Process for testing the density of castings.
The casting is filled with helium, a helium-containing gas mixture or a forming or testing gas, closed and pressurized. In case of leakage, e.g. in a loosened structural area or cold shut, the pressure gradient between the interior of the casting and the ambient air leads to a gas current through the leakage channel. At the exterior wall of the casting, a sniffer probe is applied at an operating speed that has been defined by means of a test leakage. If the sniffer probe passes a leakage, the incoming helium is detected at the leakage detection unit.
The flow resistance of the sniffer probe and the vacuum system of the leakage detection provide the necessary pressure reduction from the atmospheric pressure to the operating pressure of the mass spectrometer of under 10ˆ’4 mbar.
This sniffer test enables high spatial resolution during the search for leakages and respectively faultless identification of defects at castings. Low overpressure also ensures low force on thin walls and enables to carry out the test at castings with very thin walls.
A variation of the process also enables to measure and evaluate the increase in concentration in helium in a shell around the sample. This process does not offer spatial resolution, however, a quantitative statement on whether leakages are present.
The helium concentration in the test gas, the test pressure and the natural helium base in the air (~5 ppm) limit the sensitivity of the analytic process. The detection limit is approx. 10-7 mbar·l/s. The efficiency of the process depends on the operator and cannot be easily calibrated. For this reason, the sniffer test process us often used as qualitative test process.
Differential pressure test, Water bath test