Humidity Sensor

It sounds as though you are not really wanting to measure relative humidity (A relatively inexact science) for which there are electronic devices. (I can find you the name of a manufacturer in Chicago, but not right now. If you simply want to know if water is present, perhaps a simple galvanometer will show wet or dry by conductivity. (Resistance meter)

Sensor contamination by road salt, pollution (such as 'acid' rain, for example), dirt, dust, motor-vehicle offcasting, and other environmental influences can defeat a conductance-based moisture sensor in no time. Such contaminants can significantly increase the electrical conductivity of the medium-under-test - in this case the roadbed material - and fool the sensor into reporting a far greater moisture content than is actually the case. Even if the sensor design does not require the device to be in direct contact with the medium (as in the case of some 'rain sensors' used in certain automatic sprinkler-system designs, for example) the sensor must still be exposed to the elements in order to function properly and will eventually become contaminated by the roadside environment.

Consequently, moisture sensors that depend on electrical conductance, alone, cannot differentiate between the moisture itself and the effects of contamination. A different approach must be used; one which recognizes moisture specifically and can determine its content independently of other factors. I'm just guessing here, of course, but there are probably a number of other, reliable moisture-sensing technologies based on a variety of techniques. For example, a sensor design might possibly be based on spectroscopic techniques, on energy absorption by or resonance of water molecules at specific microwave frequencies, or even perhaps on some rudimentary form of NMR (nuclear magnetic resonance). Whether such sensing technologies are actually available and can be used in practical day-to-day applications needs to be determined. And, naturally, there's always the ever-present issue of cost; but unless and until one determines which is the right kind of sensor for the job, there's nothing to actually buy!

Just speculation on my part, to be sure, but hopefully not so far-fetched as to not be worth a good looking into. Hope this has been helpful to you (even if only a little).

-E

Thank you for your comment.

The thing we have to consider here is , we have to use the sensor undrneath the water proof membrane of a concrete slab deck bridge that means we have to make hole , groove or attach somewhere someway some kind of device to monitor the humidity.

Simply gavanometer is good but I think we can not use it for this particular case.

Just regular humidity sensors are very bad about going out of calibration so some type of moisture indicator is what you are looking for.

Honeywell makes a polmer and platinum humidity sensor that is contamination resistant. Common solvents, dirt, oil, and other pollutants do not affect the stability or accuracy of the sensor.