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As the demand for water resources continues to grow in the U.S. and abroad, the ability to assess the impact of urban development and agriculture on water resources is more important than ever. To meet this growing demand, water and wastewater industry professionals have a greater need to monitor well levels with water level data loggers in order to document baseline and changing water levels over time.
Water level loggers typically incorporate built-in microprocessing, pressure sensors and battery power in a rugged enclosure designed for long-term underwater deployment. They can be deployed and left unattended for months at a time, collecting water level data at user-defined intervals and storing it digitally into logger memory. By operating in a continuous 24/7 monitoring mode, water level loggers eliminate many of the hassles of manual data collection approaches and facilitate monitoring of multiple locations at the same time.
Water level loggers also automate the process of archiving and reporting data. Users can simply offload the logger data to an office or laptop PC and create detailed graphs or tables with the click of a mouse. The charts can be easily printed for documentation purposes while the electronic data is automatically archived.
Although water level loggers have become the data collection instruments of choice for an increasing number of water industry professionals, the multitude of product choices available today can make it difficult to determine which product is right for the application. The following are some of the most important variables to consider during the product selection process, along with some tips on specific product features to look for.
When evaluating water level accuracy, there are a number of points to be aware of with respect to specifications.
Helpful questions for the manufacturer include:
In addition to these questions, be sure to ask the manufacturer if the logger’s accuracy has been verified or measured against NIST-traceable standards. Some companies stand behind their accuracy specifications by providing a calibration certificate of accuracy with each logger.
Vented vs. non-vented loggers
There are two primary types of water level loggers—vented and non-vented.
Vented loggers incorporate a vent tube built into the cable that enables them to automatically compensate for atmospheric pressure changes. By equalizing these changes on both sides of the pressure sensor, a well-designed and maintained vented water level logger can provide high-accuracy water level data.
Non-vented loggers do not use vent tubes. Instead, these loggers can be barometrically compensated using a barometric pressure logger and a simple software function to perform the mathematics. Barometric pressure values can also be obtained from nearby weather stations within a 10-mile radius.
When comparing vented and non-vented loggers, be aware that while vented loggers have the potential to provide the greatest accuracy, they have a number of limitations that cause problems and result in bad data and/or loss of data.
Vented loggers are bulkier than non-vented loggers. This makes transporting them out to field sites more difficult—especially when several units need to be deployed. In many cases, the bulkiness of a vent cable can also become a problem when trying to fit the logger down a narrow well opening. The cable must be protected when extended over sharp casing edges, and the end must be stored in a watertight location while the logger is deployed.
Most vented loggers require the use of desiccants for moisture protection. While desiccants can effectively keep moisture out of the logger, they typically need to be changed on a regular basis. This adds to the amount of logger maintenance required, which increases the total cost of ownership of the logger.
Vent tubes with contaminant-resistant material must be used if contaminants are present in the groundwater or surface water being monitored. This can add to the cost of a water level logger as well. Additionally, if a logger has been deployed in contaminated water, it must be decontaminated before it can be redeployed. This may take a considerable amount of time since vent tubes are typically 25 ft in length or longer.
Vented loggers are not flexible when it comes to deploying them at various depths. Their cables cannot be lengthened without sending them back to the supplier, and cables typically cost several dollars per foot. Shortening the cable requires the user to delicately coil the cable without creating any kinks.
Condensation can easily build up in vented loggers, which can lead to accuracy problems.
If the end of the vented logger cable is inundated by rising water, all subsequent data are compromised due to unknown pressure compensation dynamics during the flood event. This is a significant problem when monitoring water levels of streams and rivers during storm events.
These limitations highlight the advantages of a non-vented logger. Non-vented loggers are more compact, require minimal maintenance, can be easily deployed in wells of varying depths and are not affected by floodwater.
Just as water level loggers can vary considerably from model to model, so too can the graphing and analysis software applications that accompany them. In terms of specific features, make sure the logger software supports the following:
Buying vs. renting
In the past, the relatively high price of water level loggers has prevented many potential users from purchasing their own supply of loggers.
Many have chosen to rent loggers instead. While renting can be a convenient option for short-term deployments, it is important to be aware that the price of water level loggers has recently come down to the point where purchasing is more cost-effective for many applications.
When choosing a water level logger, make sure the logger’s PC interface enables quick and easy hook-up to a laptop or office computer. When using a PC equipped with USB ports (Most computers manufactured today have them.), choose a logger with a direct USB interface. Direct USB enables plug-and-play ease-of-use, which can be particularly useful when offloading data in the field. Direct USB also enables users to offload data in a matter of seconds compared to the minutes it takes via serial communications.
Loggers that rely on mechanical plug-in connectors can be damaged by water in the field and cause logger failures. Water level loggers with an optical interface that is completely sealed within the logger’s housing eliminate the possibility of water-related damage and/or failures.