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
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:
- 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
- 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.