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do_measurementprinciple [31/03/2015 13:31]
pieter [Membrane thickness and cathode area] 		do_measurementprinciple [31/03/2015 13:34]
pieter [Temperature effects on the probe output]
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   * Teflon offers resistance to oxygen diffusion. Thus, at a given temperature and a fixed concentration of oxygen at the water-to-Teflon interface, the number of oxygen molecules arriving at the cathode per unit of time is inversely proportional to membrane thickness (D).   * Teflon offers resistance to oxygen diffusion. Thus, at a given temperature and a fixed concentration of oxygen at the water-to-Teflon interface, the number of oxygen molecules arriving at the cathode per unit of time is inversely proportional to membrane thickness (D).
  
-//N////t//// = f [O////2//// x (1/D)]//+**//N////t//// = f [O////2//// x (1/D)]//**
  
 From these relationships, it is evident that in order to have maximum sensitivity for the probe it is necessary to make the cathode area as large as is practical and to make the Teflon membrane as thin as is practical. The practical consideration for cathode size is its relationship to overall probe size, which usually is determined by where the probe must be placed. Membrane thickness must recognise desired response rate and rugged performance. Thin membranes provide quick response in addition to sensitivity because diffusion equilibrium is reached more quickly, but thicker membranes are tougher and will provide longer service. From these relationships, it is evident that in order to have maximum sensitivity for the probe it is necessary to make the cathode area as large as is practical and to make the Teflon membrane as thin as is practical. The practical consideration for cathode size is its relationship to overall probe size, which usually is determined by where the probe must be placed. Membrane thickness must recognise desired response rate and rugged performance. Thin membranes provide quick response in addition to sensitivity because diffusion equilibrium is reached more quickly, but thicker membranes are tougher and will provide longer service.
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 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.
-//•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, compensation for the overall temperature effect must be accomplished. 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, compensation for the overall temperature effect must be accomplished.
do_measurementprinciple.txt · Last modified: 31/03/2015 13:35 by pieter

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