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Solinst has always listened to customer feedback in order to offer the best selection of groundwater and surface water monitoring instrumentation. As a result, after many years and many requests, vented water level dataloggers were included in our product line to meet the needs of our clients.
Now that there are two options, what are the differences and which one do you choose?
Let’s answer this by discussing the differences in measurement fundamentals, operation, installation, maintenance, and accuracy.
Solinst Water Level Datalogger Measurement Fundamentals
Absolute Pressure Transducer
The Levelogger Series of dataloggers features an absolute pressure transducer. Absolute sensors are sealed; therefore, they measure all the pressure they are under, both water (hydraulic head) and air acting on the surface of the water (barometric or atmospheric pressure). As such, data must be compensated for barometric effects to gain hydraulic head measurements only.
Barometric compensation is most easily done using a second datalogger (a Barologger) which records air pressure only. Barometric compensation of Levelogger data is performed using the Barologger data and the Data Wizard in Solinst Levelogger Software.
Vented Pressure Transducer
The Solinst LevelVent and AquaVent feature a vented pressure transducer, also known as a gauged pressure transducer. Gauged pressure sensors are open to the atmosphere by use of a vented communication cable. The cable has a small vent tube, which runs the length of the cable from the surface, and terminates behind the pressure transducer. This vent tube acts as a channel for barometric pressure change at the surface, and allows the barometric pressure on the water column to be cancelled out by the pressure transmitted in the tube. The data is automatically compensated for barometric effects, as well as altitude.
Programming and Operation of Water Level Dataloggers
Although measurement fundamentals differ, Levelogger and vented datalogger operation and programming options are similar. Both are primarily programmed using Levelogger PC Software, through a USB connection. They are set to record readings at a specific rate, and then installed in the field for the monitoring period.
When finished recording, the readings are downloaded to your PC, or to a field data transfer device. Both also have options to integrate into a telemetry system or a third party datalogger or PLC for remote water level data transmission.
Installing Water Level Dataloggers
Because they are all-in-one units, Levelogger absolute water level dataloggers tend to be easier to install. They have the option of being suspended simply by using a stainless steel wireline or Kevlar cord. Installation, therefore, can also be less costly.
The LevelVent and AquaVent pressure transducers must be deployed using vented cable, which can be cumbersome – especially when dealing with longer lengths. Transporting and installation can be tough in more remote or isolated areas. You also have to make sure the vent tube inside the cable is not kinked or nicked while being handled.
Maintenance of Water Level Dataloggers
A major advantage of absolute dataloggers is that they are virtually maintenance-free. Save for periodic field visits for cleaning and maintenance, Leveloggers can be left in the field for extended periods with little worry. See our document about Levelogger Maintenance for more details.
Traditional vented cable systems have desiccants that need to be replaced regularly to ensure the vent tube remains dry. However, Solinst designed the LevelVent and AquaVent so maintenance is at a minimum, with hydrophobic filters and desiccants that provide moisture protection for the lifetime of the instruments. A custom tool to blow out the vent tube with dry nitrogen, before use or storage, was also designed.
Check out our Technical Bulletin that discusses the unique, low-maintenance features of the AquaVent water level datalogger.
Accuracy of Water Level Readings
Generally, vented transducers can provide more accurate water level data in shallow applications, where the pressure sensor is placed close to the water surface. However, the deeper you go, or the higher the expected water level fluctuation, it is recommended to use an absolute pressure sensor. Vented transducers may be subject to slow responses to small changes in barometric pressure, especially when they are at greater depths.
The barometric data that absolute pressure sensors record is actually an advantage. The barometric data can be used to determine barometric efficiency, which can be very significant, especially in deeper confined aquifers. Vented pressure sensors assume 100% barometric efficiency, which is generally not accurate, and provides no barometric data to determine any different.
The lack of barometric data also makes it difficult to confirm if the vented transducer is recording correctly. A crimped, damp, or cut vent tube can cause erroneous barometric compensation, and therefore inaccurate data.
In shallow applications, the vent tube does work well to transmit any changes in barometric pressure directly to the sensor, resulting in instant true water level readings. With absolute sensors, you have to take into account any inaccuracies in both the level sensor and barometric sensor when looking at the final water level data, even in shallow applications.
Absolute vs Vented: Your Water Level Datalogger Choice
Ultimately, the decision is up to you. Each type of water level datalogger is better suited to specific applications and conditions, and each has its advantages and disadvantages.
Vented water level dataloggers work well in very shallow applications with low expected water level fluctuations. They are also ideal when it is not convenient to install a barometric datalogger nearby.
Because vented water level dataloggers provide data that is automatically compensated, they can come in handy when real-time data is required, especially in short-term projects like pumping, and other aquifer characterization tests. Vented water level dataloggers can also save you time, reducing post collection data processing.
Absolute pressure sensors also have many advantages; due to their simpler designs, they are more convenient for transporting and installing in remote or hard to reach locations. They also tend to be lower in cost and require less maintenance.
Absolute pressure sensors are suited for a broader range of applications, and have more tolerance and accuracy at greater depths and with high water level fluctuations. Absolute water level dataloggers are also a better choice in areas that are humid, prone to flooding, or exposed to long term freezing conditions.
To learn more about the Solinst Levelogger, LevelVent or AquaVent, be sure to check out all the resources available on Solinst.com, or call to speak with one of our technical representatives.