Water Level loggersVented vs Absolute

DefinitionsAbsolute: 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 freeEasy deploymentLow cost, particularly for multiple installationsPurpose made intuitive interfacesEasy re-deployment/VersatilityDiscrete

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