Why use self-cleaning water quality probes?

Written by Jakab András on .


It has become obvious by now that, as compared to traditional approaches such as collecting occasional grab samples and analyzing them under laboratory conditions, or performing measurements using hand-held instrumentation, much trusted monitoring data may be obtained by permanent deployment of water quality probes in either surface or groundwater monitoring locations. Of course, the traditional approaches have their undisputed technical and economical benefits as well.

The main advantages of permanent systems include, but are not limited to:

  • high measurement frequency,
  • measurement data relevant to exactly the same location,
  • acclimatized sensors,
  • no human intervention is needed to collect measurement data,
  • etc.

Disadvantages may include:

  • higher investment costs,
  • security and
  • maintenance issues

The higher investment costs are not a true disadvantage as the amount of data obtained and the information in it are probably a greater value than the cost of the system. Moreover, such systems are capable of generating alarms, therefore, they provide you with important and timely information for preventive actions to be taken. Security issues can be relatively easily solved with minimal investment costs.

As far as maintenance issues are concerned, the most important requirement – besides calibration – for the reliable operation of permanently deployed electrodes is to avoid the formation of physical, chemical and biological coatings on their surfaces. These problems are more apparent in the case of deployment in groundwater and/or non-flowing surface water bodies, but they may be a concern also in rivers.

Fortunately, the above problems can be easily sorted out by using Aquaread's AP-7000 multi-parameter probe that is equipped with a self-cleaning mechanism, which can be operated at the beginning of each sampling cycle. As far its efficiency is concerned, an AP-7000 probe that was removed for inspection two weeks after deployment is shown on the figures above and below. The probe is equipped with 7 electrodes, which measure 9 parameters. Two additional parameters (depth and temperature) are measured directly by the probe itself, and 4 parameters are calculated based on the measured ones. Looking at the probe, it becomes clear from a glance that conditions at the monitoring point are far from ideal. A significant biofilm layer has formed on both the probe and sensor surfaces during a relatively short time period. Nonetheless, the surfaces of the actual sensor points of both the optical and ion-selective electrodes are quite clear. This is due to the relatively frequent self-cleaning, i.e., the system performs one measurement cycle every 10 minutes, which is preceded by a complete cleaning cycle performed by the two-brush self-cleaning mechanism.

AP-7000 detail

AP-7000 Aquaprobe pulled out from its measuring location with its protective sleeve removed.

It can be seen that the ion-selective nitrate sensor right from the lower brush of the self-cleaning mechanism, followed by the CDOM (yellow light on), blue-green algae (red light off) optical sensors, and by the combined pH/ORP sensor are perfectly clean, warranting reliable long term usage.

Regular maintenance checks indicate (checks are performed roughly on a monthly basis) that calibration didn't go off by more than 10% for any of the electrodes. In contrary, much lower deviations were typical. In the meantime, the total energy consumption of the system is low enough to allow enough battery capacity for the duration of a complete monitoring cycle. This site is a fishpond, and the monitoring cycle/season is of approximately 5-6 months duration.