A new type of thermal moisture sensor for determining material moisture and initial applications are presented. The sensor is practically insensitive to typical disturbance variables such as electromagnetic fields or varying salinity due to the thermal measuring method. The measuring range of the sensor can be limited to very thin layers without further effort by means of suitable signal evaluation. This makes the sensor particularly suitable for measuring the water content of textile layers and skin close to the body. The system can be constructed very compactly and the energy consumption is low, so that portable applications - wearables - can be realized.

The human moisture balance plays a very important role in sport and general health. (Smith & Havenith, 2011). A water loss of just 2% of body weight leads to a noticeable decrease in physical and mental performance. A loss of 5% or more can reduce the ability to work by around 30% (Jeukendrup, 2010, p. 32). A water loss of 10% of body weight is ultimately life-threatening. For good reason, there are countless apps and fitness trackers that remind you to drink enough. The disadvantage is that these applications usually have no information on the current moisture content in the body or on moisture loss through sweating. This information can be supplemented with the help of moisture measurements taken close to the body. This makes it possible to provide older people and active endurance athletes with targeted additional information on their current water balance and/or to remind them to drink.

The thermal humidity sensor developed at Kaiserslautern University of Applied Sciences (Schönfisch et al., 2020) offers optimal conditions for moisture measurements close to the body. The micro-engineered THMS sensor (THMS for Transient-Heat Moisture-Sensing) is based on a very thin foil heater (approx. 8 µm), which is heated by a few degrees for a very short period (~0.3 s). At the same time, its temperature rise is recorded and evaluated. This temperature behavior is dependent on the moisture content of the environment. The energy consumption is low and allows operation with a small battery such as those typically used in smart watches.

Moisture measurements in clothing worn close to the body make it possible to measure the moisture input caused by sweating directly and, in conjunction with other data, to extract information on the current sweat rate. The data can be transmitted directly via a wireless interface to a cell phone for recording or further analysis.

In the first pilot tests on skin moisture measurements, it was shown that the THMS sensor can detect different moisture levels in the outer skin layer (stratum corneum).

Although the moisture content of the stratum corneum does not yet provide any information about possible dehydration of the body, it demonstrates the suitability of the THMS sensor for measurements in the skin. Experiments are currently being carried out to measure deeper layers of the epidermis and to correlate the measurements with moisture loss after sporting activity.

Literature list:

Jeukendrup, A. (2010). Sports Nutrition - From Lab to Kitchen. Meyer & Meyer Sport.

Schönfisch, D., Göddel, M., Blinn, J., Heyde, C., Schlarb, H., Deferme, W., & Picard, A. (2020). Miniaturized and Thermal-Based Measurement System to Measure Moisture in Textile Materials. Physica Status Solidi (A) Applications and Materials Science, 1900835. doi.org/10.1002/pssa.201900835

Silbernagl, S., & Draguhn, A. (2018). Pocket atlas of physiology. Thieme.

Smith, C. J., & Havenith, G. (2011). Body mapping of sweating patterns in male athletes in mild exercise-induced hyperthermia. European Journal of Applied Physiology, 111(7), 1391-1404. doi.org/10.1007/s00421-010-1744-8