Dissolved oxygen (DO) is the free oxygen available in solutions and is consumed in the metabolism of all aerobic organisms. DO concentration in water is used as an indicator of the health of ecosystems, and it varies with solution temperature, agitation, salinity, injection rate, and barometric pressure (Al-Rawahy et al., 2019; Wei et al., 2019). High temperatures and salinities decrease the capacity for DO. Most DO meters have temperature ranges of 0 to 40 °C and salinity ranges of 0 to 70 g⋅L–1 (typically from NaCl). These ranges encompass environments ranging from arctic to tropical and from freshwater to seawater.
Demand for DO in respiration increases with biological density, metabolic rate, growth stage, and temperature. High biological demand requires greater inputs of oxygen to sustain homeostasis (Ben-Noah and Friedman, 2018). The nutrient solution of liquid hydroponics benefits from levels of DO close to saturation near 8 mg⋅L–1. This promotes healthy root respiration and minimizes hypoxic microsites or the development of anaerobic bacteria (Schroeder and Lieth, 2004). DO concentrations less than 5 mg⋅L–1 can lead to reduced root respiration and can result in the development of adventitious roots (Holtman et al., 2014). DO is necessary for nitrification in aquaponic systems; concentrations near saturation promote aerobic bacteria and nitrification whereas depleted levels promote anaerobic bacteria and denitrification (Wongkiew et al., 2017; Zhen et al., 2015). Accurate measurement of DO helps identify hypoxic zones even in seemingly well-aerated solutions.
DO can be measured using three methods: iodometric titration, membrane diffusion (electrochemical), and fluorescence quenching (optical) (Tai et al., 2011; Zaitsev et al., 2018). Iodometric titration is a colorimetric titration and is labor intensive. More than eight companies sell a DO meter that uses optical technology, and more than 12 companies sell a meter with electrochemical technology. The meters tested in our review represent about 20% of the electrochemical DO meters available. Meters using electrochemical technology are less expensive than optical meters. With electrochemical meters, DO crosses a semipermeable membrane, is reduced at the cathode, and produces a voltage difference with the anode that is proportional to oxygen concentration (Parra et al., 2018).
There are two types of electrochemical technology. Polarographic sensors have a gold cathode and silver anode that are polarized temporarily at 0.8 V during measurement. Galvanic sensors have a silver cathode and zinc anode that are polarized constantly even when the meter is powered off (Wei et al., 2019). Polarographic electrodes require a warmup period of up to 15 min. Galvanic electrodes do not require a warmup period, but constant polarization decreases the lifetime of the electrode.
Sensor technology determines price. Galvanic sensors are typically less expensive than polarographic sensors, which are less expensive than optical sensors. Optical sensors typically have a faster response and greater resolution than polarographic and galvanic sensors. We evaluated DO meters at three price points for the accuracy, stability, and response time across salinity and temperature.
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