Humidity sensor for automated hydroponic system

Hello everyone,

I'm working on an automated hydroponic system and one of its functions would be a watering cycle. The theory is that a sensor measures the relative humidity in the root area, once it drops to a certain percentage the pump activates and waters the roots, effectively raising the relative humidity to 100%.
The issue is that I don't know if there is a type of sensor that can measure such a thing. All I could find were these kind of sensors: http://www.amazon.com/Arduino-compatible-Analog-Temperature-Humidity/dp/B00AF22GDC/ref=sr_1_cc_1?s=aps&ie=UTF8&qid=1397620003&sr=1-1-catcorr&keywords=humidity+sensor+arduino

or like this one:

One thing I couldn't find an answer on is whether these sensors can come in contact with actual water, instead of only the water vapor in the air.
Does anyone have experience with these type of sensors in relation to hydroponics? What type of sensor do I need? Is there a better way of building a feedback-based watering system?

Thanks for your time!

Attached is a datasheet for a water level detector that works using two metal probes very close together . This can be made using
a cork with two small holes punched through it or some material that you poke the probes through and hot glue them to the material at the holes. They should be as close together as you can get without actually touching (1 to 3 mm).
Look at figure 3 & 4 on page 5 for example circuits. I have used this chip and it works well. There are a bunch of example circuits on those pages.

The DHT11 Sensor has a range up to 90% humidity.

LM1830.pdf (121 KB)

DHT11.pdf (842 KB)

Hi.

There is also this model:

http://www.ebay.com/itm/P4PM-Soil-Hygrometer-Humidity-Detection-Module-Moisture-Water-Sensor-for-Arduino-/350931846485?pt=LH_DefaultDomain_2&hash=item51b52b0555

Regards Torben.

I guess you really can't beat the price on that one...

I... What you want to see the level of water if its a Hydroponic system? sorry but Im working in almost the same and I think that what you need is change the water or looking the conductivity and pH, but not the level. In any case if is a full hydroponic system (100% water) what you have to use is a Water level sensor, and in the other hand, if you are ussing a mix of Hydroponic system (sand + water) you can use the moisture sensor that the guys give you... Send me an email if you need help or a PM!

The LM1830 is a water level sensor.