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can view the sample during solidi- fication to see if bubbles form on the surface. In practice, the reduced pressure test is conducted two ways: 1. Low Gas Contents: Some-


times low gas metal may be required, such as needed for high-end aerospace castings or safety criti- cal components. In these cases, the best procedure is to use a low test pressure (high vacuum) and to count bubbles forming on the surface. When less than two or three bubbles appear on the surface of the casting, gas content is low. 2. Medium to High Gas Con-


tents: Sometimes higher gas levels are beneficial, such as in permanent mold castings, where higher gas level can help avoid shrinkage. In this case, a higher test pressure (lower vacuum) is used so that the gas remains inside the solidifying sample. Te test result is usually measured by determining


Fig. 3. A basic RPT system includes a pump, a gauge and a control system to regulate the pressure.


the density or specific gravity of the RPT sample. In either case, it’s a good idea to


train operators to look at the surface of the samples, both during the test and afterward. Two RPT samples are shown in Fig. 3. The sample on the right is of better metal quality, having fewer oxides and presumably lower gas content.


It’s important to choose the cor-


rect test pressure for the RPT test. To do this, one needs to know a little more about how pressures are measured and how pressure deter- mines when porosity is found during solidification. Most of the gauges used on RPT testers are differential, measuring the difference in pressure between the vacuum test chamber


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