Field and continuous monitoring instruments

An overview into the techniques used by managers, researchers and manufacturers as the demand for effective remote monitoring increases.

Demand for field measurement in the wake of new legislation and environmental concerns may have increased over recent decades, but the requirements of researchers have remained largely unchanged. They still need the data quickly and access to it almost on demand. They still want it to be accurate. They need to be sure that chunks of data won’t go missing. It’s important that the process of monitoring has very little impact on the environment.

Ideally, the researcher wants a fully functional laboratory in his pocket!

Laboratory instruments are able to provide meaningful data from small samples, each analysis achieved by powerful equipment unencumbered by the need to be small, portable or more than a metre from a fully backed-up PC and with a ready supply of uninterrupted electricity! The biggest headache for analysts is the potential for contamination of the sample during transit to the lab. A problem that is not insurmountable.

The growth of the environmental industry and the need to see how the objects of our research react to daily changes has led to a significant shift from lab analysis to field studies and long-term monitoring. Greater reliance on the data for decision making has led to an increased demand for immediate availability. Environmental managers have no less a need for timely information to aid decision making than do City executives, but how to obtain that information?

Making frequent trips for repeated sampling is not only costly, but also risks a delicate eco-system or sensitive area being trampled under-foot for the purposes of monitoring it.

In the last twenty years several factors have combined to enable the long-term field monitoring of a growing number of parameters, to be an increasingly more viable option. First came the reduction in size of electronic equipment giving rise to truly portable devices, even if in the early days the size of the power supplies still limited the number of applications.

The second factor was that manufacturers, by and large, had split into two camps, sensor manufacture and data acquisition module manufacture. This allowed each camp to greatly improve their specialist products, each taking advantage of new technologies. The gap between the accuracy of field measurements and those taken in the lab had now narrowed sufficiently to gain acceptance across science, industry and government.

This in turn led to the third factor, a demand for data gathered in the field to be made readily available to any number of interested parties and even placed in the public domain.

Today, field measurement can be carried out at remote and extreme sites, with accurate data gathered over long periods – even continuously – by completely autonomous stations or networks.

A little knowledge can be – the tip of the iceberg…

It’s fair to say that much of the continuous monitoring undertaken leads to a wider understanding of our world or particular eco-systems, such as the effects of climate change on the polar regions. It raises our awareness and creates a demand for more knowledge, or to monitor our attempts to combat potential emergencies uncovered by earlier research. Ultimately, legislative changes as a result of these findings invariably demand more monitoring and greater control.

This is evident in the field of climate research. The Institut für Physische Geographie (IPG) of the Universität Freiburg (Germany) has been working since 1992 on the Antarctic Peninsula in close co-operation with partners from Argentina, Brazil and Chile. The major objectives of the climate change research activities comprise monitoring of snow cover dynamics and glaciers by means of remote sensing data, energy and mass balance measurements and modelling.

Due to the remoteness and the logistical difficulties of obtaining data on a regular basis, remote sensing techniques provide an effective tool to monitor the Antarctic glaciers. However, the research is also supplemented with fieldwork, which is an integral part of the IPG Antarctic research activities.

At the other pole, researchers from the Swedish Meteorological and Hydrological Institute combined measurements taken from four pyranometers mounted on the deck of an ice-breaker, with data recorded by a datalogger and sensors left to drift on an ice floe in the arctic currents. Both sets of data were stamped with date, time and GPS location.

Over recent years satellite data has been used to map the extent of the open ice coverage. The objective of the SMHI project is to make a detailed study of the relationship between radar data measured by satellite and the albedo distribution along a north-south gradient in the Arctic and at the same time collect data about the properties and distribution of ice and snow.

None of these polar research projects (and hundreds of similar ones) would have been possible without the availability of instruments capable of withstanding extreme conditions and operating on very little power. Continuous monitoring for environmental purposes has moved on a pace over the last ten years. Whilst there are many polar studies, where answers to bigger questions are sought, the ability of continuous monitoring of our environment to provide time-critical information is also helping to save lives.

Sentinels

Stand-alone data acquisition systems have been deployed alongside waterways, providing information on water level and flow rates. Invariably, the systems also monitor meteorological parameters, providing frequently updated data, either separately, or as is increasingly the case, a complete network. Staff observing the data on a PC are able to issue flood warnings without the need for on-site reports. With a system or network configured with the ALERT protocols, stations can be programmed so that when pre-set levels are reached, the ALERT system on the station can send warnings by SMS, RF or even satellite to key personnel.

People and property are often at risk from environmental events, but many communities and their long-term health can also suffer indirectly when crops or livestock are affected. Olive farms in Spain can be ravaged by swarms of olive fruit flies (Bactrocera oleae), which are a serious pest in most of the countries around the Mediterranean Sea. The larvae are monophagous and feed exclusively on olive fruits. Adults feed on nectar, honey dew, and other opportunistic sources of liquid or semi-liquid food. The damage caused by tunnelling of larvae in the fruit results in about 30 percent loss of the olive crop in Mediterranean countries and especially in Greece and Italy where large commercial production occurs.

The olive fruit fly reproduces under certain conditions of temperature and relative humidity. They are unable to fly great distances and rely on winds to move the swarm to new areas. Automatic weather stations installed by Campbell Scientific in Spain are continually measuring temperature, relative humidity, solar radiation and rainfall. The stations provide a continuous monitor of weather conditions that may affect the development and distribution of the olive fruit fly and provide a warning mechanism to farmers, allowing them to take action.

The biting midges of the Scottish Highlands (Culicoides impunctatus), are a major problem for the Scottish economy. A survey carried out by Dr Alison Blackwell (Edinburgh University) found that 49 percent of tourists vowed to stay away from Scotland at the height of summer to avoid the blood-sucking creatures! That equates to a ¤420 million loss of revenue. Her field research utilised a low-cost weather station from Skye Instruments to better observe a pattern in the lifespan and activities of ‘the little beasties’. Dr Blackwell, who also runs In-Phage Ltd, out of Edinburgh University, warned that the midge is a permanent feature in Scotland despite years of efforts to kill them off.

Using data in real time…

When monitoring inaccessible sites it makes sense to use instruments that can do more than simply count and allow the devices to take actions in accordance with the readings provided by the sensors.

Offshore data buoys assessing suitable sites for wind farms include data loggers capable of operating beacons and foghorns.

High in the Alps, where avalanches and rock falls can pose serious threats to human life, Dr Hansueli Gubler of AlpuG uses a wide range of instruments in some of the harshest conditions on earth. Dr Gubler has combined a range of sensors and dataloggers to produce systems that actively save lives.

AlpuG are experts in obtaining data from many specially adapted sensors including snow surface temperature, snow height, snow temperatures, wind, microwave snow profiles, seismic signals from avalanches, ground surface wetness and drifting snow gauges. All of these measurements are logged by low temperature tested dataloggers and transmitted hourly by radio to a base station. AlpuG’s alarm stations for avalanches, mudflow or rock fall are set to automatically stop endangered road traffic and mountain railways should an incident occur higher up the mountain, with the datalogger controlling traffic signals on the roads or tracks below.

Technological advances

Field researchers now have a choice between intelligent sensors and flexible data loggers. Whilst it’s fair to say that a robust measurement and control system will reap dividends over long periods of varied research, sensor manufacturers have moved on a pace to produce highly specialised sensors that are able to operate effectively without the need for a logger to carry out a great deal of processing – effectively meaning a cheaper data logger. These intelligent sensors utilise digital signal processing, transmitting signals consisting of numerical values in addition to status signals.

Intelligent sensors are an extension of traditional sensors towards those with advanced learning and adaptation capabilities. They must also be re-configurable and perform the necessary data interpretation, fusion of data from multiple sensors and the validation of local and remotely collected data. Programmable data loggers often include commands to check power supply and ignore or highlight suspect data strings from the sensor. Any truly intelligent sensors therefore must contain embedded processing functionality that provides the computational resources to perform complex sensing and actuating tasks along with high-level applications.

It’s clear that sensor technologies continue to be developed in response to greater demands from the monitoring world. A range of measurement techniques exist providing choices in monitoring styles for researchers. Whilst for many applications different techniques for monitoring may be available, the tendency is for researchers to stay with tried and tested methods. Understandably data validity is a key factor and risks should not be taken unnecessarily. However, this can mean an increased amount of time to field test new sensors and bring them to market, and in turn this makes new sensor development a serious financial undertaking.

One standard that emerged from the hydrological monitoring sector in the eighties was the SDI-12 protocol, which provided an output platform for sensor manufacturers and data logger manufacturers to provide simple connectivity and ease of use. For reasons stated previously, the range of SDI-12 sensors has been slow to grow beyond the hydrological sphere. Only now are we seeing some real progress. Gill Instruments has developed an ultrasonic wind sensor using SDI 12. Data loggers with an SDI-12 interface provide increased accuracy and tremendous system savings, with up to ten SDI-12 sensors connecting to the same interface port.

Measurement techniques

Environmental managers, business owners and process engineers need to be aware of the range of methods available for environmental monitoring. Whilst most techniques are perfectly valid, some may turn out to be inflexible, costly, or just not provide the data and level of control required to truly benefit from the system. The task of finding the amount of water in soil is a good example of an application with a healthy range of measurement techniques…

For almost two decades, researchers, consultants and large landowners have used neutron probes to measure soil moisture. The radioactive device is expensive and its use requires an Atomic Energy Commission (AEC) permit. Also, the operator has to wear/use a radiation detection film badge to determine exposure levels, which is monitored by the AEC.

The neutron method of measuring soil water content uses the principle of neutron thermalisation. When both hydrogen and oxygen are considered, water has a marked effect on slowing or thermalising neutrons. Thermal neutron density is easily measured with a detector if the capture cross-section remains constant then the thermal neutron density may be calibrated against water concentration on a volume basis.

Recently, Australian company Sentek has developed a range of soil moisture probes that use the capacitance method to measure soil moisture content. A high frequency electrical field, created around each sensor, extends through a plastic access tube into the soil, to provide an extremely accurate soil moisture measurement in a 360o field. Probes such as the EasyAG®, EnviroSMARTTM and TriSCAN have been designed to give a true picture of soil conditions, including salinity, to depths of up to two metres and at preset levels in-between. Crop growers can schedule irrigation events to maximise yield and save water, whilst a true picture of root growth also helps researchers.

The principles behind tensiometers, such as those manufactured by Soil Monitoring Engineering, have been around since the 1920s, but such instruments are still in wide use today. A conventional tensiometer comprises a sealed tube defining a chamber, which is normally completely filled with water, a hollow porous tip on one end of the tube and a vacuum gauge connected to the water chamber. The porous tip is inserted into the soil and establishes liquid contact between the water in the tube and moisture in the soil surrounding the tip. Relatively dry soil tends to pull water from the tube through the porous tip.

However since the tube is sealed, only a minute amount of water is actually withdrawn. Accordingly, the water in the tube is placed under tension by the pulling effect of the dry soil, thus creating a measurable subatmospheric pressure in the tube. Higher moisture contents in the soil produce correspondingly less vacuum in the tube and completely saturated soil registers substantially zero vacuum or atmospheric pressure.

The adage “There’s more than one way to break an egg” is justifiably applied to methods of taking field measurements for the same application. The same is also true of the number of applications that are monitored using Time Domain Reflectometry (TDR).

TDR technology was for a while believed to be an expensive measurement option, a fact that is no longer true! TDR works by sending a pulse of energy down a cable. When that pulse reaches the end of the cable, or a fault along the cable, part or all of the pulse energy is reflected back to the instrument. The TDR device measures the time it takes for the signal to travel down the cable, see the problem, and reflect back. This is then converted to distance and displays the information as a waveform and/or distance reading.

Campbell Scientific’s CS616 water content reflectometer uses TDR to monitor – surprise, surprise! – soil moisture content and, as if proof of how inexpensive TDR monitoring has become, it is being used to monitor whole sections of rail tracks for land movements and track disturbances. Given that TDR measurements are non-destructive and offer excellent accuracy and precision, it’s no wonder the technique is being investigated as a possible monitoring method by a growing number in industry and all areas of environmental research.

The Centre for Research on Indoor Climate and Health at Glasgow Caledonian University is undertaking a research project for the Engineering and Physical Sciences Research Council, to investigate new ways of measuring and monitoring moisture in buildings. The three-year study is being undertaken in collaboration with the University College London.

One of the techniques being investigated is TDR and the team currently developing sensors are confident that a working system will be developed and validated soon. Mark Phillipson of Glasgow Caledonian University reports: “No previous work has been undertaken in the UK to look at the application of TDR to building physics; the physical principle is straightforward, the challenge lies in developing a system for consistent installation of sensors into building materials. Over the course of the next year this system will be refined, before facing the final challenge of demonstrating its capabilities on real buildings. The team are also working on an advanced electrical measurement approach for moisture contents, and colleagues at University College London are working on an exciting heat pulse method of assessing moisture content.”

The overall objective of this project is to introduce sophisticated innovative approaches to measuring moisture content in a variety of building materials and evaluate the relative benefits of these compared to traditional methods.

All this because, as our world changes, so do the demands placed on industry and environment for new methods of making long-term studies and obtaining better results. Instruments of the future are emerging today.

Where the need for continuous monitoring remains, the evolution of monitoring instruments remains continuous

Author

Stuart Cresswell is Marketing Manager at Campbell Scientific Ltd.

[email protected] Campbell Scientific Ltd Campbell Park,

80 Hathern Road, Shepshed, Leics. England, LE12 9GX

Tel: +44 (0)1509 601141

Fax: +44 (0)1509 601091

[email protected]

www.campbellsci.co.uk

Published: 10th Mar 2006 in AWE International