Meet your Decagon Soil Moisture Sensor
- Measurements:
Read the helpful tips below before using your Decagon soil moisture sensors for the first time.
1. Testing sensor readings
The first thing that most people do when they receive their sensors is stick them into a cup of water expecting a "working" sensor to read %100 VWC. When the sensors do not read 100%, we receive distressed calls about broken sensors. Remember that the sensors and software utilize a factory calibration that we performed in REAL SOIL. "Air" and "Water" points are not contained in the factory calibration so the sensors will read slightly negative in air and between 50%-98% (depending on the sensor) in water. You can watch our soil moisture sensor calibration video to get a better idea of how we do these calibrations.
2. Comparing one sensor to another: sensor to sensor variability
After the "water" test, many people want to see how one sensor compares to another. The common test for this is to stick a hanful of sensors in a bucket of sand, dump some water into the sand bucket, and watch the readings. This is one of the worst ways of checking sensor to sensor variability second only to putting sensors out into the field to do this test. Heterogeneity in soils, even a bucket of seemingly homogeneous sand, will cause sensors to read differently from one another because they are reading actual differences in the water content in the soils.
We check each sensor before it leaves Decagon to make sure that one sensor will read the same as another in the same situations. We encourage you to do your own testing, but use a technique that removes the effects of heterogeneity and bulk density. One method that we recommend is comparing sensor outputs in antifreeze. Most antifreezes have a dielectric permittivity similar to that of saturated soils so the value falls within the calibrations that we do on the sensors. Because all antifreezes are different, we don't have a specific number in mind for what the sensor reads, but all the sensors should read about the same thing in the same antifreeze.
Learn more about sensor to sensor variability in our article Suprise! The sensor is correct.
3. Checking the accuracy of the sensors
Just like a thermometer uses predictible changes in mercury volume to give you a temperature, Decagon sensors use predictable changes in dielectric permittivity (the ability of something to hold a charge) to give you volumetric water content. Our sensors are only as good as our ability to measure dielectric permittivity. Because of this, the only REAL way to check the sensor accuracy (and not the accuracy of the relationship between dielectric permittivity and volumetric water content) is to check out the sensor output in a solution of known dielectric permittivity.
That being said, we don't expect you to actually do this just like the thermometer companies don't expect you to actually look at how much the mercury inside a thermometer changes in volume. A more useful test may be to check our factory calibration. Using the methods from the soil moisture sensor calibration video, create a few points with your soils and see where they fall in our factory calibration data set. The data set has been published in the Journal of Hydrology and covers soils from clay to sand with a variety of electrical conductivities.
Learn more about sensor accuracy in our article Why Does My Sensor Read Negative?
4. Installing the sensors
If you install your sensors improperly, your data may end up being useless. If you install your sensors perfectly, but leave the cables above ground unprotected, rodents or machinery may make collecting data impossible. There is no one "right" way to install the sensors, but we've tried to give you some good recommendations in our soil moisture installation video.
Questions? Visit our forum to ask a question, read the answers to common questions, and answers from Decagon researchers and other users. You can always call us to talk with our research scientists at 1-800-755-2751.
Or, you can always go directly to our Soil Moisture Sensors.