We are often asked about the differences between vented and non-vented (or absolute pressure) groundwater dataloggers, and which is better. This article presents a summary discussion of the information we typically provide on this frequently asked question.
The primary difference between these two types of dataloggers is that the vented variety has a tube running from the datalogger to ground surface, while the non-vented datalogger does not. The tube connects the vented datalogger's transducer to ground surface, thereby providing connection to the atmosphere.
Which type of datalogger is better for a given application varies depending on a number of parameters. These include operational differences between the two types of dataloggers, as well monitoring well conditions such as depth to water and expected water level fluctuations. We look at how these parameters affect three main considerations for datalogger performance: installation, operation and data accuracy.
From an installation perspective, the non-vented datalogger is easier to deploy. It involves simply lowering the instrument into a monitoring well on an accurately measured length of cable. The vented datalogger involves handling both a cable and vent tube. During deployment, care must be taken to ensure that the vent tube is not damaged, and the pairing of the cable and tube can lead to errors in accurately determining the depth of the datalogger. For shallow applications, the added complexity and potential wear and tear brought on by the vented datalogger can be minimal, but as deeper monitoring is required the potential for problems increases.
Once either type of datalogger is placed in a monitoring well, their operation is essentially the same. They collect readings while unattended, and can be downloaded remotely or by retrieval. Retrieval of a datalogger is essentially a repeat of installation, so in this case the non-vented datalogger is preferable for the same reasons as stated above. For remote downloading there is little difference between the two formats. However, from an ongoing operational standpoint, a vented transducer requires more maintenance including desiccant and regular cleaning.
Data collected from a non-vented datalogger requires barometric compensation, and therefore needs to be paired with simultaneously collected atmospheric data. For most applications this requires one barometric datalogger for any number of groundwater dataloggers in a region. Barometric compensation calculations are normally carried out using standard data management software, so although it represents an increase in operations, the process is automated. Data collected from a vented datalogger does not require barometric compensation.
One significant difference between the two types of dataloggers is that atmospheric data can be used to determine barometric efficiency, which can be very significant, especially in confined aquifers. This is a key advantage for non-vented dataloggers as vented pressure sensors assume 100% barometric efficiency and provide no data to investigate otherwise.
Data accuracy is paramount to successful groundwater projects, and it often starts with confidence in the data collected using a datalogger. Accuracy is an in-depth topic requiring a broad discussion beyond the scope of this article. But there are a few very general guidelines that can be used for deciding which type of datalogger provides better accuracy given different monitoring well conditions. In shallow applications where the datalogger can be located very close to the water surface, a vented datalogger can be expected to have marginally higher accuracy. However, in applications where the datalogger needs to be submerged at greater depth, due to expected water level fluctuation or otherwise, the non-vented datalogger will generally have increasingly better accuracy as the depth of submergence increases.
We recommend Diver non-vented dataloggers for our client's groundwater monitoring needs. As described above, non-vented dataloggers are easier to deploy, require less maintenance and provide better accuracy over a wider variety of conditions.