Article written by: Jason Danaher Ph.D.
Dissolved oxygen is one of the most important water quality parameters in aquaculture because it directly affects fish respiration, feeding behavior, growth rates, and survival. When your fish or aquatic organisms experience sub-optimal dissolved oxygen concentration you can expect reduced feeding, poor feed conversion ratios, immunosuppression, and chronic/sub-chronic mortality events. In many aquaculture systems, oxygen is the first limiting factor for biomass and that means your dissolved oxygen readings must be accurate and reliable. A handheld oxygen meter with the appropriate oxygen probe is an ideal technology for all aquatic facilities to quickly measure the oxygen concentration in their system water. Oxygen meters may come with one of three types of dissolved oxygen probes: optical, polarographic, or galvanic. Here is a brief explanation of each probe.
The first and modern-day gold standard is the optical dissolved oxygen probe. An optical probe functions by using a luminescent dye cap and a light to excite the dye. The presence of oxygen reduces the light signal. The probe measures the reduction of the light signal and an algorithm converts the measured light signal to a dissolved oxygen concentration. Advantages of an optical probe include: no oxygen consumption, minimal dependence on water flow passing by the probe, reduced maintenance, and stable readings. Optical probes are excellent for continuous monitoring scenarios. Some disadvantages of an optical probe are the greater upfront cost to purchase the technology and eventually, as with all monitoring equipment, the sensor cap will need to be replaced.
The second option is the polarographic dissolved oxygen probe. A polarographic probe functions using an electrochemical sensor. Oxygen diffuses through a membrane where it is reduced by a cathode and encounters a polarizing voltage. The current generated from the reduction of oxygen is proportional to the actual dissolved oxygen concentration present in the water. A polarographic dissolved oxygen probe has a lower upfront purchase price compared to an optical probe; however, some disadvantages of a polarographic probe are the required maintenance of the membrane and electrolyte solution associated with the probe, the requirement for consistent water flow to the probe for accurate measurement of oxygen concentration, and a required warm-up time of 10-15 minutes before use to obtain accurate and reliable results.
The third and final option is a galvanic dissolved oxygen probe. Like the polarographic probe, an electrochemical reaction generates voltage, but unlike the polarographic probe no external polarization from the meter itself is needed. A galvanic probe is self-powered using dissimilar metals (i.e. zinc/lead anode and silver cathode) to create the internal voltage that reduces the oxygen molecule. The advantage of a galvanic probe is its simplicity in design, it is economical compared to the optical and polarographic probes, and it requires little to no warm-up time before it is capable of supplying the operator with accurate and reliable results. Disadvantages of the galvanic probe are the anode and cathode do not stop reacting with one another when the meter is off. This results in less operation time before replacement of the probe is necessary and like the polarographic probe it too requires constant water flow to measure oxygen. Also, the stability of the galvanic probe reading is often less than the polarographic and optical probes.
Both the polarographic and galvanic probes consume oxygen during the electrochemical reduction process, but it is minuscule compared to that of other biotic factors in the aquatic system. The main concern with the consumption of oxygen is if water flow around the probe is insufficient, then the operator may obtain a false low reading, when in fact there is sufficient oxygen concentration in the water. If your monitoring program is to take place in a tank or area of the system with low flow, then the optical probe would be your best choice. Table 1 below provides a quick side-by-side comparison of the three different oxygen probes discussed above.
Table 1. Dissolved oxygen probe comparison table.
If budget allows, then optical probes are the best long-term investment for continuous monitoring in intensive production facilities like hatcheries and recirculating aquaculture systems. The galvanic probe may be more convenient than polarographic for spot checking dissolved oxygen in your aquatic system if the budget is limited.
A handheld meter and dissolved oxygen probe are an important life-support monitor for your facility. Good oxygen management and records improve your aquatic animal growth, reduce stress, and protect your investment by preventing catastrophic loss that could have been prevented. Feel free to contact Aquatic Equipment and Design, Inc. at 407-995-6490 or email info@aquaticed.com to determine which technology may be best for your monitoring and operational goals .