Appendix
Accuracy and Reproducibility of the Oxygen Activity Measurement
According to chemical principles, the logarithm of the oxy- gen activity versus the inverse of the absolute temperature should give a linear relationship.8,28
This is illustrated in Fig-
ure 23 for a melt with and without magnesium. The melt is cooled in the induction furnace in the absence of power. In both cases, all data fit very well to a straight line. The stand- ard deviation σ of the data for the melt without magnesium is defined as the logarithm of the oxygen activity. Hence the value of σ changes when expressed as the oxygen activity:
The same analysis for the magnesium treated melt is not ap- propriate because the magnesium content changes too much while holding. Instead, several measurements have been quickly carried out at a constant melt temperature of 1420°C (2588F) as shown in Figure 24. The curve in the Figure represents a second order polynomial regression which ap- proximates magnesium decrease of oxygen activity increase during holding. During this experiment magnesium drops from 0,047% to 0.022%. In this case the standard deviation becomes 3,73 ppb at an oxygen activity of 104 ppb which is nearly the same as listed above in the Table for 100 ppb. Since the oxygen activity is based on the measurement of both electro magnetic frequency and temperature, the values listed for σ encompass fluctuations of both parameters.
Figure 24. Oxygen activities measured in a melt containing magnesium (melt holding temperature 1420°C).
Figure 23. Oxygen activity measurements in melts with and without magnesium, at variable temperature.8 the absolute temperature.
T is
International Journal of Metalcasting/Spring 10
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