VERSION 1.2

JUNE 1994

VITEL, INC.

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VITEL, INC,

TABLE OF CONTENTS

PRINCIPLE OF OPERATION AND OVERVIEW...............................................................................................2

GENERAL PROBE CARE AND HANDLING......................................................................................................3

                Mechanical Stress....................................................................................................................................3

                Environmental Stress................................................................................................................................3

GENERAL INSTALLATION CONCERNS...........................................................................................................4

SPECIFIC IIYDRA PROBE INSTALLATION RECOMMENDATIONS.............................................................4

                Semi-Permanent Installation Procedure.....................................................................................................4

                One Measurement Installation...................................................................................................................6

ELECTRICAL CONNECTIONS AND MEASUREMENTS.................................................................................6

POWER REQUIREMENTS....................................................................................................................................6

MEASUREMENT CHANNELS.............................................................................................................................7

USE OF THE HYDRA PROBE WITH THE CR-10................................................................................................8

IR DROP AND LONG HYDRA PROBE CABLE LENGTHS................................................................................8

AVOIDING DAMAGE............................................................................................................................................9

MEASUREMENT UNITS AND ACCURACY.......................................................................................................9

SOIL MOISTURE....................................................................................................................................................9

                Calibration and Accuracy.........................................................................................................................10

SOIL SALINITY....................................................................................................................................................10

                Calibration and Accuracy.........................................................................................................................11

TEMPERATURE....................................................................................................................................................11

REAL AND IMAGINARY DIELECTRIC CONSTANTS.....................................................................................12

                Calibration and Accuracy.........................................................................................................................12

CONDUCTIVITY..................................................................................................................................................13

                Calibration and Accuracy.........................................................................................................................13

NOTE ON USE IN HIGH SALINITY SOILS.......................................................................................................13

CONVERTING RAW PROBE OUTPUT TO SOIL PARAMETERS....................................................................14

HYDRA.EXE..........................................................................................................................................................14

HYD_FILE.EXE.....................................................................................................................................................14

NOTE ON SOIL TYPE..........................................................................................................................................15

TROUBLESHOOTING...........................................................................................................................................15

USE OF PROBE IN FREEZING SOILS ................................................................................................................16

PRINCIPLE OF OPERATION AND OVERVIEW

The Hydra soil moisture probe determines soil moisture and salinity by making a high frequency (50 MHz) complex dielectric constant measurement. A complex dielectric constant measurement resolves Simultaneously the capacitive and conductive parts of a soil's electrical response. The capacitive part of the response is most indicative of soil moisture while the conductive part reflects predominantly soil salinity. Temperature is determined from a calibrated thermistor incorporated into the probe head.

As a soil is wetted, the low dielectric constant component, air, is replaced by water with its much higher dielectric constant. Thus as a soil is wetted, the capacitive response (which depends upon the real dielectric constant) increases steadily. Through the use of appropriate calibration curves, the dielectric constant measurement can be directly related to soil moisture.

Pure water, soil particles, and air all have a very low electrical conductivity. However, natural or man made salts (fertilizers, for example) present in a soil dissolve into the soil water. These dissolved salts dramatically increase the conductivity of the water and thus the soil. A measurement of soil conductivity, combined with the capacitive response, can be used to determine soil salinity.

The dielectric constant of moist soil has a small, but significant, dependence on soil temperature while soil conductivity varies strongly with temperature. The soil temperature measurement that the Hydra probe makes is used to remove most of the temperature effects.

The Hydra probe has three main structural components, a multiconductor cable, a probe head, and probe sensing tines. The direct burial multiconductor cable serves to connect the power and data recording site with the soil measurement site. The cable supplies power as well as providing the output data channels. All voltages carried on the cable are DC voltages. The probe head contains the necessary electronics to generate the 50 MHz stimulus and generate DC data channel voltages that reflect the soil's electrical properties. The three outer and one center tine form the sensing volume of soil which the probes measured electrical response reflects.

In addition to the probe hardware, software is included to rapidly and conveniently convert the raw data channel voltages to soil parameter such as water content, salinity, and temperature. Two programs are provided for use with IBM PC compatible computers running DOS which allow for both interactive and batch file processing of data.

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Mechanical Stress

Avoid putting undue mechanical stress on the probe. Do not allow the tines to be bent as this will distort the probe data. After removing the probe from the soil, any remaining soil should be removed with careful scraping. Do not place an object between the tines and pry to remove soil. Do not rock the probe in a side to side manner when installing as this will unduly stress the tines (as well result in a poor installation). While the probe is quite shock resistant, avoid dropping the probe onto a hard surface.

The cabling from the probe head is quite tough and is firmly anchored in the probe head. Nevertheless, pulling on the cable to remove the probe from soil is not recommended. The cable is direct burial cable and will tolerate long term burial in moist soil. The outer cable covering however is susceptible to abrasions and or cuts. Should the outer sheath be broken and the inner wires exposed the inner wire insulation should be inspected and repaired if necessary. The cable should then be sealed with a water-resistant flexible sealant such as RTV, and after drying, wrapped in a water-resistant tape. Duct tape will often work quite nicely. The free end of the cable should be protected from moisture or contaminates as water can wick down the cabling from this end.

Moderate scratches or nicks to the stainless steel tines or the PVC probe head housing will not affect the probe's performance. Very deep scratches or cracks in the probe head can breach this cover and expose the electronics contained in the probe to water and other contaminants. The inner probe electronics are permanently sealed in a water-resistant electrical potting compound to reduce the possibility of water induced failure of the probe. Common sense dictates that significant damage to the probe head be avoided.

The Hydra probe is designed to tolerate a field environment for an extended period. As such, the probe will withstand full water submersion indefinitely. The passivated stainless steel and PVC construction will protect the probe from corrosion in all but the most hostile environments. However, exposure of the probe or cabling to solvents or oils/grease or strong oxidizing or reducing agents such as concentrated fertilizer or cleaning agents is not recommended. Any such exposure should be followed as soon as possible with preferably soap and water cleaning to prevent damage or the contaminants from affecting the soil upon installation.

The probe will operate over a temperature range of -10°C to +65°C and give accurate measurements. Below -10°C, probe output may become unreliable but no damage will result. Operation of the probe at temperatures in excess of +65°C can result in permanent damage. The probe can be stored (or left in the ground) through a temperature range of ~0°C to +70°C. The probe can be left in soils over winter where freezing will occur.

In fact, the probe can be used to monitor the progression and extent of soil freezing. See the Use of Probe in Freezing Soil section for additional details.

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VITEL, INC,

GENERAL INSTALLATION CONCERNS

As with the installation of any soil moisture measuring instrument, there are two prime considerations: the location the probe is to be installed at, and the installation technique. First, the probe installation site should be chosen carefully so that the measured soil parameters are "characteristic" of the site. In a site with varying soil types, vegetative cover, topography, exposure to sunlight, etc., a single point measurement is highly unlikely to be representative of the site as a whole. A clear picture of soil parameters in a highly variable site will typically require many probes carefully positioned.

In more uniform sites with little topography, relatively uniform soil type and vegetative cover, common sense dictates probe placement. The probe should not be placed in the one spot bare of vegetative cover, near a boulder, etc. However, one should bear in mind that even in what appears to be a uniform site, will in general have significant variations in soil properties such as soil moisture and salinity.

Second, the installation technique should minimize disruption to the site as much as possible so that the probe measurement reflects the "undisturbed site" as much as possible. To effect this, the following guidelines should be observed. When installing the probe, use the smallest hole practical. The hole should provide adequate working room but should not be larger than necessary. Excessive foot or vehicular traffic near the site which will knock down the vegetative cover and produce soil compaction should be avoided. Remove soil and vegetation from the installation site and place it on a tarp, in order removed, so that after probe placement, the soil structure can be recreated. This is particularly important in soils with pronounced vertical gradation or soil horizons. Care should be exercised to attempt to repack the soil to the undisturbed density.

SPECIFIC IIYDRA PROBE INSTALLATION RECOMMENDATIONS

The probe can be installed in one of two manners, a "semi- permanent installation", and a brief one measurement installation. The "semi-permanent" installation is used for the long term monitoring of a particular site where the probe is left in the ground for a period of days to years and periodically interrogated. The brief one measurement installation is employed when the probe is used in a "roving manner", i.e. a site is selected, the probe is installed, a measurement is made and the probe is removed.

The semi-permanent installation has a number of advantages over the one measurement installation and is recommended. While the probe is mechanically quite strong, repeated installation and removal will age the probe particularly in rocky soils. The use of a one measurement installation technique to monitor a site can be quite labor intensive. The foot traffic and digging of installation holes can also markedly affect the site over time. In addition, since the temperature measurement is made in the probe head, a one time installation will typically produce temperature measurements that do not reflect the actual soil temperature. Correction factors to soil moisture and salinity will also typically be non-optimal.

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In a semi-permanent installation, the issue of site disruption is particularly relevant. The probe will be left in the ground for long periods of time and any disruption of the site will, over time, result in an altered soil state at the probe.

Besides observing the guidelines regarding not disturbing the site, specific recommendations for the installation of the Hydra probe are as follows. While the probe can be installed in any orientation, vertical or horizontal, a horizontal installation is recommended, particularly in locations near the soil surface or where strong soil moisture gradients are encountered. This is because the effective sensing volume is a cylinder approximately 2.5 cm in diameter and 6 cm in length bounded by the three outer tines, the probe head, and the "free" end of the tines. The resulting probe output parameters reflect an average value over this sensing volume. By installing the probe horizontally, the longest dimension of the probe sensing volume will be parallel to the soil surface and perpendicular to the direction of, typically, the strongest soil moisture gradients. This allows the probe to give the best approximation to a "point measurement". In addition, because the probe is made of materials with a thermal conductivity large compared to soil, a vertical installation can form a thermal pathway from the soil surface to soil at depth thus altering the soil in the probe. Even in relatively extreme cases, this is generally a concern only for probe placement at depths of 25 cm or less. One important advantage of the well localized sensing volume is that the probe can be installed very near the soil surface without a surface induced error. A horizontal installation with the tines covered with a centimeter of soil will give accurate measurements.

After digging the access hole as described earlier, a section of the hole wall, or bottom, should be made relatively flat. A paint scraper works well for this. The probe should then be carefully inserted into the prepared hole section. The probe should be placed into the soil without any side to side motion which will result in soil compression and air gaps between the tines and subsequent measurement inaccuracies. The probe should be inserted far enough that the plane formed where the tines join the probe head is flush with the soil surface. Do not insert the probe any further as this will result in soil compression within the sensing volume. If a rock is encountered do not force the probe. Try installing the probe in a spot immediately adjacent. Under no circumstances should a force in excess of 20 kilograms be employed during installation.

In hard, very dry, or rocky soils installation may be difficult. A Hydra Probe Jig is recommended in these instances. The probe jigger will make pilot holes for the probe and allow for easy installation in difficult soils.

After placing the probe in the soil, the access hole should be refilled as described earlier. Particularly in near soil surface installations, one should avoid routing the cable from the probe head directly to the surface. Usually, after probe installation the soil will undergo settling which may form air gaps between the cabling and the soil. Under certain conditions this will form water and thermal conductivity pathways not originally present in the soil. A horizontal cable run of 20 cm between the probe head and the beginning of a vertical cable orientation in near soil surface installations is recommended.

In soils where burrowing rodents are present, it may be desirable to shield the cable in conduit to prevent rodents from chewing through the cable. When conduit is used, particularly metal conduit, particular care should be exercised to avoid introducing water and thermal conductivity pathways.

Because the probe will be interrogated immediately after installation, site disruption is of much less concern except where long term intensive monitoring of a small area is anticipated, and where consequently, the cumulative effect of the many separate installations will alter the site. However, because the probe will be installed and removed many times, particular care must be used to avoid mechanically damaging the probe.

In a one measurement installation, the probe can be installed either by digging an access hole, or by installing the probe vertically from the surface if near surface soil parameters are desired. In both cases, a roughly flat surface should be made prior to insertion and the probe inserted as described in the Semi-Permanent Installation Section. If the probe is installed vertically from the surface, the measurement will reflect an average value over a sensing volume (described in the semi-permanent installation section) that may contain a strong gradient in soil properties.

Because the temperature measurement is made in the probe head, the temperature values are not generally accurate in a one time installation, particularly in very hot or cold air temperatures. Correction factors based upon temperature measurements will consequently be incorrect. Except in extreme conditions, the temperature correction for soil moisture is slight. The temperature effects on the calculated soil salinity temperature correction, temperature corrected imaginary dielectric constant, temperature corrected soil conductivity, and soil water salinity can however be significant. Avoid making one measurement installations where the air temperature differs markedly from the soil temperature.

Because of the mechanical stress repeated installation and removal places on the probe, care should be exercised to avoid damage particularly in rocky soils. The use of the Hydra Probe Jig in one measurement installations in all but the "softest" soils is strongly recommended.

The Hydra probe cable consists of seven color coded copper 18 AWG wires. Only six of the seven wires are currently used except in the case of probes with cable lengths of 50 feet or greater (see the "IR Drop and Long Hydra probe Cable Length" section).

The required electrical power and measurements are Simple to make. The following instructions should be followed to give the best results.

POWER REQUIREMENTS

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Although a supply voltage up to 30 volts can be used, a supply voltage of 7-12 volts is recommended for two reasons. First, since the current draw is independent of supply voltage, high supply voltages waste power. Second, the wasted power is dissipated in the probe head producing a rise in temperature. Although the dissipated power poses no risk to the electronics, the temperature measurement is made in the probe head and can be biased by this dissipated power. A lower supply voltage minimizes this effect. Typically, a 12V supply voltage will bias the temperature measurement by about +0.2°C after a minute. Steady state temperature elevations produced by a 12 volt supply are typically 1-5OC depending on the soil and moisture content. Since a measurement can be made in a few seconds or less, this temperature rise is inconsequential if a switched power supply is used. For this reason, as well as to conserve power, it is recommended that power be applied to the probe only when actually making a measurement.

MEASUREMENT CHANNELS

The other four wires (blue, brown, green, white) serve as analog data channels for the probe. The output consists of a O-5V DC signal. The voltages of the blue, brown, green, and white wires will henceforth be referred to, respectively, as V1, V2, V3, V4 (the numbers are in the same order as the alphabetical order of the colors).

V1, V2, V3 are used to determine the capacitive and conductive response, and hence water content and salinity, of the soil. V4 produces a signal that is used to determine temperature. All of these voltages are to be measured with respect to the ground used in the power supply (the black wire). The voltages can be measured using a Simple multi meter or a variety of data acquisition boards or devices. The only requirement is that the input impedance of the voltage measuring device be 1 megaohm or greater (a typical bench top multimeter has an input impedance in excess of 10 megaohm). This requirement is usually easily satisfied.

In some commercial data acquisition systems, the inputs to the A/D converters are protected by transorbs (or other components). Although typically this will not cause problems, in some Systems these protection circuits can drop the effective impedance enough to distort the voltage measurement. This can easily be ascertained by using the following procedure. Connect the probe to power and connect the probe output wires to the data acquisition device. With a separate multimeter, measure one of the four voltages while connected to the data acquisition system and then immediately disconnect the wire from the data acquisition system and use the multimeter to repeat the measurement. The two values should agree to within a few millivolts. Any greater deviation is indicative of too low an input impedance and will result in a degradation in the measurement accuracy.

In an automated data acquisition system, power to the probe should be provided to the probe for at least two seconds prior to reading the data channels in order to insure the output levels are settled.

V1, V2, V3, and V4 should then be read either Simultaneously or if sequentially, within 0.1 seconds of each other, and, preferably in this order. If the voltages are to be read by hand using a multimeter or if the data acquisition system requires greater than 0.1 seconds to poll an input channel, power should be applied for approximately 10 seconds before measuring the output voltages. For optimal accuracy, voltages should be read to the nearest millivolt. In data acquisition systems this corresponds to a 12 bit A/D conversion over the range O to 5 volts.

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Campbell Scientific's CR-lO is widely used in automated data collection installations. Unfortunately, the CR-10 is limited to measuring directly +/- 2.500 volts. However, there are a number of ways to connect the Hydra Probe to a CR-10. While the four channels of the Hydra Probe are strictly speaking 0-5 volts, the first three channels V1, V2, V3 are typically 2.5 volts or less. Furthermore, V1 will be greater than or equal to both V2 and V3. In addition, voltage V1 will drop with increasing tldielectric loadlt, i.e., V1 will decrease when probe is placed in a soil (from air) and as the soil moisture increases. Thus under almost all conditions, the CR-10 A/D ports can be used to directly measure V1, V2, and V3. To check this, power the Hydra Probe (in air) and measure V1. If V1 is 2.500 volts or less, V1, V2, and V3 will always be within the CR-IO's input range. If V1 in air is greater than 2.500 volts, place the probe in a soil at the lowest soil moisture value at which soil moisture data is desired. If V1 is less than 2.500 volts, the Hydra Probe channels V1, V2, and V3 can be used directly with the CR-lO.

The analog channel V4 will typically range from 1 to 4 volts with varying temperature. For reliable operation with the CR-1O, a voltage division circuit must be used. The resistance of the voltage division circuit should be at least two megaohm. It is recommend that the circuit use two precision 1 Megaohm resistors in series to ground with the voltage V4 being applied at the top of this circuit and the input to the CR-IO coming from between the two resistors. This circuit will divide V4 in half insuring compatibility with the CR-IO.

If desired (or necessary) a voltage division circuit analogous to that required for V4 can be used with V1, V2, and V3. It is very important that the voltage division circuit used is very precise particularly with V1, V2, and V3. Ideally, the precision of the voltage division circuit should be +/- 0.05% or better over the expected temperature range. Of course, if a voltage division circuit is used, before data processing, the voltages should be corrected to reflect their values before voltage division.

IR DROP AND LONG HYDRA PROBE CABLE LENGTHS

In order to achieve the highest accuracy possible with the Hydra Probe, the user should be aware of issues related to use of the probe with long cable lengths and the effect of resistive drops.

Since the Hydra Probe draws a significant amount of current (max. 40 mA), there is a voltage drop that occurs along the power and ground lines (the voltage drops across the analog channel lines are insignificant because of the extremely low current draw of a good" voltage meter or A/D system). This results in the ground level at the probe being slightly higher than the ground at the point where the analog channel voltages are measured and a subsequent loss of probe accuracy due to a distortion in the measured voltages.

The Hydra Probe cabling consists of 18 gauge copper wiring to reduce this effect. Probes of length 50 feet or greater, utilize the yellow wire in addition to the black wire for the ground connection. Probes supplied with connectors will have this connection made in the connector housing. When using the "bare wires" probe, join the black and yellow wire together to form the ground connection (the Hydra Probe in the bare wires version, will be shipped with the yellow and black wire connected. This approach will limit IR drop errors in the analog channel voltages to one millivolt over Hydra Probe cable lengths up to a full 100 feet.

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If the Hydra Probe is not connected directly to a volt meter or A/D system, any cabling used to connect the Hydra Probe cable to the voltmeter should limit IR drops on the ground line to no more than one millivolt. This is particularly important if more than one Hydra Probe is being used or if another high current drawing sensor or system is on Simultaneously. Thus it is highly desirable that the ground line on the Hydra Probe be tied directly into the ground (or reference) input for the voltmeter or A/D system with as little intervening cabling as possible. In systems with a large overall current draw, limiting IR drops to one millivolt with intervening cables can be difficult. For example, if the overall system draws one amp and ten feet of 10 gauge copper wire is used to connect the AID or voltmeter ground to the ground lines on the Hydra Probe, the IR drop will be approximately 10 millivolts which is unacceptably high.

AVOIDING DAMAGE

Avoiding damage to the Hydra probe electronics is relatively Simple. Observe the power supply limitations mentioned earlier. Avoid applying voltages to the measurement channels. While the electronics are fairly tough, applying voltage, particularly large voltages to the output wires can possibly damage the probe. Try to minimize spikes, transients, electrostatic discharges that may be applied to either the power or measurement wires. Direct lightning strikes may cause damage particularly if the cable is struck.

SOIL MOISTURE

The output of the data Conversion program is water fraction by volume (wfv). For example a water content of 0.20-wfv means that a one liter soil sample contains 200 ml of water. Full saturation (all the soil pore spaces filled with water) occurs typically between 0.3-0.45 wfv and is quite soil dependent.

There are a number of other units used to measure soil moisture. They include % water by weight, % field capacity, % available (to a crop), and tension (or pressure). They are all inter-related in the sense that for a particular soil, knowledge of the soil moisture in any one of these units, allows the soil moisture level in any of the other unit systems to be determined. It is important to remember that the conversion between units can be highly soil dependent.

The unit of water fraction by volume (wfv) was chosen for the Hydra probe for a number of important reasons. First, the physics behind the soil moisture measurement dictates a response that is most closely tied with the wfv content of the soil. Second, without specific knowledge of the soil, one can not convert from wfv to the other unit systems. Third, the unit wfv allows for direct comparison between readings in different soils. A 0.20 wfv clay contains the same amount of water as a 0.20 wfv sand. However, the same thing can not be said about the other measurement units. For example to use the unit common in tensiometer measurements, a one Bar sand and a one Bar clay will have vastly different water contents.

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The wfv unit can also be readily used to estimate the effects of precipitation or irrigation. For example consider a soil that is initially 0.20 wfv, and assume a 5 cm rainfall that is distributed uniformly through the upper one meter of soil. What will the resultant soil moisture in the upper one meter of soil be? 5 cm is 0.05 of one meter so the rainfall will increase the soil moisture by 0.05 wfv to result in a 0.25 wfv soil. For other units, this calculation can be much less straightforward, particularly when soil moisture is measured as a tension.

If one prefers to operate in another system of units, a direct relationship between the Hydra probe determined soil moisture unit and any of the other units can be established for a particular soil.

While the design of the Hydra probe and the data reduction algorithms were carefully chosen to minimize the effects of variations in soil type on measurement accuracy, there are some important things to keep in mind. The response of the Hydra probe to soils with identical wfv values will vary slightly from soil to soil. Typically, without any knowledge of the soil type, the accuracy is +/- 0.03 wfv. For example a measured value of 0.20 wfv could correspond to an actual wfv of 0.17 to 0.23 depending on soil type. With a crude knowledge of soil type (sand, silt, clay classification), the uncertainty drops to typically +/- 0.015-0.020 wfv to give a range of soil moisture for our example of 0.18-0.22 wfv. If a soil specific calibration for the particular soil is performed, the uncertainty drops to less than +/- 0.005 wfv, and for our example, the range is down to 0.195-0.205 wfv. The remaining uncertainty is predominantly due to inaccuracies in the calibration process and the basic soil electrical properties measurement.

It is important to note that because the reproducibility of the Hydra probe is typically +/- 0.003 wfv, or better, the ability to measures changes and characterize a particular soil is very good. For example, if we have no knowledge of soil type and we have a soil moisture reading of 0.20 wfv, then we have a range of actual soil moisture values that this reading may reflect as discussed in the previous example. However, suppose after a one month period accompanied by periods of rainfall and drying we again have a Hydra reading of 0.20 wfv. We can be assured that the soil moisture content of the soil is identical to that obtained one month prior to within the reproducibility of the measurement, i.e. 0.003 wfv. Simply stated, the relative accuracy is higher (by about an order of magnitude) than the absolute accuracy.

Soil salinity is expressed in units of total Sodium Chloride (NaCl) burden (grams) per unit volume (liter). The salt content of the soil water can be calculated by multiplying the soil salinity by the wfv. For example, a soil that has a soil salinity of 0.1 g NaCVliter and a wfv of 0.20 will have a (1/0.2) X 0.1 g NaCVliter=0.5 g NaCl/liter dissolved salt in the soil water. It is important to keep the distinction clear. Barring any salt migration in a soil, the soil salinity (or total salt burden) stays fixed. As the soil wets and dries the soil water salinity will, respectively, drop and rise, as the fixed salt content is diluted or concentrated.

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It is also important to note that salinity is expressed in terms of a NaCl burden. In actual soils, a wide variety of different natural and man made salts may be present in soil in addition to NaCl. The total burden (by weight) will then in general be different from the value calculated based on NaCl. For example, the conductivity of Potassium Nitrate (KNO3) by weight in water is approximately 67% of the conductivity of an equal weight of NaCl. Thus, a NaCl burden of 0.1 g/liter corresponds to a burden of (0.1 g KNO3/0.67 g NaCl) x 0.1 g NaCl/liter=0.15 g KNO3/liter.

In order to be consistent with other soil salinity measuring techniques, the temperature corrected soil water conductivity is also calculated. This is a measure of the conductivity of the water in the soil pores as influenced by the dissolved salts. It should be noted again that for a soil with no salt migration, as the soil water content rises, the salts present will be diluted resulting in a falling soil water conductivity. The temperature correction amounts to calculating the effective conductivity at 25°C. As with all other conductivity measurements, the unit is the standard MKS unit S/m or mhos/m.

The two different salinity measurements, soil salinity and temperature corrected soil water conductivity, have very different absolute accuracies and calibration requirements. As we have seen earlier in this section, the soil salinity expressed as a NaCl burden will be strictly accurate only in soils that have a salinity due to NaCl. Thus the absolute accuracy of this measurement can be quite poor if the soil salinity is due to a salt (or mixture of salts) with a very different conductivity by dissolved weight, i.e. the salts presents in the soil will weigh more or less than the indicated NaCl weight. However, the relative accuracy and reproducibility of the measurement is much better. Regardless of the nature of the salts present in the soil, the soil salinity will track changes in these salt levels. For a soil whose salinity is due to NaCl, the accuracy of the soil salinity value is +/- 20%. For soils of unknown salt composition, the accuracy is approximately -40%,+/- 100%. Reproducibility- is typically +/- 2%.

The other salinity measurement, temperature corrected soil water conductivity (referenced to 25°C), is independent of the nature of the salts present in the soil. As a practical matter, soil salinities are most often measured by measuring the conductivity of a soil/water paste or slurry followed by appropriate dilution corrections. The soil water conductivity measurement is designed to allow comparisons between these two salinity measurement techniques. The accuracy of this measurement is +/- 20%. The largest source of uncertainty is due to variations in the response of particular soil types. The reproducibility of this measurement is typically +/- 2%.

The accuracy of both soil salinity indications, is best in sand or silts, and at moderate to high moisture contents. At low soil moisture levels, wfv of 0.10 or less, the salinity measurements will read low. Both soil salinity measurements accuracies can be improved to about +/- 5% with soil specific calibration.

TEMPERATURE

The temperature measurement is in degrees Celsius (or Centigrade). The standard accuracy is +/0.6°C throughout the full operating range of -10°C to +65°C. A high accuracy option is available with an accuracy of +/- 0.1°C. Reproducibility is to +/- 1°C for the standard temperature option

REAL AND IMAGINARY DIELECTRIC CONSTANTS

The measured raw electrical parameters determined by the Hydra soil probe are the real and imaginary dielectric constants. These two parameters serve to fully characterize the electrical response of the soil (at the frequency of operation, 50 MHz). These are both dimensionless quantities.

Most people are more familiar with the concept of a real dielectric constant. The imaginary dielectric constant is directly related to the Conductivity of the medium, the higher the imaginary dielectric constant the higher the Conductivity. The soil conductivity is also calculated by the two computer programs mentioned in the Converting Raw Probe Output to Soil Parameters section for those interested in this quantity.

Because both the real and imaginary dielectric constants will vary somewhat with temperature, a temperature correction using the measured soil temperature is applied to produce temperature corrected values for the real and imaginary dielectric constant. The temperature correction amounts to calculating what the dielectric constants should be at 25°C.

The calculated water content is based on the temperature corrected real dielectric constant while the soil salinity, soil water conductivity, soil conductivity, and temperature corrected soil conductivity are all based upon both the temperature corrected real and imaginary dielectric constants

Calibration and Accuracy

Since all soil measurement parameters (except temperature) are determined from the real and imaginary dielectric constant measurements, great care has been taken to insure the accuracy of these measurements. -The Hydra probe has been carefully designed to need no further calibration in dielectric constant measurements. All necessary calibrations are contained in the accompanying software and are independent of the particular Hydra probe being used, i.e. the probes are interchangeable with no individual probe calibration constants needed.

The accuracy of the real and imaginary dielectric constants (as measured at the prevailing soil temperature) are typically +/- 1% or 0.5, whichever is greater. Particularly in cases where one component is much larger than the other (5 times or more), the smaller component will have an accuracy that will be degraded. In general, the accuracy of the smaller component will be uncertain by 3-5% of the value of the larger value. Accuracy can also be degraded when used in extremely saline soils (see section Note On Use In High Salinity Soils).

CONDUCTIVITY

There are three conductivity measurements included in the computed soil parameters: soil conductivity, temperature corrected soil conductivity, and temperature corrected soil water conductivity. The unit of measurement is the MKS unit S/m (mhos/m). Temperature corrected soil water conductivity is covered in the Soil Salinity section and will not discussed here.

The soil conductivity and temperature corrected soil conductivity are both based directly on, respectively, the imaginary dielectric constant and the temperature corrected imaginary dielectric. constant. The accuracy of these two conductivity values is thus directly tied to the accuracy of the respective dielectric constants. Typical accuracy in the soil conductivity (as measured at the prevailing soil temperature) is +/- 0.0014 S/m or +/- 1%, whichever is greatest. However, in cases where the imaginary dielectric constant is small compared to the real dielectric constant, the accuracy will be less. See the section on Real And Imaginary Dielectric Constants. The accuracy of the temperature corrected soil conductivity is +/- 0.0014 S/m or +/- 5% over the temperature range of 0°C to 35°C.

NOTE ON USE IN HIGH SALINITY SOILS

In very saline soils measurement accuracy of a number of parameters can be degraded. When the imaginary dielectric constant exceeds the real dielectric constant by a factor of two or greater as it will in very saline soils, the accuracy of the real dielectric constant will be degraded. The accuracy of the soil salinity and soil water salinity values will also be degraded, albeit to a lesser extent. The soil conductivity and temperature corrected soil conductivity accuracy will not be degraded. In extremely saline soils where the imaginary dielectric constant is in excess of five times the real dielectric constant, or the imaginary dielectric constant exceeds 150, the real dielectric constant and salinity measurements become increasingly inaccurate.

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The raw Hydra probe output voltages are not related to the processed soil data (water content, salinity, temperature) in a Simple fashion. In order to allow the user to easily convert these raw values, two programs that will run on an IBM PC compatible computer, as well as two sample data files, are included in a sleeve on the back cover of the manual.

The first program, HYDRA.EXE, will prompt the user for a soil type and the four measured voltages V1, V2, V3, V4 (these voltages are defined in the Measurement Channels section). The program will then generate the following output, real and imaginary dielectric constant, temperature, temperature corrected real and imaginary dielectric constants, water content, soil salinity, soil conductivity, temperature corrected soil conductivity, and temperature corrected soil water conductivity. See the Units and Measurement Accuracy section for definitions of these values. The program will continue to operate until the user exits by typing <Control> C, i.e. holding down the 'control' key while Simultaneously pressing the "C" key.

The second program, HYD_FILE.EXE, is designed to operate on a PC file of raw input data, and output a file consisting of the processed data. This program is primarily designed to allow the user to quickly process large amounts of data from automated data collection Systems.

The input data file should have the following format for each of the records:

STATION#<Space>SOlL<Space>V1 < Space>V2<Space>V3<Space>V4

where STATION# is a short integer which serves as a label for the data point, SOIL is a short integer that indicates the soil type (l--sand, 2--silt, 3--clay), V1, V2, V3, V4 are all floating point numbers which are the measured probe voltages as defined earlier. An example of the input data file, DATA_IN, is included on the disk. The <Space> indicates that the values are delimited by a single space. There is no limit to the number of records in the data file.

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successfully converts the input file, the program will terminate displaying a Conversion Completed" message.

The output file will have the following record & structure:

STATION#<Space>SOIL<Space>ER<Space>EI<Space>TEMP<Space>ER_CO<Space>EI_COR< Space>WATER<Space>SALlNITY<Space>SOIL_COND

<Space>SOIL_COND_COR<Space>WATER_COND_COR

where STATION# is a short integer (the same value as the input file), SOIL is a short integer (the same value as the input file),ER is a floating point value (the real dielectric constant), E1 is a floating point value (the imaginary dielectric constant), TEMP is a floating point value (the temperature), ER_COR is a floating point value (the temperature corrected real dielectric constant), EI_COR is a floating point value (the temperature corrected imaginary dielectric constant), WATER is a floating point value (soil moisture), SALINITY is a floating point value (the soil salinity), SOIL_COND is a floating point value (the soil conductivity), SOIL_COND_COR is a floating point value (the temperature corrected soil conductivity), and WATER_COND_COR is a floating point value (the temperature corrected conductivity of the water in the soil. All of the output parameters computed using HYDRA.EXE are also produced by HYD_FILE.EXE. For a discussion of definition and units employed see the Measurement and Units section. A sample data output file DATA_OUT is also included on the disk and was (and can be) generated using the file DATA_IN.

In both of the two programs, the user is required to enter a soil type. The choices are sand, silt, or clay. Although this Simple soil classification is rather crude, it has two important advantages. First, the classification is relatively easy to make in a field setting by persons of limited experience in soil science. Second, the electrical response of soils to moisture and salinity can be classified fairly well by this Simple scheme.

If the user is uncertain as to the soil classification (or has no knowledge whatsoever as to the soil type), it is recommended that a soil type of silt be chosen. The choice of soil type will affect only the calculated value of soil moisture. All other values are unchanged.

While the Hydra probe is designed to work reliably for long periods in the field, if the probe data does not seem reasonable or a complete failure is apparent the following procedure will help to quickly identify the problem.

First, if an automated data collection system is being employed, disconnect the data collection system and apply power to the probe. Make sure the applied power is between 7-30 volts DC; supply voltages outside this range will cause erratic or erroneous output. After applying power, read the measurement channels with a voltmeter. If they differ significantly from the data acquisition system values (by more than a few millivolts) the problem most likely lies in the data acquisition system. Either it is malfunctioning or the input impedance is too low (see the remarks in the Measurement Channels section).

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Second, connect an ammeter in series with the probe power supply and measure the current. It should be in the range of 30 to 45 mA. If it is zero, most likely the cable has been damaged and one or more of the wires cut (this can arise well after installation if burrowing rodents are present). If the current is in excess of 45 mA, the cable is most likely damaged and some of the wires have been shorted together.

Third, check the resistance between the ground (black wire) and all of the remaining wires except the red power line. They should all be less than 300K. A value in excess of this is indicative of a partial or total break in one or more of the wires.

Fourth, check the resistance between all possible pairs of conductors. They should all be in excess of 5K. If any resistance is below this value, most likely the cable has been damaged and one or more wires have been shorted together.

If the any of the second, third or, fourth, checks has failed, the entire cable should be inspected for signs of damage and repaired if necessary. If this fails to fix the problem, or there is no visible cable breaks, the internal probe electronics have been damaged and the probe needs to be replaced. If the probe has passed all of these four checks, precede to the fifth check.

Fifth, remove the probe from the soil, rinse off any soil, and place it in a small container of distilled water at room temperature. Do not use a metal container and hang the probe so it is at least 2 cm from any side or the bottom. Apply power and read the output voltages. Run the computer program to perform data reduction calculations. The real dielectric constant should in the range 70-85 and the imaginary dielectric constant should be less than 5. If this check passes, the probe is almost certainly operating correctly. If it fails, and the cable is not damaged nor the probe tines obviously bent, the internal electronics are damaged and the probe needs to be replaced. This last check is particularly useful if the failure is not "catastrophic" but a slow degradation of probe accuracy is suspected.

USE OF PROBE IN FREEZING SOILS

As mentioned in the Environmental Stress section, the Hydra probe can be left installed in soils subject to freezing. In addition, during the freezing process, the Hydra probe can be used to determine a number of important parameters. As a soil begins to cools to 0°C, the moisture present in the soil may begin to freeze. However, super cooling to -1°C to -2OC, may occur before the water present in a soil begins to freeze. Hence, a Simple temperature measurement is not sufficient to determine whether a soil has begun to freeze. In addition, a Simple temperature measurement can only possibly detect the beginning of soil freezing and cannot resolve the fraction of the soil moisture that is frozen since the temperature stays essentially fixed while the soil freezes.

However, as a soil freezes, the electrical properties of the soil change dramatically. The real dielectric constant of water drops from near 88 to approx. 4 as the water freezes. The real dielectric constant of the soil will also reflect the fall in the real dielectric constant of the water present. For example, a moderately wet soil with a real dielectric constant of 20 will undergo a drop in the real dielectric constant to approx. 3-4 as the soil freezes.

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Roughly, the drop in the real dielectric constant is proportional to the fraction of the soil water that is frozen. In the example mentioned earlier, a real dielectric constant of 12 would be indicative of approximately half the water content of the soil being frozen. In addition to the real dielectric constant falling as freezing occurs, the imaginary dielectric constant will also fall.

The marked change in the electrical properties of freezing soil make it relatively Simple to distinguish soil freezing from a drop in soil moisture. One should strongly suspect soil freezing is occurring when either l)the soil temperature is near 0°C or below, or 2) the real dielectric and imaginary dielectric constants both begin to fall.

It should be noted that when freezing occurs in the soil, calculated soil moisture values, temperature corrected real and imaginary dielectric constants, temperature corrected soil conductivity, soil salinity and soil water conductivity will lose their meaning as they are predicated on the water present in the soil being a liquid. The raw electrical parameters such as dielectric constants and soil conductivity, as well as temperature, retain their relevancy. As a practical matter, particularly when the Hydra probe is installed at a depth in excess of 50 cm, once freezing commences the water present in the soil remains fairly fixed. Thus the last measured soil moisture value obtained before freezing is likely to be a good estimate of the water content of the frozen soil.

Because of the rugged design of the Hydra probe, and wealth of provided data, the probe is suitable for use in a number of media besides soil. For instance, it can be used to measure the conductivity and temperature of stream, lakes, or agricultural runoff water.

Before using the Hydra probe in a medium besides soil, one should carefully review the General Probe Care and Handling section to insure that the intended application will not result in damage to the probe. In additiori, if the probe is to be used in a highly conductive material (greater than 0.40 S/m) some degradation in accuracy will occur. Use in media with conductivities greater than 0.80 S/m is not recommended. The same measurement accuracy effects mentioned in the Note On Use In High Salinity Soils section apply.

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Office phone 208-754-4704

John & Bob (software techs)

208-322-2717