Dissolved oxygen levels in water are dependent on several environmental factors, including temperature, altitude (barometric pressure), salinity. There are a variety of meter types available for use in the field these days, from the simple dissolved oxygen and temperature to the all-inclusive multi-parameter units.
Let's talk dissolved oxygen. It is, without a doubt, one of the most important water quality parameters to follow in aquaculture systems. The technology has come a long way, from the radio box-looking analog meters that I started out using to the newer optical meters with immediate readings and require very little maintenance.
There are a variety of brands out there. However, this article will focus on the types of meters and probes that are currently available.
In the early days of aquaculture, there were not a lot of options for measuring dissolved oxygen. Readings were made via what chemistry. As things progressed, electrochemistry meters became available such as the Model 51 from YSI. These were analog meters and were workhorses for several decades before the more modern digital meters started coming available.
Dissolved oxygen levels in water are dependent on several environmental factors, including temperature, altitude (barometric pressure), and salinity. Early versions of DO meters required manual compensation for these.
Today's models almost always include temperature compensation and can consist of salinity and altitude compensation.
As the industry grew, so did the investment from several companies back into better technology for farmers and environmentalists to use.
There are a variety of meters available for use in the field these days, from the simple dissolved oxygen and temperature to the all-inclusive multi-parameter units.
One of the factors to consider when purchasing a DO meter is what style sensor will be used. There are the older style electrochemical-based galvanic and polygraphic sensors and the newer optical-base sensor.
Polarographic Sensor Style
Polarographic sensors are the original electrochemistry sensor. This style probe uses a silver medal anode and a gold medal cathode. Polarographic senors require the user to allow the sensor to warm up or polarize. Once the sensor is warmed up or polarized, the meter’s readings are quick.
Galvanic Sensor Style
Galvanic sensors generally use a lead-based android and a silver cathode. These sensors are self-polarizing and require very little warm-up time, so they do not require a warm-up period.
Both Galvanic and Polarographic require flow past the membrane to provide accurate readings. Both style sensors use a removable membrane that the oxygen passes through to react with the sensor to determine oxygen concentration through a chemical reduction reaction.
Both styles require periodic membrane and solution changes and need to be calibrated often. Thankfully, most sensor manufacturers have adopted a screw cap membrane that is much easier to change than the old-style membrane and O-ring. The probe must be appropriately maintained to have accurate readings.
This includes scouring the probes’ exterior portions to remove any buildup and making sure the new membrane is completely full of the probe solution with no air bubbles.
Optical Sensor Style
These have become all the rage for some very good reasons! The optical sense style sensor uses a phosphorescence coating on the sensor cap.
The probe emits a pulsing light that causes the sensor cap to phosphoresce based on the concentration of dissolved oxygen in the water. Meters that use the style of sensor have calibration memory that requires very little calibration and doesn't react to dissolve gases in the water source.
These are more expensive than traditional sensors, but the readings are more stable, require less calibration and maintenance, and do not require flow or the use to agitate the water around the probe. These are great options for meters that are in constant use in the field.
The meter side…
All of these probes are available with several options and terms of the user interface. Some of the more economical ones are not waterproof, while the more expensive models include waterproof casings, internal memory, and optional downloads to software that help keep the trending information available, allowing for better management of aquatic systems.
Higher-end models include additional sensors such as pH, ORP, Conductivity, and other common water quality parameters. The ultimate decision on which model to choose depends on the application.
At the end of the day, it is very important that facilities have a way of tracking oxygen in their systems to avoid catastrophic losses do to variable DO in their systems.