The article presents new Polish probes SRDN-3, developed at the Institute of Nuclear Chemistry and Technology in Warsaw, equipped
with a semi-conductor detector used for continuous measurements of 222Rn activity concentration. Due to a relatively high lower detection limit, the device is dedicated for use in underground
spaces—caves, adits, mines, tourist routes in strongholds, pyramids, etc. Its structure allows for difficult conditions in
which the device is transported to the measurement site, as well as hard operating conditions caused chiefly by large ambient
relative humidity, reaching up to 100%. The authors present calibration results of these appliances, as well as the results
of their work in a cave and an adit in the Sudetes (SW Poland). After almost 2 years of working in difficult conditions, the
probes displayed high reliability. No defects of the semi-conductor detectors or the electronics were observed, which ensured
problem-free communication of the probe-programmer-PC set. Thanks to this, the authors have a 2 year stock of data, recorded
hourly by five probes, at their disposal. The only element that did not withstand the test of extreme operating conditions
was one of the combined relative humidity and temperature sensors. No powering problems were observed either, and the batteries
were replaced once a year, before the winter season. Also the programmer functioned faultlessly, enabling data transmission
to a PC, which, being much more sensitive to operating conditions, had been placed away from the site of probe exposure. After
using more sensitive temperature, relative humidity and pressure sensors, SRDN-3 probes will certainly prove an excellent
tool for microclimate measurements (including measurement of air-atmosphere exchange) in caves and other underground sites.
Even nowadays they are already a satisfactory tool for monitoring 222Rn concentration in underground spaces.