Critical Success Factors in Water Level Monitoring

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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



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ACCURACY TEST

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10

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25

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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



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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



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LARGE DRIFT ERROR EXAMPLE

14.5

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15.5

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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

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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



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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


Vented vs. Non-Vented

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Vented logger

Non-vented logger

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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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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


Logger Materials Considerations

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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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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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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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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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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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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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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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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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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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Stilling Well Examples

For more information see www.onsetcomp.com/water_level_tech_notes

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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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Critical Success Factors in Water Level Monitoring

Technology

Transcript of Critical Success Factors in Water Level Monitoring