Post on 26-Dec-2015
Water Level loggersVented vs Absolute
Definitions
Absolute: Measures total pressure (weight) above the sensor. Atmospheric pressure must be removed from this value to reveal actual water pressure (water column).
Vented: Measures only water pressure above sensor. Atmospheric vent accounts for atmospheric pressure.
The mystery of barometric compensation
Pressure in cmH₂OLevel logger: 1500cmH₂O – Baro: 1000cmH₂O = 500cmH₂O
= 0.5m above the pressure sensor
ALWAYS take a manual dip measurement!
How do they work?5cm
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Baro
Absolute
2 x Errors
Pressure ranges
1m --------------------------------------------------------------------------------------
10m --------------------------------------------------------------------------------------
30m --------------------------------------------------------------------------------------
Sensitivity varies with manufacturer. Accuracy based on full scale.
One size does not fit all…..High Accuracy?
100m range?
Low maintenance?
Easy to deploy?
Multiple interface options?
Long battery life?
Low cost?
120
UbUnobtanium
If vented loggers are so good, why would I choose not to use them?
Vented cable: installation, vandalism, weather protection, tailor length cables/re-deployment
Cost
User interface
Maintenance
When should I use a vented logger?
Critical data
External logger or telemetry?
Single installation?
Why would I choose to use an absolute logger?
Maintenance free
Easy deployment
Low cost, particularly for multiple installations
Purpose made intuitive interfaces
Easy re-deployment/Versatility
Discrete
What else to consider?Site access?
Equipment placement (above and below water)
Environmental/human risk?
Return visiting?
GPRS signal? (NOT THE SAME AS CALL SIGNAL)
Critical data?
Expected fluctuation?
Other parameters?
Budget?
Conclusion
Is vented better than absolute? … Not necessarily
Consider your application carefully