MIT), consists of two parallel lines of straight pins as the two probe electrodes. This is an intrusive probe that must be mounted within the system being tested. The Thermocouple Probe is also intrusive. Its use assumes that thermocouple junctions exist at known locations in the specimen for temperature measurements. The probe electronics allows high-frequency impedance measurements between thermocouples without interfering with their temperature measurement function. The third probe, the Planar Probe, is nonintrusive and consists of a series of equally spaced plate electrodes in a box that sits on the surface of the roof. These electrodes provide several independent electrical signals from overlapping depths within the roof system. An analysis procedure was developed to identify the electrical properties of successively deeper layers of material.
Prototypes of each of these probes have been constructed and promising preliminary data are available. Probe calibration, circuit stability, and range of applicability are major outstanding issues that must still be addressed.