Application and Development of Soil Measurement Sensing Technology

Dr John Newstead at Delta-T Devices talks to AZoSensors about the application and development of the PR2 soil measurement sensing technology.

One of Delta-T Devices’ key innovations has been the development of the PR2 Profile Probe. Please can you introduce this instrument and explain why it is has become a key product for you?

The PR2 Profile Probe offers a highly convenient single instrument solution for accurate and concurrent measurement of soil water content at various depths down to one metre. Installation is via an access tube - which is inserted into a pre-augered 27mm diameter hole in the soil. This allows for minimal soil disturbance.

The system offers two lengths of probe, with either 4 or 6 fixed depth soil moisture sensors (along the length of the probe) that allow the user to easily monitor the distribution and amount of water through a soil profile. The PR2 can be used with our hand held readout meter for spot measurements or, for time series analysis, can be connected to a logger such as our new GP2 Data Logger and Controller.

The PR2 is a powerful research grade tool that can generate large amounts of accurate information quickly and easily. As such it has proved to have a wide range of applications in academic and industrial environments, from crop water balance and catchment scale studies to geotechnical, forest and viticulture applications. It has even been used to monitor relative changes in moisture content in stone walls.

PR2 Soil Moisture Profile Probe with hand-held readout unit.

What are the main features of this probe?

The PR2 is built around patented sensing technology which provides research grade performance in all soil types. It offers low salinity and temperature sensitivity and uses bespoke access tubes for easy probe insertion and removal. The PR2 system is adaptable to different applications. The fact that it is a single probe with multiple sensors removes the need to purchase multiple sensors and install/monitor them individually.

The PR2 is notably easy to use – whether with our hand-held meter or connected to a data logger system.

The PR2/4 Probe measures soil moisture at 4 depths down to 40cm - the PR2/6 Probe measures at 6 depths down to 100cm.

How does this probe compare to other soil moisture profile probes currently on the market?

The PR2 has emerged from the demanding world of soil science research, where accuracy and reliability are paramount. This gives it the edge over other profiling type probes on the market. The PR2 is relatively small, lightweight, portable, and easy to use. It also has a unique ability to test the quality of the installation against the presence of stones or voids next to a sensor. This gives the operator the choice to re-site the access tube and probe if needed - to achieve optimum measuring conditions.

Combined, these attributes have made the PR2 a first choice for many researchers who are looking for a field-proven solution. Over several years the PR2 has established a strong reputation in research circles for enabling quick, accurate, and easy monitoring of large numbers of access tubes across a site.

Taking a portable reading with PR2 soil moisture probe.

How does it work?

Firstly, an operator will auger a hole using our dedicated augering kit - and insert an access tube. The PR2 probe is then inserted into this tube. Research has shown that one of the greatest sources of error in any profiling system is poor installation. Delta-T Devices has spent a lot of time developing an installation system that mitigates these error sources. For instance, our auger kits include a stabilisation plate that ensures the production of a vertical hole that is not funnel shaped (a common fault with augering). Funnel shaped holes allow air or water to ingress down the side of the access tube and bias water content readings. The stabilisation kit is a simple, elegant solution and one that is very effective.

Multiple access tubes can be placed around a research site. When an operator visits such a site the PR2 can be inserted into each individual access tube - one at a time. When connected to our HH2 readout unit, the operator simply pushes the read button to view the measurement. The mV output of each sensor is converted using calibration tables to water content and is displayed on the screen. An operator then has the choice of saving the data for later download. Typically before they leave a site, many customers will change the connecting cable and leave the PR2 in an access tube attached to a data logger. Thus they are able to cover large research sites very quickly with spot readings using a single PR2 and gather time series data between visits.  

Installed Profile Probe with representation of sensed zones.

Are there any recent case studies that have tested the functional capacity of this probe and generated ideas on how to develop the instrument?

Yes, Profile Probes have been installed at four sites across the U.K. to provide soil moisture data as part of FutMon, an EU Life+ project. The aim of FutMon is to create a Europe-wide long term forest monitoring system. The FutMon partner organisation in the UK is Forest Research (Centre for Forestry and Climate Change). The data required for FutMon are being collected at existing UK long-term forest monitoring plots.

At each site, a single Delta-T Profile Probe (connected to a Delta-T Data Logger) is used to carry out continuous monitoring of soil moisture down to a depth of 40cms. Once a month, when the site is visited to collect data, the Profile Probe is removed from its access tube and connected to a handheld readout meter. In this mode it is used to make instant readings at four nearby measurement points, each with its own access tube installed.

After the measurements have been made, the Profile Probe is simply repositioned in its original access tube and reconnected to the logger, ready for another month of unattended recording. The advantage for Forest Research is being able to use a single Profile Probe for continuous monitoring as well as for checking spatial variability. It minimises the cost of equipping each site, it means fewer cables and it reduces security risks.

What data is generated from this probe and how is this used by the end-user?

The PR2 system generates volumetric moisture content values at each of the sensor positions along the length of the Profile Probe. Delta-T data loggers use DeltaLINK 3.0 software to graphically display PR2 outputs, alternatively all the data can be imported from the hand held unit or data logger into Excel type packages for further manipulation or graphical representation. GPRS communication systems are also available from Delta-T Devices which allow remote monitoring of the data logger.

Where can we find further information on your products and services? 

The PR2 system and associated products such as data loggers are featured on our website. Our technical sales team is available to answer any questions on our instruments and potential applications. They can be contacted via [email protected].

About Dr John Newstead

John has a BSc in Soils and the Environment and a PhD in Soil Science and Climate Change. Since joining Delta-T Devices John has used his expertise and practical experience in the soil sciences arena to help customers plan and execute successful soil moisture research projects.