Published on March 20, 2014
GROUNDWATER DATALOGGER LEVEL MEASUREMENT BASICS 2014 ©HydroG Resources Group Inc.
Dataloggers are now frequently used for water level measurement by hydrogeologists and technicians. This course introduces some basic concepts and methods for beginners.
• Introduce basic concepts1 • Review standard protocols2 • Illustrate practical field methods3 Course Outline
Knowledge Base We are assuming you are familiar with : • common terms such as pressure; water level measurement; elevation; aquifer; unconfined; water table; etc. • concepts of quality control and quality assurance. For simplicity all our examples deal with the water table in an unconfined aquifer.
BASIC DATALOGGER CONCEPTS
What Is A Datalogger? “Datalogger” is the term used in this course, however they are also known by a variety of other terms (including pressure sensors, transducers, etc). Dataloggers are available in a wide variety of makes and models with a large range of capabilities and associated cost scale.
What Is A Datalogger? Dataloggers allow automated water level measurements to be obtained at rapid rates and/or in remote locations. They improve our capability to monitor water level. To do this Dataloggers measure water or air PRESSURE, record those measurements as digital time series files, and, allows the files to be retrieved as needed.
Why Measure Pressure? The Datalogger measures pressure, but is used to obtain water level measurements. The pressure reading is used as an INDIRECT measure of water level. It is important to note that the Datalogger pressure readings must be CONVERTED to an equivalent water level measurement.
Why Measure Pressure? • Water pressure increases with depth. • Pressure values can be converted to an equivalent depth of water. • However there are a number of practical factors to consider. we want to measure the pressure due to water column (PH20 ) water level but atmospheric air pressure (Patm) is also acting on the water column therefore total pressure at this depth PT= PH20 + Patm
Datalogger Pressure Measurement Some Dataloggers have a vent tube to the atmosphere which allow “Gauge Pressure” measurements of PH2O directly. Some Dataloggers are sealed units that measure Total (or “Absolute”) Pressure (PT). In order to obtain PH2O you must also measure Patm (usually with a separate Barologger) and subtract that value from your readings. PH20 water level Patm PT Vented Datalogger Measures PH20 directly. Non-vented Datalogger Measures PT Sensor is open to air pressure Barologger Measures Patm
Datalogger Construction Basics • Two component groups to consider: 1. The pressure sensor, and, 2. The internal electronics (including battery). • The pressure sensor has a diaphragm, one side of which is exposed to water. • The diaphragm changes shape as pressure changes. pressure to be measured diaphragm electronics (not shown)
Datalogger Construction Basics • The changing shape also changes the electrical resistance across the diaphragm – this provides a range of voltage “signals” that corresponds to equivalent pressures. • For each measurement the internal electronics provides voltage to the sensor, then converts the returning “signal” to a pressure reading, and records the data.
venting and communication cable communication port Datalogger Construction Example This is a (very) old example of a vented sensor.
Datalogger Construction Example electronicspressure sensor diaphragm vent tube Sensor disassembled to show components
Datalogger Type Review Vented Non-vented Patm PatmPH2O Pconstant PatmPH2O diaphragm atmospheric pressure “cancels out” across diaphragm PH2O measured directly sealed unit measures PT PH2O is calculated
Standard Method SOP’s A few datalogger specific Standard Operating Procedures do exist. Two examples are: • USGS • ISO 2005
United States Geological Survey Use of Submersible Pressure Transducers in Water-resources Investigations, USGS Techniques of Water- Resources Investigations Book 8, Chapter A3 (2004). Provides a detailed explanation of theory, and, construction/operation of Dataloggers. Please refer to that publication for more details.
ISO 2005 Reference: International Standard ISO/TR 23211, Hydrometry – Measuring the water level in a well using automated pressure transducer methods. Similar to USGS paper and methodology, please refer to the publication for more details.
Summary • Detailed theoretical background information and SOP’s exist, these deal with a wide variety of instrument types and conditions. • However for most practical applications using modern commercially available complete Datalogger units, some basic understanding and common sense use should result in good data. • We encourage you to develop a practical SOP for your organization.
PRACTICAL FIELD METHODS
Try To Avoid These Situations! (mistakes we have already made, or found, in the field) confusion (poor planning) unanticipated conditions (that were in fact predictable)
(more mistakes ….) installations that really don’t measure anything (can you tell why?) messy installations (again, poor planning)
Datalogger Success All of the commercially available Dataloggers that are typically in use, vented or non-vented, can provide accurate and valid water level data – provided they are installed and used correctly. Operator training is essential for success.
Basic Considerations To ensure you are collecting good (accurate) data, you should consider a number of factors when using a Datalogger – including : • Datalogger choice • Datalogger use • Datalogger installation • Converting Datalogger readings to water levels
Datalogger Choice • Type (Vented vs. Non-vented) • Measurement range • Rated accuracy • Measurement frequency • Installation method • Communication method • Battery life • Construction material • Reliability • Durability • User options • Software • Technical support • Warranty • Flexibility • Special conditions There are many factors to consider when choosing a Datalogger for your project. Some of these factors include:
Datalogger Choice The final choice of Datalogger will depend on your project and organizational needs, specific recommendations are beyond the scope of this course. The key is to plan each Datalogger installation starting with why you need it, what data you need to collect, what conditions the Datalogger will encounter at the well, how will it be installed, and, how it will be maintained and downloaded.
Datalogger Use Once you choose your Datalogger follow the manufacturer’s instructions regarding communication, programing and use. In addition you should: • Test each Datalogger before it goes into the field. • Develop a QA/QC program that starts when you install the Datalogger. • Regularly maintain / download the Datalogger. • Review your data – check for errors.
Datalogger Use Remember you are using the Datalogger to calculate equivalent water level measurements. So you need to “calibrate” the measurement and conversion process. • Obtain regular manual water level measurements to compare to your Datalogger data. • Check regularly to ensure the Datalogger is responding correctly to water level change. • Remember – no Datalogger will last forever, monitor parameters that predict failure (DRIFT, battery levels, other anomalies, etc.) before it happens so that you can replace it in time.
Datalogger Installation Basics The Datalogger is hung (set) on a cable to a specific depth in the well below a reference (measuring) point. The depth setting should be below water and remain constant . If needed, the Barologger is set above the water. Well Screen Well casing Top of well (measuring point) Water Level Cable Datalogger Barologger (if needed)
Installation Considerations • Depth to water and well construction (e.g. total well depth). • Expected water level variation (e.g. avoid a dry Datalogger). • Maximum pressure / depth (exceeding damages the sensor). • Type of cable used (stainless, direct read, vented, etc.). • How cable is secured at the top of the well. • Need for Barologger to measure atmospheric pressure. • Other equipment installed in well. There are many factors to consider when installing your Datalogger. Some of these factors include:
Some Installation Tips • Vented Dataloggers need a Vented communication cable, install according to manufacturer’s instructions. • Direct read cables can also be used for Non- vented Dataloggers, again install according to manufacture’s instructions. • Many Non-vented Dataloggers are installed on simple cables or lines. These need to be removed (pulled) from the well at each download.
Our Preferred Method held securely in place lowered into well to set depth stainless cable stainless swage sleeves stainless hose clamp stainless quick links ready to install
Some Installation Tips Please refer to our website for more details on recommended cable materials and installation methods for Non-Vented Dataloggers. hydrogresources.com
CalculationsTop of well (measuring point) Water Level Barologger measures Patm Datalogger Vented measures PH2O Non-vented measures PT watercolumn(W) Dataloggerdepthsetting(D) First find W (length, in metres). A Vented Datalogger may give you depth of water directly (m or cm). For a Non-vented Datalogger subtract Patm from PT . Programming your Datalogger to read in depth of water (e.g. cm of H2O) makes things easier. Subtracting W from D gives a depth to water from the top of the well. Keeping D constant over time becomes very important!
Data Accuracy • The accuracy of your Datalogger will be much greater than the accuracy of your installation depth setting measurement or manual water levels (used to check the validity of your data) • Manual water level measurements are generally ±1 cm • Improving the accuracy of your depth setting measurement is one of the most important methods to improve your data accuracy • Avoid the tendency to determine the depth setting based on Datalogger measurements, measure your depth setting! • Datalogger depth setting (D) changes over time result in DRIFT There are many factors that affect the accuracy of your measurements. Some factors to consider:
Like everything – the care you take in your Datalogger installation and use will be reflected in the accuracy of your data. Planning the installation, and recording as much information as possible in the process, helps immensely. There is no substitute for common sense, so adapt to your own situation, and good luck!
Response tests are completed at two wells at the same time using separate Dataloggers. When you are trying to do 10 things at once……. Some consistency helps!
WE HOPE YOU ENJOYED THIS COURSE – PLEASE ALSO SEE OUR COURSES ON ELIMINATING DATALOGGER ERRORS 2014 ©HydroG Resources Group Inc.
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