1 What is the water depth?
When selecting a water level logger one of the first things to consider is the water level at the monitoring location. To determine this a knowledge of the potential change in level at each location will be needed. For example, if a borehole has 100m of water but the change will only be 5m it is not necessary to have a 100m range water level logger. Potentially, a 10m range model would be sufficient. However, if the variation in level is greater, for example, 80m, then a large range water level logger would be required.
2 How accurate?
After determining the range of the logger, the next thing to consider is the required accuracy. The lower the range of the water level logger the greater the accuracy. Bell Environmental's 9m rugged Troll 100, has an accuracy of +/-9mm. However, in comparison, the 76m version has an accuracy of +/-76mm. If monitoring a surface water application, which typically has lower levels, Bell recommends using a model with a lower level range to provide the best possible accuracy.
3 Saline/acidic water
The potential salinity and acidity of the water are important to consider when choosing a water level logger. If the application is likely to be acidic or saline consider certain materials for the water level logger. For these applications, a titanium bodied product would be recommended, with a ceramic pressure sensor to ensure the unit is not corroded or damaged as these materials offer high levels of resistance. In the case of a clean water application stainless steel or titanium products can be used.
4 Is flooding an issue?
Determining if the location is likely to flood is crucial and can be the difference between having a system which works accurately and a system becoming damaged. In applications prone to flooding an absolute (non-vented) water level logger is recommended. The reason for selecting the absolute version simply comes down the way the sensor works. With a vented system there is a breather tube, with a desiccant connected to the top of the sensor cable. If this desiccant and breather were to become submerged there is potential for water ingress into the water level logger and potential damage to the sensor. Absolute water level loggers are completely sealed meaning there is no risk of water ingress via a vent tube.
5 Vented or Absolute?
It may be the case that the points above have highlighted key aspects of applications, which in turn have confirmed the model needed. If not, then an overview of both the key advantages and disadvantages of both a vented and absolute water logger should help further.
For the highest level of accuracy, the vented option is recommended. In most cases, this will also be more accurate than an absolute system. An example of this is shallow applications, as the vented tube directly transmits barometric data to the water level logger pressure transducer. With an absolute sensor, users need to consider both the inaccuracies of the water level logger and the barometric logger. However, in deep monitoring applications or those with a greater level variation it may be advantageous to use an absolute sensor, where the measurement of individual barometric data alongside level data allows the determination of the barometric efficiency.
6 What is the budget?
Cost is another factor, typically absolute water level loggers are lower in cost, especially when monitoring with multiple sensors in one location, as a single barometric logger can be used to compensate multiple water level loggers. The costs could be comparable to monitoring a single point, however, this would be determined by the cable lengths. As well as purchase cost, the maintenance of absolute water level loggers is a lot less as there is no desiccant or breather tube to protect from water ingress.