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Measuring Dissolved Oxygen Concentration in Aquaculture

by the Fish Site Editor
09 January 2006, at 12:00am

By John A. Hargreaves and Craig S. Tucker, Mississippi State University and published by the Southern Regional Agricultural Center and the Texas Aquaculture Extension Service - Dissolved oxygen concentration(DO) is considered the most important water quality variable in fish culture. In the broadest sense, however, dissolved oxygen concentration is no more important than other environmental variables because any factor that is outside the range tolerated by fish cancause stress or death.

Measuring Dissolved Oxygen Concentration in Aquaculture - John A. Hargreaves and Craig S. Tucker, Mississippi State University and published by the Southern Regional Agricultural Center and the Texas Aquaculture Extension Service - Dissolved oxygen concentration (DO) is considered the most important water quality variable in fish culture. In the broadest sense, however, dissolved oxygen concentration is no more important than other environmental variables because any factor that is outside the range tolerated by fish can cause stress or death. Southern Regional Agricultural Center

What makes dissolved oxygen concentration so important in intensive fish culture is the speed with which it can change. Over a matter of hours, or sometimes even minutes, DO can change from optimum to lethal levels. No other critical environmental variable in fish culture is so dynamic.

The dynamic nature of dissolved oxygen results from the interaction of three factors. First, oxygen is not very soluble in water so water has only a limited capacity to hold oxygen. Second, the rate of oxygen use by fish, plankton and organisms living in the pond mud can be high. Third, oxygen diffuses very slowly from the atmosphere into undisturbed water. The combination of these three factorslimited solubility, rapid use and slow replenishmentcan cause rapid changes in dissolved oxygen concentrations. Dissolved oxygen levels can be managed with aeration, but the response time for taking corrective measures is short. This makes it critical to have a rapid and reliable method of measuring dissolved oxygen concentrations so that aeration devices can be activated when needed.

There are a number of ways to measure dissolved oxygen concentration. Select a method based on 1) the number of ponds or tanks to be measured, 2) the level of accuracy required, and 3) the cost of the measurement technique. The titration-based drop count method fairly rapidly assesses whether or not there is sufficient dissolved oxygen in water. The drop count method is inexpensive and appropriate if DO concentration is to be measured infrequently in a few ponds or tanks. However, on commercial fish farms or in any other situation where DO measurement of many ponds or culture units is routine, a dissolved oxygen meter is an indispensable piece of equipment.

What is an oxygen meter?

An oxygen meter has two components the sensor (sometimes called the probe) and the meter. Various types of sensors are available, but they all operate in basically the same way: the sensor reacts with oxygen and an electrical signal is produced in proportion to the oxygen concentration.

The signal is then amplified, translated into concentration units, and displayed by the meter. The meter circuitry also compensates the reading for changes in temperature, altitude or salinity. The meter circuitry may also include features to aid in calibration. Most DO sensors operate as electrochemical cells with a positive electrode (cathode) and a negative electrode (anode) connected by a salt bridge consisting of a saturated electrolyte solution. In most sensors, oxygen passes through a permeable membrane and is chemically reduced within the sensor. The chemical reduction of oxygen generates an electrical current that is processed by the electronic components within the meter and displayed as a DO concentration. The current is proportional to the concentration. Thus, DO meters do not measure oxygen concentration directly, but measure a voltage that is produced by the chemical reactions of oxygen with the various components of the sensor.

Types of dissolved oxygen sensors

Polarographic or Clark sensors use gold or platinum as the cathode and silver as the anode (Fig. 1). Polarizing voltage is applied to the cathode to cause the reduction of oxygen within the sensor. Oxygen is consumed at the cathode according to the reaction: O2 + 2H2O + 4e- 4OH-. In response to the production of hydroxyl ions (OH-) at the anode, and in order to preserve the charge balance of the electrolyte (saturated KCl) solution, chloride ions react with silver at the anode according to the reaction: Ago + Cl- AgCl. Therefore, the chloride ions in the electrolyte solution function as a carrier of the electric potential. Galvanic sensors use silver or platinum as the cathode and lead, iron or zinc as the anode.

Application of a polarizing voltage is not necessary because the reduction of oxygen in the presence of the sensor materials is spontaneous. Thus, a galvanic sensor is like a battery (fuel cell) that is fueled by oxygen. Galvanic sensors typically have faster response times than polarographic sensors and are more expensive.

Fiber optic oxygen sensors consist of an optical fiber with a sensor tip that contains a thin layer of oxygen- sensitive fluorescent dye dissolved in pure silicon. The optical fiber carries blue light from a lightemitting diode (LED) to the sensor. This stimulates the dye to emit fluorescent light that travels back up the optical fiber to a photodetector. Oxygen diffusing into the sensor tip binds to the fluorescent dye, which reduces (quenches) the intensity of light emission. The extent of quenching is directly related to oxygen concentration. Fiber optic sensors are very sensitive at low DO concentrations. Fiber optic sensors are sensitive to ambient light, but this problem can be overcome by coating the sensor tip with silicon. However, this silicon overcoat will reduce probe response time.


Figure 1. A cross section of a typical polarographic dissolved oxygen sensor.

Which oxygen meter is best?

Many different oxygen meters are commercially available (see list of manufacturers below), and each model has a unique combination of features that makes it more or less suitable for a particular application. The best meter for occasional use in an indoor setting, such as a hatchery, will be quite different from the one that is best for regular use under rough, outdoor conditions.

The purchase decision is further complicated by the fact that good meter systems are expensive, primarily because precious metals are used in the construction of many sensors. Before buying a meter, consult with other fish farmers, Extension specialists, aquaculture supply companies, and meter manufacturers to identify the most suitable one.

Some of the desirable features of a dissolved oxygen meter suitable for making field measurements include:

  • accuracy
  • rapid response
  • ease of calibration
  • water resistance
  • sturdy, rugged construction
  • automatic temperature compensation
  • manual salinity compensation
  • manual barometric pressure compensation
  • measures from 0 to 200 percent saturation
  • easily changed cable or probe
Other desirable features may include:
  • at least a 25-foot cable
  • a digital, liquid crystal display that can be read in bright sunlight or in total darkness
  • an integral membrane cap assembly
  • a built-in calibration chamber/storage sleeve
  • storage of measured values in memory within the meter (datalogging)
  • an RS-232 personal computer interface
  • a hold or auto-read function indicating that a stable reading has been attained
  • a battery charger

Operating an oxygen meter

It is beyond the scope of this publication to provide detailed operational instructions for each of the many kinds of meters. Carefully read the instructions that come with the meter to understand how it works and how to use it properly. Remember, the number displayed by the meter is not necessarily accurate. The number on the display is correct only when the meter has been properly calibrated, the measurement is made correctly, and the sensor and meter have been properly maintained. One common step in the use of most meters is calibration, and some details of this process are presented in the next section.

Further Information

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Source: Southern Regional Agricultural Center and the Texas Aquaculture Extension Service - Taken from site - January 2006

the Fish Site Editor