One of the first questions raised when selecting a ground water data logger is Vented or Non – vented, also called gauge or absolute. To fully understand which will be best for your application, first we must explain the differences between the two technologies.
Non Vented Water Level Data logger
A non-vented data logger utilises an absolute pressure sensor that measures the pressure of everything above it, this commonly is the water as well as the barometric pressure being exerted by the atmosphere on the pressure sensor. Therefore to achieve accurate water level measurement, the water level data recorded by this type of instrument must be adjusted for barometric effects. The image below shows a typical setup
Vented Water Level Data Logger
A vented data logger operates with a vent tube from the surface, allowing for continuous barometric pressure to exert back pressure on the transducer, removing atmospheric influences from the measurements. When selecting this type of data logger you must ensure the integrity of the vented cable.
So which type should I use?
Below we have compiled a list of the key factors and application details to consider, before making your decision.
Number of monitoring locations
If you application is to monitor only one well, then typically vented logger could be more cost effective and more simple to use, as an additional barometric sensor would not be required. If a series of wells / locations are being monitored and all wells are located in the same 20 km area, then a non-vented solution would be your best option, as a non-vented sensor is typical more cost effective and you can deploy a barometric sensor centrally and use that data to compensate all your collected data.
Depth of Well
Vented water level loggers are a good selection for large / shallow. Typically this types of application responds immediately to barometric pressure changes. When monitoring deeper wells a better selection would be a non-vented logger as barometric influences are much less and you have a wide range compensation options.
What Data is being collected?
For pump tests or slug tests, both types of logger would work very well. Generally these tests are of short time period, so no significant barometric change should occur during the sampling periods so barometric compensation of the data is not essential, but can be used if required.
For long term monitoring where barometric changes would vary, you should choose the type of logger you need based on other relevant factors, including a solution for barometric pressure compensation.
Monitoring location conditions
Vented water level loggers incorporate a vent tube, which is open to the atmospheric pressure. This tube is typically sealed with a water impermeable barrier and includes desiccant. If this is not properly maintained a potential issue can be water intrusion. Water intrusion will not only effect water level readings been taken by the sensor, it will also cause the battery to drain quickly and cause problems with the electronics, making your data irrecoverable.
Our advice would be if you know your loggers will be used in a highly humid area or location prone to flooding, you should ALWAYS select a non-vented solution.
Type of Aquifer
Confined aquifers have an instant reaction to barometric pressure changes, while non-confined aquifers generally have a delayed reaction. If using a vented logger in a non-confined aquifer it is not able to recognize or adjust for the delayed response in your data.
The final consideration will be the monitoring site. If this is in a remote / inaccessible area, we would recommend you choose the non-vented loggers. A non-vented water level logger will require little maintenance and can be deployed for long periods of monitoring without human intervention. Vented loggers are more inconvenient to transport as the vented cables than their non-vented alternative.
We hope the above information proves helpful in aiding with your water level logger selection. If you do require any further information please feel free to contact us on firstname.lastname@example.org and we will be happy to help.