A microwave device for detecting moisture and salt content in frescoes and walls

Kurzfassung

An Italian research group of an Institute of Applied Physics has developed a microwave sensor and an instrument for the diagnostics of frescoes and bare walls. The instrument is capable of measuring the sub-superficial moisture content, and allows detecting the presence of soluble salts in a wall up to a depth of about 2 cm. The group is looking for companies interested in industrial implementation of the device.

Details

The instrument is called SUSI, acronym of "Strumento per la misura di Umidità e Salinità Integrato" (literally: integrated instrument for measuring humidity and salinity). SUSI has been developed to investigate the health status of frescoes by measuring the sub-superficial moisture content and detecting the presence of soluble salts up to a depth of about 2 cm. The measurement is based on the detection of water and salts as a consequence of their influence on the microwave dielectric properties of the investigated wall.
The device consists of a resonant sensor connected to a microwave analyser and, optionally, of a notebook computer allowing more complex elaborations on the SUSI response. The sensor has been designed to operate on solid flat surfaces, and it consists of a coaxial head and of a microstrip cavity. The microwave analyser contains a synthesised generator, a microwave power meter, and ancillary electronics for signal elaboration. The instrument displays the measured data on a LCD display, and it allows data to be acquired by a personal computer via a serial port. SUSI allows both absolute and relative measurements of moisture and salt content.
The measurement is performed non-invasively, with the sensor in direct contact with the wall. Data are elaborated in real time.
SUSI is a useful tool for early diagnostics of frescoes, for periodic monitoring, and for quantitative investigation of the dynamics of water diffusion in porous materials like plaster, mortar or wood. It can thus be of interest for companies developing electronic instrumentation for the field of cultural heritage diagnostics and preservation.

Innovative Aspects:
- SUSI allows sub-surface moisture content to be detected and quantified, whilst other conventional methods based on infrared radiation are superficial.
- It allows detecting the presence of soluble salts, an almost unique feature of SUSI.
- The sensor is composed of two sections, (1) a coaxial probe and (2) a microstrip cavity. The two parts can be easily and independently substituted: (1) can be substituted to investigate larger or smaller regions; (2) can be changed to operate in a different frequency range.
- The sensor probe is equipped with a "spring" contact, making the measurement reliable and not influenced by the presence of air between probe and wall.