Moisture climate monitoring in confined spaces using percolation sensors

It is a well-known fact that inappropriate levels of moisture can harm different kinds of structures, especially also in cultural heritage. This does not only regard hygroscopic materials but all assemblies where small quantities of water can change the physical properties of materials and structures. Damage modes range from crack formation due to swelling pressure, migration of harmful agents such as salts, up to corrosion and microbial damage. A specific concern is not only a certain moisture level as such, but also the periodic moisture variations as they can even be related to mechanical fatigue processes when alternating hydration pressures apply. Therefore, moisture monitoring is in many cases unavoidable and a multitude of commercial and experimental set-ups exist ranging from simple point measurements using hygrometers up to sophisticated time-domain reflectometry that provides humidity profiles at long ranges. Inspired by an analogous problem in aircraft structures suffering e.g. from moisture ingress in floor structures, a sensing material to detect the moisture climate was developed that is based on the percolation effect. Here, an electrical conductive material is changing into an isolator when a certain humidity level is reached. The material presented is based on hygroscopic poly-vinyl alcohol and electrically conductive titanium carbonitrides. The resulting material shows a characteristic change of the resistance over many orders of magnitude when a humidity level of 80% is reached which can be fine-tuned depending on the actual application. The sensing material is then applied on a cable with a small diameter and in this way, a versatile sensor that can be used especially in confined spaces is obtained. Data transmission can start with simple read-out by a multimeter, but also a network of wireless nodes were tested as well as RFID technology. The developed sensor PercoSens® is for the moment being applied in chemical installations and on an experimental level also in concrete structures enabling extended sensing ranges and a high independency on interfering with environmental parameters. Very recent tests showed the potential of this sensor to indicate water ingress in damaged building structures. For the latter, a sensor was embedded in concrete and after imitated crack formation, the subsequent water ingress was determined quickly.