A vented datalogger (above) employs a vent tube from the surface to allow the barometric pressure to exert back pressure on the transducer to remove atmospheric influences from the measurements. If choosing this type of datalogger you must ensure the integrity of the vented cable as a kink or pinch can prevent the vent tube from performing correctly.
A non-vented datalogger (above) makes use of an absolute pressure sensor that measures the pressure of everything above it, water as well as the barometric pressure being exerted by the atmosphere. The water level data recorded by this type of instrument must be adjusted for barometric effects after it has been downloaded to achieve accurate water levels.
How to choose the right Heron dipperLog for the job:
Type of Aquifer
Confined aquifers has an immediate response to barometric pressure changes, while non-confined aquifers can have a delayed response. If using a vented logger in a non-confined aquifer you are unable to recognize or adjust for the delayed response in your data.
Depth of Well
Vented loggers are an excellent choice for large wells that are not too deep. These types of wells respond immediately to barometric pressure changes. Deep wells are better served with a non-vented logger as they are not as susceptible to barometric influences and you have more compensation options.
Number of Wells Being Monitored
If you are monitoring only one well, then a vented logger could be more cost effective and more simple to use. If multiple wells are being monitored for your project and all wells are located in the same 20 km area, then non-vented cable would be your best option. You could deploy a Heron barLog centrally and use that data to compensate all your collected data. This would be the most economical solution.
Type of Data Being Collected
For pump tests or slug tests, either type of logger would work extremely well. Since these tests are of short duration no significant barometric change is likely to occur during the sampling periods so barometric compensation of the data is not essential.
For long term monitoring where barometric changes would be relevant, you should choose the type of logger you need based on other relevant factors.
Vented loggers incorporate a vent tube that is open to the atmosphere. Although this tube is sealed with a water impermeable barrier and incorporates desiccant, if not properly maintained they can be subject to water intrusion. This water intrusion will not only compromise any water level readings but will also cause the battery to drain and the electronics to corrode, making your data irrecoverable.
If you are going to be deploying your loggers in a high humidity area or one prone to flooding, you should ALWAYS choose a non-vented option.
If your monitoring site is in a remote or seasonably inaccessible area, you should choose the non-vented loggers. This type of logger requires no maintenance and can be deployed for extended periods of time without any oversight. Please note that vented loggers are much more inconvenient to transport as the vented suspension cables are much thicker and heavier than their non-vented counterparts and these cables can prove bulky, heavy and difficult to work with.
Either type of Heron Instruments dipperLog is an excellent choice as a source of accurate and reliable data for your project. The choice of vented or non-vented is mostly a matter of personal preference and familiarity.