This shows you the differences between two versions of the page.
Both sides previous revision Previous revision Next revision | Previous revision | ||
do_measurementprinciple [31/03/2015 13:32] pieter |
do_measurementprinciple [31/03/2015 13:35] (current) pieter [Galvanic probe] |
||
---|---|---|---|
Line 2: | Line 2: | ||
The dissolved Oxygen electrode is a galvanic MPOD (Membrane-covered Polarographic Oxygen Detector) system. | The dissolved Oxygen electrode is a galvanic MPOD (Membrane-covered Polarographic Oxygen Detector) system. | ||
- | =====Measurement | + | =====Galvanic |
A galvanic MPOD consists internally of two metals of different nobility which serve as electrodes. The more noble metal is the cathode (Silver, Platinum or Gold), the other (Lead or Zinc) is the anode. In the presence of an electrolyte, | A galvanic MPOD consists internally of two metals of different nobility which serve as electrodes. The more noble metal is the cathode (Silver, Platinum or Gold), the other (Lead or Zinc) is the anode. In the presence of an electrolyte, | ||
Molecules of oxygen dissolved in the electrolyte will diffuse to the surface of the cathode and pick up electrons which, in combination with water, will produce hydroxyl ions. At essentially the same time, hydroxyl ions will give up electrons at the anode and form an oxide. The resulting transfer of electrons establishes a current flow through an internal resistance which is measured as a voltage by the oxygen meter. | Molecules of oxygen dissolved in the electrolyte will diffuse to the surface of the cathode and pick up electrons which, in combination with water, will produce hydroxyl ions. At essentially the same time, hydroxyl ions will give up electrons at the anode and form an oxide. The resulting transfer of electrons establishes a current flow through an internal resistance which is measured as a voltage by the oxygen meter. | ||
- | The membraned | + | The membrane |
* The membrane encloses the two electrodes in a captured volume of electrolyte, | * The membrane encloses the two electrodes in a captured volume of electrolyte, | ||
Line 44: | Line 44: | ||
There are two factors related to temperature which must be recognised in order to correlate the output of the dissolved oxygen probe with concentration of molecular oxygen in the sample. | There are two factors related to temperature which must be recognised in order to correlate the output of the dissolved oxygen probe with concentration of molecular oxygen in the sample. | ||
- | //•As the temperature of water decreases, kinetic energy of water and oxygen molecules decreases and molecular attraction increases. As a result, the concentration of oxygen which must be present in the water to establish a particular concentration of oxygen at the water-to-Teflon interface increases.// | + | * As the temperature of water decreases, kinetic energy of water and oxygen molecules decreases and molecular attraction increases. As a result, the concentration of oxygen which must be present in the water to establish a particular concentration of oxygen at the water-to-Teflon interface increases. |
- | + | | |
- | //•The resistance to oxygen diffusion through the Teflon membrane increases as temperature decreases.// | + | |
Both of the temperature factors serve to decrease the rate at which oxygen molecules reach the surface of the cathode as temperature decreases. Therefore, if the read-out from the dissolved oxygen meter is to display the correct reading of oxygen concentration for all samples which have the same concentration of oxygen but are at different temperatures, | Both of the temperature factors serve to decrease the rate at which oxygen molecules reach the surface of the cathode as temperature decreases. Therefore, if the read-out from the dissolved oxygen meter is to display the correct reading of oxygen concentration for all samples which have the same concentration of oxygen but are at different temperatures, |