Dissolved Oxygen Meters Professional Instruments for a Variety of Applications Dissolved Oxygen Theory and Measurement Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in a system. Measurements are usually taken in water using a DO probe and meter. Henry’s Law states that the concentration of gas in a solution is directly proportional to the partial pressure of that gas above the solution. Henry’s Law constant is a factor of proportionality, and so is specic to the gas in the solvent being measured. The partial pressure of oxygen is in fact a measurement of the thermodynamic activity of its molecules. The rate at which oxygen dissolves, diuses, and reacts is not determined by its concentration, but by its partial pressure. The Earth’s atmosphere is composed of 20.9% oxygen, and at sea level the atmosphere is 100% saturated with oxygen. Percent saturation is the amount of DO present per amount of DO possible at a given temperature and pressure. Percent saturation is a common unit for DO measurement since it is based upon the partial pressure of a gas; thus it is correct for determination in any solvent. Concentration measurements of DO can also use the units of parts per million (ppm) or milligrams per liter (mg/L). In meters that report DO concentration in ppm or mg/L, the solvent is always assumed to be water. In other solvents such as oils or acids, the Henry’s Law constant would be dierent. In those cases, percent saturation should be used as it is incorrect to use ppm or mg/L. Eects of Temperature and Pressure As the temperature of a solution increases, the particle movement within that solution increases. With greater particle motion, dissolved gases escape more readily from solution. In warm water, oxygen is less soluble while in cold water, oxygen is more soluble. DO concentration in air saturated waters decreases with increasing temperature. Atmospheric pressure decreases as altitude increases. Since there is lower partial pressure, oxygen is less soluble at higher altitudes. DO concentration in air saturated waters decreases with increasing elevations. Applications Water quality measurements are vital to environmental monitoring. In quiescent lakes and rivers, the decay of organic matter can cause bacteria levels to increase. The aerobic bacteria consume oxygen, triggering a deciency that can cause a water body "to die," killing aquatic plants and animals. Aquaculture is the breeding, rearing, and harvesting of plants and animals in all types of water environments. Dissolved oxygen is needed by sh, zooplankton, and plants to survive and reproduce. DO measurements are used to monitor and control the environment required for success. Wastewater treatment plants rely on bacteria to break down the organic compounds found in water. If the amount of dissolved oxygen in the wastewater is too low, these bacteria will die and septic conditions will occur. The amount of DO must be consistently monitored to ensure proper waste treatment. Wine and beer are both aected by oxygen at various stages during production and storage. DO is an important parameter to monitor for those who wish to produce consistent, high quality products. Laboratory Monitoring of BOD, OUR and SOUR BOD (Biochemical Oxygen Demand) is an empirical test that determines the relative oxygen requirements of wastewater, e£uent, and polluted waters. BOD measures the rate of oxygen uptake by microorganisms in a water sample at a xed temperature over a given period of time. To ensure that all other conditions are equal, a very small amount of microorganism seed is added to each sample being tested. The samples are kept at 20°C in the dark for ve days. The loss of dissolved oxygen during incubation is called the BOD5. OUR (Oxygen Uptake Rate) measurement indicates the rate of metabolic processes in sludge treatment, helping operators determine the stability of solids after digestion. It is dened as the milligrams per liter of oxygen consumed per hour. SOUR (Specic Oxygen Uptake Rate) also determines the oxygen consumption of a system, but is dened as the milligrams of oxygen consumed per gram of volatile suspended solids (VSS) per hour. Types of Dissolved Oxygen Probes Hanna Instruments oers three types of Dissolved Oxygen sensors. Polarographic DO probes consist of a working electrode (cathode) and a counter electrode (anode). A polarizing voltage is applied to these electrodes that is specic for the reduction of oxygen. A thin, gas permeable membrane isolates the sensor elements from the water sample but allows oxygen to pass through. The oxygen that passes through the membrane is reduced at the cathode, causing a current from which the oxygen concentration is determined. Two-electrode polarographic probes use the anode as a reference electrode. Galvanic DO probes consist of a working electrode (cathode) and a counter electrode (anode) that act as a battery to produce a voltage specic for the reduction of oxygen. A thin, gas permeable membrane isolates the sensor elements from the water sample but allows oxygen to pass through. The oxygen that passes through the membrane is reduced at the cathode, causing a current from which the oxygen concentration is determined. Optical DO probes are based on the principle of ¦uorescence quenching. The sensing method typically has an immobilized luminophore that is excited by a light and emits a light at another wavelength. Oxygen quenches this excitation. A photodetector measures the light, and it is used to calculate the dissolved oxygen concentration. 6 Dissolved Oxygen 6.2 | www.hannainst.com introduction
RkJQdWJsaXNoZXIy MTI3NTM=