Specifically, is there a low-cost reader and tag configuration available that will detect moisture from a distance of approximately 3 feet?
Most RFID-based moisture sensors are active tags that have a battery and can transmit over longer distances (100 feet or more).
RF Code offers a sensor tag for real-time leak and fluid detection. There are also a variety of mesh-network sensors that can detect moisture. These small devices pass data from one to the next until it reaches a gateway that then transmits the information to a host system. Battery-powered devices such as these cost $50 or more apiece, and might not be what you would consider "low-cost."
RFID solutions providers have been working to come up with lower-cost passive sensors. We recently wrote about the use of passive moisture and corrosion sensors within a parking garage built in the German city of Boppard (see German Parking Garage Installs Moisture- and Corrosion-Sensing RFID Tags). The passive tags with sensors were placed below the cement's surface and above reinforcement steel at select spots throughout the garage. The tags can be read using a handheld device to detect the onset of corrosion or excessive moisture, thus enabling the city to repair the problem before it can harm the reinforcement steel.
The CorroDec corrosion-monitoring system was developed by BS2 Sicherheitssysteme GmbH, a manufacturer of security and construction-monitoring solutions, in cooperation with engineering consulting firm Selfsan Consult GmbH. The city of Boppard spent €8,000 ($10,400) on the monitoring system.
I am unaware of other passive moisture sensors, but if any of our readers have information about such sensors, they are welcome to post links below.
—Mark Roberti, Founder and Editor, RFID Journal