วันพุธที่ 12 เมษายน พ.ศ. 2560

Dissolved Oxygen Meters Reviews.

Dissolved Oxygen Meters

    Dissolved oxygen meters are a instruments that measure the quantity of oxygen dissolved in water. In normal condition at normal temperature and pressure in an aqueous solution that is allowed to equilibrate with room air, oxygen constitutes about twenty point nine percent of the total dissolved gas with nitrogen being about seventy-eight percent(some carbon  dioxide is present as well). These proportions are similar in air and in solution equibrated with room air. The atmospheric pressure on the water surface along with temperature are factors that modify the concentrations of DO and therefore have to be compensated for a correct reading.

                 Since dissolved oxygen determination is based on measurement of a current that is established between the anode and cathode, the salt content of water is another factor that can influence readings and must also be compensated.

      Applications of Dissolved Oxygen

              Dissolved oxygen is an important indicator of the degree of usefulness of a sample of water for a specific application. Dissolved oxygen can exist in water in different concentration and is important for the respiration of a wide variety of animals and aerobic bacteria in the aquatic environment. Other applications include: water treatment plants, sewage treatment works, effluent activated sludge process, river monitoring, fish farming, and generally any other field where water quality is important.

               Biotechnological processes, are another area of applications where dissolved oxygen measurements are essential to maintain quality of the finished product.

        In water quality applications, such as aquaculture and fish farming, the level of DO must be kept high, if the DO level falls too low the fish will suffocate. For waste water treatment, in sewage treatment, bacteria decompose the solids, if the DO level is too low, the bacteria will die and decomposition stops : the DO level is high, there is no need to spend energy in the aeration of the water.

           For industrial applications like boilers or cooling towers, low DO levels prevent corrosion and scale build-up which inhibits heat transfer.

Polarographic and Galvanic Probes

        Hanna dissolved oxygen meters utilize one of the two common types of dissolved oxygen sensing probes: polarographic sensors and galvanic sensor. Polarographic sonsor technology is based on the Ross and Clark polarographic measurement method. An oxygen probe is composed of a platinum electrode and a solid silver electrode. A concentrated potassium chloride solution is held in place over the surfaces of the electrodes by a PTFE membrane. An external voltage creates a different in potential between the cathode and anode(less than zero point five volts).

       The external voltage applied to the platinum electrode cathode, silver electrode anode, potassium chloride solution and gas-permeable membrane material establishes a current that is proportional with the concentration of oxygen.

        In contrast, a galvanic probe requires no external voltage. The difference in potential between the cathode and anode is greater than zero point five volts. Galvanic DO sensors consist  of two electrodes, a Zinc or lead anode and silver cathode, both of which are immersed in electrolyte. An oxygen permeable membrane separates the anode and cathode from the water being measured. Oxygen diffuses across the membrane and interacts with the probe internals to produce an electrical current.


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