1. Water Level loggers Vented vs Absolute

2. 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.

3. The mystery of barometric compensation Pressure in cmH ₂ O Level logger: 1500cmH ₂ O – Baro: 1000cmH ₂ O = 500cmH ₂ O = 0.5m above the pressure sensor ALWAYS take a manual dip measurement!

4. How do they work? 5cm 0 ------------------------------------------------------------------------------ ----------------------- --------------------

5. ----------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------- ---------------------------------------- Baro Absolute 2 x Errors

6. Pressure ranges 1m -------------------------------------------------------------------------------------- 10m -------------------------------------------------------------------------------------- 30m -------------------------------------------------------------------------------------- Sensitivity varies with manufacturer. Accuracy based on full scale.

7. One size does not fit all….. High Accuracy? 100m range? Low maintenance? Easy to deploy? Multiple interface options? Long battery life? Low cost? 120 Ub Unobtanium

8. 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

9. When should I use a vented logger? Critical data External logger or telemetry? Single installation?

10. 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

11. 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?

12. Conclusion Is vented better than absolute? … Not necessarily Consider your application carefully