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Critical Success Factors inWater Level Monitoring
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Presenters:
Richard Kaiser, Onset Engineer and Water Level/Flow Monitoring Specialist
Nick Lowell, Onset Chief Engineer
May 16, 2006
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I. Considerations in choosing a pressure-based water level logger Comparing specifications Vented vs. non-vented Materials Software & communications
II. Deployment tips and considerations In water wells In streams, lakes, estuaries, channels and
wetlands
III. Questions
Topics for Discussion
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Does the specified accuracy relate to the sensor or entire logger?
• Potential sources of logger accuracy errors:– Sensor error
– ADC/Voltage reference
– Temperature error
– Case/Sensor mounting
• Barometric pressure
Sensor
Thermal
ADC/Voltagereference
Case/Sensor
mounting
Total Error: .4% (typical)
0.1%
0.1%
0.1%
0.1%
Choosing a Water Level Logger
Demystifying Accuracy Specifications
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Choosing a Logger
Do sensor accuracy specifications reflect real-world effects such as:
• Single point vs. entire pressure range
• Linearity
• Repeatability
• Hysteresis
Choosing a Water Level Logger
Demystifying Accuracy Specifications
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ACCURACY TEST
5
10
15
20
25
30
35
11/17/2005
13:40
11/17/2005
13:48
11/17/2005
13:55
11/17/2005
14:02
11/17/2005
14:09
11/17/2005
14:16
11/17/2005
14:24
PS
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Do temperature variations cause additional error outside the accuracy spec?
• Temperature changes affect all pressure-based sensors
• Compensation for temp changes required
• Temp should be stabilized prior to reference measurement
TEMP DEPTH ERROR at 20 PSIA
FIGURE 2
-0.2000
-0.1500
-0.1000
-0.0500
0.0000
0.0500
0.1000
0 10 20 30 40
TEMPERATURE C
DE
PT
H E
RR
OR
(%
of
FS
)
HOBO U20
EXAMPLE
Choosing a Water Level Logger
Demystifying Accuracy Specifications
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What is the impact of drift errors?
• All pressure sensors drift over time
• Important in long-term deployments where no recent reference measurements are available
• Drift is quantifiable through manual measurements at beginning and end of deployments
Choosing a Water Level Logger
Demystifying Accuracy Specifications
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LARGE DRIFT ERROR EXAMPLE
14.5
15
15.5
16
16.5
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12/9/04 12/19/04 12/29/04 1/8/05 1/18/05 1/28/05 2/7/05
DATE / TIME
PS
I
ACTUAL DRIFT ERROR
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Logger calibration and testing considerations:
• Has the logger been calibrated and tested against NIST-traceable standards?
• Has the logger been factory-calibrated over multiple points?
– 1-point eliminates offset errors
– 2-point eliminates offset and gain errors
– Multi-point eliminates offset, gain and linearity errors
– Pressure/temperature calibration ensures accuracy when temperature changes
Choosing a Water Level Logger
Demystifying Accuracy Specifications
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High-level considerations:
• All pressure-based readings require barometric compensation
• Vented loggers rely on built-in vent tubes
• Non-vented loggers use barometric pressure data from external source
Choosing a Water Level Logger
Vented vs. Non-Vented
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Vented logger
Non-vented logger
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Choosing a Water Level Logger
Vented vs. Non-Vented
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Vented Loggers
• Highest accuracy possible
• More expensive
• Significant reliability/maintenance issues
- Problems from condensation
- Must be above flood level
- Bulky, hard to transport, conceal
- Easily damaged, hard to clean
- Requires desiccant replacement
Non-Vented Loggers
• Easy to deploy
• Fewer maintenance issues
• Barometric pressure data required
• Theoretically not as accurate
- Wider-range sensor to measure
same depths
- Error potential with separate
barometric pressure measurement
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Choosing a Water Level Logger
Logger Materials Considerations
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• Will the environment corrode the logger?
• Will the logger and/or vent tube contaminate the well water? (is it easy to clean?)
• Could the logger be frozen in ice?
• Is there a temperature extreme?
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Housing: 316 stainless steel is suitable for many applications (stable, non-corrosive, non-contaminating)
Titanium is recommended in salt water or other corrosive environments
Sealed bladder is an alternative option
Must also consider other wetted materials such as o-rings, end caps, and vent tubes
Sensor: Stainless steel sensors are common but can be damaged by improper handling, shock, corrosion or expansion of freezing water
Properly-mounted ceramic sensors are more robust and not damaged by freezing, corrosion or shock
Choosing a Water Level Logger
Logger Materials Considerations
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Choosing a Water Level Logger
Communications Considerations
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• Does the logger offer a USB or serial interface?
• Is the interface optical or mechanical?
• Is real-time data necessary?
• How often do loggers need to be offloaded?
• What are the options for data offload?
- Bring logger back to PC
- Take laptop into the field
- Data shuttle
- Telemetry
• Will the logger need to be removed or is there an access cable option?
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Choosing a Water Level Logger
Software Considerations
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• Is the software easy-to-use and intuitive?
- Set up
- Readout and Level Conversion
- Plotting
- Data Export
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Choosing a Water Level Logger
Software Considerations
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Set-up
• Is logger setup intuitive and repeatable?
• What sampling modes are available?
- Multiple user-defined logging intervals
- Logarithmic sampling
- Event based
• How do I synchronize multiple loggers?
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Choosing a Water Level Logger
Software Considerations
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Readout and converting pressure to water level
• Is readout fast and reliable?
• Can I enter my own fluid density?
• How do I enter a reference level?
• Can I import barometric pressure files from other loggers or local weather stations? (Not required for vented loggers.)
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Choosing a Water Level Logger
Software Considerations
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Plotting
• Can I get a quick preview view of the data?
• Is the plot format easy to manipulate?
• Does the software allow for plotting of multiple series?
• Can I easily do min/max/average data filtering?
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Choosing a Water Level Logger
Software Considerations
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Data Export
• How do I export data from the logger software to my groundwater modeling software package?
• Does the export function allow for flexibility in data formats?
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Choosing a Water Level Logger
Deployment Tips and Considerations
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In general…
• Use stranded, stainless-steel wire that will not stretch or kink during deployment(www.onsetcomp.com/water_level_tech_notes)
• Use Teflon-coated wire where ease-of-cleaning is important
• Be careful lowering the logger into water to avoid sensor shock
• Always take reference measurements to optimize accuracy
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Choosing a Water Level Logger
Deployment Tips and Considerations
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Water Well Deployments
• Deploy logger, wait 10 minutes for temperature stabilization, take manual measurement
• Synchronize PC and field clocks
• Take manual measurement before removing or offloading logger to check for drift
• Take additional manual measurement at redeployment
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Choosing a Water Level Logger
Deployment Tips and Considerations
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Deployments in streams, lakes, estuaries, channels and wetlands
• Typically, build a stilling well with PVC schedule 40 conduit and slotted pipe
• Take manual measurement at fixed lake or stream location, record date and time
• Repeat manual measurements at beginning and end of each deployment
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Choosing a Water Level Logger
Deployment Tips and Considerations
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Stilling Well Examples
For more information see www.onsetcomp.com/water_level_tech_notes
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Choosing a Water Level Logger
Deployment Tips and Considerations
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General Deployment Tips
• Be sure to log temperature as well as pressure for accurate temperature compensation
• Consider mounting logger in pipe within well
• Deploy barometric pressure logger in area where temperature fluctuations are minimal, such as in a well
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Questions and Discussion
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For additional information, contact:
Richard Kaiser, Onset Computer [email protected]
Nick Lowell, Onset Computer [email protected]
www.onsetcomp.com/webinar/waterlevel
Thank You!
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