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Soil water sensors reveal subsoil conditions – Part 4: Frequency delay measurements by means of capacitance and heat pulse measurements

by Frikkie Koegelenberg, Pr Eng and Hendrik Jordaan

Frequency delay measurement is based on the measurement of the dielectrical constant of soil or materials. The frequency delay measurement is also called the radio frequency (RF) capacitance technique.

We thank the ARC Agricultural Engineering in South Africa for making their manual on soil water sensors available to the readers of ProAgri Zambia.

Capacitance technique is usually referred to as it measures the soil’s capacitance. Two, three or four electrode probes are inserted in the soil. The probes are collectively connected to a test pin and the probes of some types (Delta-T-probe) can screw in if it has to be replaced. The soil acts as a dielectricum by completing the capacitance circuit, which forms part of the feedback circuit loop of the high frequency transistor-oscillator. Highfrequency radio waves of between 100 and 150 MHz are also pulsed through the capacitance circuit. A natural resonant frequency is thereby established, which is dependent on the capacitance of the soil. The soil’s capacitance is related to the dielectricum constant, which is created by the geometry of the electrical field around the electrodes. A number of commercial apparatus using this technique are available, namely: Netafim Flori; SDEC’s HMS 9 000, Delta-T’s ThetaProbe and the Aquaterr probe.

The PR2/4 from Delta-T devices can
take moisture readings on four levels.
Photo: delta-t.co.uk.

These capacitance probes are installed in the same way as TDR wave conductors, in the side of a profile hole.

Some manufacturers arrange the electrodes around a cylindrical test probe (rod) at different distances. The test probe is then lowered into an uPVC access tube into the soil. Soil water contents are then determined at the different depths according to the electrode spacing. The depths vary in increments of 100 to 200 mm, which can be specified by the user at some types and installed during manufacturing. With other types, such as the ADCON C-probe and Sentek’s EnviroSCAN, the user can change the spacing.

The Troxler Sentry 200-AP apparatus and the DIVINER 2 000 (Sentek) uses an uPVC access tube similar to that of a neutron moisture meter to determine the soil water content at different depths. The test probe of this type of apparatus fits tightly into the access tube. It ensures that the electromagnetic signal is radiated effectively. In stony soils, a paste is made from the soil. The access tube is then installed in the paste in the soil so that no vacuum exists around the access tube. The test probe takes readings while it is lowered into the soil. A natural resonant frequency or frequency movement between the radiated and received (reflected) frequency is measured by the test probe. The Diviner apparatus measures soil water content in volumetric units at preprogrammed depths while it is lowered into the soil. The data is then shown and stored on a data logger. The data can also be downloaded onto a computer and various analyses can be executed thereon.

The PR2/4 from Delta-T devices can take moisture readings on four levels.
Photo: delta-t.co.uk.

The apparatus must be calibrated for the different soils for each depth. The calibration can also be made against a calibrated neutron water meter.

The change of gross density with depth also requires calibration at every depth where the soil water content is to be determined. The sphere of influence of measurements (in the absence of vacuum) is not influenced by soil water content and is approximately 100 mm vertical and 250 mm horizontal in diameter. The apparatus is very accurate if it is correctly installed and calibrated.

A Delta-T ThetaProbe. Photo: delta-t.co.uk.

Benefits

i) Readings can be taken easily and quickly. The soil water content can be determined simultaneously at different depths. A few milliseconds are necessary for the Diviner apparatus to take the readings while the probe is sinking into the soil. The apparatus takes sixteen readings at different depths in less than two minutes.

ii) The capacitance technique is very accurate if it is correctly installed and calibrated.

iii) Accurate readings can be taken near the surface. Readings can be taken as shallow as 100 mm from the surface in increments of 100 to 200 mm.

iv) The use of the apparatus is not a health hazard.

v) Continuous data logging of the soil water content at different depths is possible. The probes can be connected to any type of data logger.

A ThetaProbe measuring kit.
Photo: harvestagri.co.uk.

Disadvantages

i) Probes must be placed in the soil very carefully and good contact must be ensured over the entire length of the probe. Vacuum along the rods can result in incorrect readings.

ii) Installation procedures of access tubes are critical. Problems are experienced with vacuum around the tube in stony ground. A paste must then be made of the soil to ensure good contact.

iii) A hand test probe must fit tightly into the access tube. Vacuum around the probe gives inaccurate readings.

iv) Systems that work at low frequencies (<20 MHz) are influenced by the soil’s salt content. Frequencies of 100 MHz are therefore normally used.

v) Capacitance probes or combination rods are expensive.

An Aquaterr probe and meter.
Photo: instrumentchoice.com.au.

Heat pulse measurements

Heat pulse sensors or Phene cells are made of porous blocks into which two electrodes are implanted. These blocks are connected to an instrument that determines the water content of the soil. Heat pulse sensors are also made of stainless steel rods of 15 cm long and 10 mm in diameter. The temperature in the sensors is read before and after a small heat pulse.

The size of the heat pulse transmitted by the soil is proportional to the water content that forms in the block. In the case of the stainless steel rod, the size of the reflected heat pulse is an indication of the water content of the sensor. This means that a wetter soil or medium will warm slower than a dry one. The increase in temperature (or cooling) is read with an accurate temperature sensor. It is calibrated at soil water content for the specific soil or sensor.

Figure 1: Example of a capacitance test probe (Theta-Probe).

Benefits

i) The heat pulse sensors (blocks) are relatively cheap and can be read with a variety of commercial resistance meters.

ii) The sensors work over the entire soil water spectrum (from wet to dry) but the accuracy is better in the dryer portion of the spectrum.

iii) The soil water content can continuously be read on the same spot for different depths. The sensors can be buried at any depth.

iv) Both temperature and soil water content can be determined by the apparatus.

v) It can be connected to a data logger to store data.

The Sentek’s EnviroSCAN test probe. Photo: sentechnologies.com.

Disadvantages

i) Sensors have a high power requirement if readings must be taken very regularly.

ii) Each block must be calibrated individually. A small difference in measurement and depth influence the readings.

iii) The heat pulse sensor must have good contact with the soil, which is not always possible. Poor contact results in incorrect readings.

iv) The thermal conduction or conductivity readings are also influenced by other soil characteristics than by the water content. Organic materials and humus content can influence the readings adversely. The higher the humus in the soil, the higher the registered water content.

v) The sensors can also not be installed near the soil surface.

A Sentek probe reads the moisture content at different soil levels. Photo: SentekTalks.Youtube.com.

A portion of the heat pulse will escape above the surface and a lower reading is then obtained.

Next month we shall discuss the calibration of instruments. Visit www.arc.agric.za for more information.

An example of a capacitance
probe.

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