Strange readings from soil moisture sensors

I've been trying the calibrate some soil moisture sensors the old fashioned way by putting them in a glass of water. Ranging from 391 to 405 the difference between them wasn't huge. However, when I inserted them into moist soil I got much [u]lower[/u] readings in the range between 225-228. Does anyone know what's going on? I know that calibration can be difficult due to the many factors that affects the readings, but this seems weird doesn't it?

Perhaps this is because moist soil is not as wet as a glass of water.

Can you elaborate a little bit on that?

Water forms a complete contact with the sensor surface while soil particles are only partial thereby resulting in dead space.

Arctic_Eddie: Water forms a complete contact with the sensor surface while soil particles are only partial thereby resulting in dead space.

Hehe, thank you for the explanation. But why does the soil seems to have better conductivity compared to the water? In the case with my sensors, 1023 means completely dry (no contact between two metal wires) and 0 means completely wet.

Soil has a high mineral content which is often more conductive than pure or tap water. If you had highly deionized water, it would look like an open circuit.

At the nuclear weapon plant where I worked(retired), we had a DIW facility that produced 14megOhm water for product cleaning. It was so good that the cleaning tanks needed an Argon cover gas to prevent CO2 from being dissolved and lowering the readings.

Arctic_Eddie: Soil has a high mineral content which is often more conductive than pure or tap water. If you had highly deionized water, it would look like an open circuit.

At the nuclear weapon plant where I worked(retired), we had a DIW facility that produced 14megOhm water for product cleaning. It was so good that the cleaning tanks needed an Argon cover gas to prevent CO2 from being dissolved and lowering the readings.

If this is true I guess I'll have to keep the hairdryer away from the soil of my plants rather than my bathtub.

Yeah, I hate getting a headful of dirty hair ...

KJohansson:
I’ve been trying the calibrate some soil moisture sensors the old fashioned way by putting them in a glass of water. Ranging from 391 to 405 the difference between them wasn’t huge. However, when I inserted them into moist soil I got much lower readings in the range between 225-228. Does anyone know what’s going on? I know that calibration can be difficult due to the many factors that affects the readings, but this seems weird doesn’t it?

Depends if a pull-up or pull-down resistor is used.

With a cheap resistive sensor, electrolysis could also be a factor.

Post a link to the sensor.
Leo…

Wawa: Depends if a pull-up or pull-down resistor is used.

With a cheap resistive sensor, electrolysis could also be a factor.

Post a link to the sensor. Leo..

Here: http://www.ebay.com/itm/310563123869?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Yep. The equivalent of two nails and a 10k pull up resistor. The chip on that board is a threshold detector for the digital output. The pot sets the threshold. Leo..

Wawa: Yep. The equivalent of two nails and a 10k pull up resistor. The chip on that board is a threshold detector for the digital output. The pot sets the threshold. Leo..

Ok, thanks for the info but I can't see how this explains the readings I'm getting. From what I understand many factors can affect the conductivity between the nails and it's very hard to accurately calibrate this type moisture sensor - guess I'll settle with that.

Hi,

At the nuclear weapon plant where I worked(retired), we had a DIW facility that produced 14megOhm water for product cleaning. It was so good that the cleaning tanks needed an Argon cover gas to prevent CO2 from being dissolved and lowering the readings.

I used to supply some components to Coal Powered Generating Company here in Victoria, the steam generator water was de-ionised. Was almost a HazChem classed chemical, used to keep corrosion in steam circuit to a minimum.

I can across an el-cheepo soil moisture meter many years ago, just a meter, battery and an [u]aluminium stalk/probe [/u], with insulated tip. Connected CRO (Cathode Ray Oscilloscope) to the probes, AC was being used, even though the meter was not designed for permanent installation.

Tom... :)