Oil is still flowing into the Gulf of Mexico, but only through vents in a new capping stack now sitting atop the leaking BP well.
Those vents will be closed one by one later Tuesday as engineers monitor the pressure in the well.
"In essence they want to confirm that there is only one leak and that is coming up through the pipe, that no oil is escaping from other spots in a damaged well," explains Louisiana State University oil expert Ed Overton.
"If we see high pressure, it's a good sign. It actually means that the flow of the oil is fully contained in the existing well," says BP Chief Operating Officer Doug Suttles. "If low pressure exists then that would indicate that potentially oil is escaping out of the casing at some point."
Engineers will also be watching for hydrates, the ice like crystals that clogged previous containment systems.
There is also hope that the cap could soon be holding back all of the oil or pumping it all to ships on the surface.
Today's tests will also help determine how to proceed with plugging up the leak from the bottom with relief wells, which remain the one permanent solution.
Tests could take another 48 hours to complete.
Meanwhile, the Obama administration has issued a revised moratorium on deepwater drilling, hoping the new wording has more success than the version struck down by the courts.