The results of three cuts made through the same Reduced Pressure sample, illustrating the potential variations that might be found because of minor segregation in the bubble formation. The alloy is 356, and the samples were taken at the Kingsville Plant of ThyssenKrupp-Stahl Co.
A similar result on a second sample with a much lower hydrogen level than Sample 1, above, and on a 319 alloy melt. If the target hydrogen production level is for a high sample density (low gas), the potential for error is obviously rather high, and scrap castings could result.
Note: In addition to the variations in bubble density that may exist between sections on the same sample, the subjective estimates of individual readers have been noted to vary by as much as two or three levels.
The differential gage on the left, as used on most current RPT testers, measures a change in vacuum level from the current atmospheric pressure. As may be seen from the graph of atmospheric pressure in Madison, WI below, the pressure may vary over an average range from about 30.8 inches of mercury down to about 28.1 inches when high and low pressure weather systems pass through the area. By lowering the chamber pressure by 26 in.Hg, as measured with a differential gage, the actual chamber vacuum might vary over a range as much 4.8 in.Hg to 2.1 in.Hg (or the equivalent reading measured in mmHg) above a perfect vacuum.
For accurate testing of aluminum melts, the answer is to use the absolute vacuum gage on the right, which ignores variations in the current atmospheric pressure. With this type of meter, the test chamber may be always evacuated to a constant level, which is a major controlling variable in the size of the induced porosity in Reduced Pressure samples. The resulting process control accuracy that is achieved is one of the primary benefits of using a T.R.P. Tester (True Reduced Pressure).
TYPICAL RANGES of ATMOSPHERIC PRESSURE
This plot shows the daily average mean sea level pressure reported at the Dane County Regional Airport for the three year period covered. The net effect of these pressure changes can introduce significant errors to the test chamber vacuum level, as may be seen below.
The graph shows the resulting chamber vacuum level after pulling down the vacuum to the -26″Hg gage reading. The colored areas are zones of control error and represent the amount of improvement achieved by the TRP Vacuum tester.