Why is the use of an absolute vacuum level important?
Other reduced pressure units measure a test vacuum that is adjusted to a level below available standard atmospheric pressure, which changes as high and low pressure weather systems move across the country. The error induced in the chamber vacuum can be more than 2.5 inHg, and will produce major changes in samples of molten aluminum solidified at these different vacuum levels. The Figure below shows the impact of changes made in the test vacuum level upon samples containing a high level of dissolved hydrogen.
