HydraProbe FAQs

Here are 7 questions and answers about using the Stevens HydraProbe.

Q: What is the difference between the terms “real dielectric constant” (RDC) and the “real dielectric permittivity”?

A: The terms real dielectric constant and real dielectric permittivity are often times used interchangeably and in a matter of speaking, are synonyms to one another. The RDC/permittivity is a physical parameter that represents the amount of electric field a material can store. It is related to capacitance. The real dielectric permittivity is the parameter that soil moisture calibrations are based on.

Technically speaking, the term “real dielectric constant” is used when the parameter is constant, such as pure water at 20 degrees C, or a pure solvent or an electrical component such as a capacitor.

In soil however, the real dielectric constant is never “constant”. And in soil it becomes a complex number with a real and an imaginary part. Therefore the RDC of soil is called the “permittivity” i.e. the ability of a material to permit the electric field.

So if it is pure water at a specified temperature, you can call it the RDC, if it is soil, call it the real dielectric permittivity.

Q: Do I need to calibrate the HydraProbe soil sensor?

A: In general, the HydraProbe doesn’t need to be recalibrated in most soils regardless of texture or soil type. There are of course some exceptions. The factory general purpose calibration called LOAM now appears in a number of scientific publications and has been found to be fairly accurate in most naturally forming and agricultural soils. The HydraProbe factory calibration was developed by the US Department of Agriculture ARS, scientifically peer reviewed and appears in the Vadose Zone Journal. About 20 soil samples were gathered from across the US representing a range of soil textures and morphologies. A calibration was made for each soil sample and the coefficients for each calibration were averaged.

There can be a lot of confusion about the nature of a calibration of a scientific instrument. In some cases, each sensor needs to be calibrated every time you use it, such as a pH meter in a laboratory. 
In the case of the HydraProbe, each HydraProbe is tuned to measure the dielectric properties exactly the same way for every probe; therefore a calibration for the HydraProbe is a polynomial that would work with any HydraProbe. In other words, the soil moisture calibration for the HydraProbe is the relationship between the dielectric properties of the soil and the water content. It is not inter-sensor dependent and will not become uncalibrated.

Creating a new calibration can be time consuming and messy, therefore the user needs to balance the need for more accuracy with the amount of effort it would take to collect good data to curve fit a new calibration.

Q: What is soil respiration?

Soil respiration is the gas exchange between soil and the atmosphere. Soil can store large amounts of carbon as plants die and decay, building up the topsoil with organic material. Once the vegetation on the soil breaks down to a point where it becomes part of the top soil or A horizon, the continual break down and oxidation slows to a steady state. If the soil is tilled for agricultural production, the carbon in the soil becomes suddenly and instantly exposed to air, and moisture, and becomes more accessible to aerobic bacteria and fungi. Once the organic material in newly tilled soil begins this accelerated breakdown, carbon dioxide and methane are released into the atmosphere. The rate at which these greenhouse gases are released is determined by a number of processes, but are highly correlated to the soil moisture and temperature.

Carbon dioxide is exchanged between plants and the atmosphere on a daily basis as photosynthesis drives cell growth during the day and subsides at night. Wind also plays a role as the main transport mechanism. Wind doesn’t travel in a straight line, but rather it is constantly curving or rotating into eddies. Eddy Covariant flux of greenhouses gasses in industrial or agricultural areas is now a major area of research addressing ecosystems, climate change, water balance and hydrological applications.

AmeriFlux is a network of managed sites measuring ecosystem CO2, water, and energy fluxes in North, Central and South America and many AmeriFlux sites rely on the HydraProbe Soil Sensor for the soil moisture data. The AmeriFlux network was launched in 1996 and helps answer important questions about the earth’s carbon and energy budgets. More information about AmeriFlux can be found at http://ameriflux.lbl.gov/

Q: Do I need to make a temperature correction to my soil moisture data?

In general, for most soils the HydraProbe soil sensor does not need a special temperature correction for soil moisture. The dielectric response to temperature is highly soil specific, therefore, if you think that your particular soil needs a temperature correction, you should first perform a test. You can test this by putting a HydraProbe in a soil sample while sealing it up to keep the soil moisture of the sample constant. Next you can look at the HydraProbe readings from the sealed up core at temperatures ranging from 2 to 30 degrees Celsius. If the soil moisture reading changes significantly, you can make the decision to curve fit a temperature correction.

The HydraProbe’s impedance-based soil moisture measurement is less sensitive to temperature than other technologies such as TDR, and capacitance based soil sensors, because it more fully characterizes the reflected energy distribution. It is also important to note that soil temperature does not fluctuate to the same extent as air temperature and at deeper depths such as 1 meter deep, the soil temperature will fluctuate very little. From our experience, in more than 99% of deployments the HydraProbe has not needed a special temperature correction.

Q: Why does my soil moisture data look “funny”?

There are times when a perfectly good HydraProbe will output data that needs to be examined more closely.

Every site is different and most of the time there is nobody that knows more about the site than the person that installed the soil sensor. If you experience noise, data dropouts, or other data anomalies, we have a checklist that can help identify the problem.

  • Check the wiring. If you have wires crossed, the SDI-12 bus can be disrupted. Check to see if the wire is snug in the terminal block. Sometimes the screw terminals can get stripped and will not tighten properly.
  • Check the voltage with a handheld voltmeter. The SDI-12 bus should have 12 volts. You can also put the leads to the voltmeter in series with the red wire on the bus and look at the current draw. While idle, each Hydra Probe will draw 1 mA.
  • Check the probes separately in an SDI-12 Transparent mode. Send a “aM!” command followed by “aD0!” in transparent mode where “a” represents the unique address of the probe. This eliminates errors that are caused by the logger’s programming. Forcing the probe to take a measurement manually in this manner is a good exercise because it can flush out other communication problems with the bus. An excellent troubleshooting tool to independently troubleshoot SDI-12 probes in the field is the Stevens SDI-12 Xplorer. The Xplorer is an SDI-12 to USB adapter and is perfect for troubleshooting any SDI-12 sensor.
  • Noisy data. Below are some of the leading causes of noise.
    • Bioturbation is common in soil. This could be a tree root growing around the sensor, a burrowing animal could be digging around the sensor causing a void space around the tines. Sometimes you will need to dig the probes up and reinstall them somewhere else.
    • Do nothing and the noise may go away on its own. When did the noisy data start? Did it just start being noisy one day? Or is this a new installation? Is the noise off and on? Try to note the history of the noise. While the tine assembly goes into undisturbed soil, the head and cable of the probe are in disturbed soil. The soil in the pit may need to settle. It is also important to be careful back filling the hole after an installation. Put the soil back into the hole the way it came out, pack the soil in the pit as you are filling it back in with a piece of wood for every 20 to 30 cm of soil being filled back in. You may want to dip the cable near the head of the probe so that the cable is not a conduit for water.
    • Scaling. Is the data really noisy? If you log data every 24 hours and have a whole years’ worth of data on the same plot, it will look noisy. But the noise is in fact due to the time step and the range of the plot. Also, data will look noisy if it is zoomed in showing 4 or 5 places beyond the decimal point. If the data looks noisy, look at the way it is plotted and the actual magnitude of the fluctuations. Most of the time noise isn’t really noise but can look like noise the way it is plotted.
    • Sometimes the soil ped will not wet up evenly and the HydraProbe may have been placed between two peds. After a rain event, water will begin moving downward and it will take the path of least resistance first. This often times is between the peds of the soil. A ped is an individual aggregate of soil that forms from drying a wetting cycles. Peds can be as big as baseballs and can have clay films that can form around the ped. They can have different shapes depending on the soil type. Some ped shape types include angular, prismatic and columnar. In a shrink/swell clay, the gaps between ped can get huge (2 cm). If the soil moisture looks jumpy following a rain event, you might be capturing the hydrology of the ped structure of the soil.
    • Microclimates. Look for trees intercepting incoming water. Slope could be a factor. Perhaps a tree fell or something got put on top of the soil over the location of the soil probes.
    • A person or animal might have tripped on the cable yanking the sensor out of position.
  • A perched water table or hardpan will hold water up keeping the area around a soil sensor wetter than normal. Also, water can be drawn upward from the water table by capillary forces. While the sensor is above the water table as it is measured in a piezometer, the soil will still be saturated from the capillary rise of water.

Q: I put a HydraProbe in water and the soil moisture reading is only 80%! Why?

The HydraProbe soil moisture calibration is mathematically curve fit to represent water in soil, not water in water. The moisture reading in water with the default calibration is over the calibration range. The dielectric/water relationship is typically a third order polynomial or exponent function below 60%. Most agricultural soils, mineral soils and woodland soils don’t saturate with water beyond 50%. Some organic soils can hold more than 60% water. If you need moisture in an organic soil that gets over 60%, contact Stevens for an organic soil calibration.

Q: I have a really old HydraProbe that I want to reuse for a new project, how do I tell if a HydraProbe is measuring property?

Clean the head and tine assembly of the HydraProbe and place it in distilled water so the head is completely submerged. Take a reading. In the SDI-12 “aM!” measurement set, parameter 6 (K) is the real dielectric permittivity. This parameter should be about 79. EC is the second parameter in the string and should be close to zero (0.00 to 0.01 S/m).

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