RC, or reverse circulation, drilling is performed when sampling is required for specific projects or decision-making requirements. Drilling companies usually use this technique on large-diameter holes because it is fast and easy to clean the hole.

The fluid and cuttings flow up the drill pipe/rod, and down the ring. In softer formations, this causes less washout and makes sampling faster and more accurate because cuttings are not tainted by sloughing formation off the hole walls. They also come up the drill pipe very fast, giving an accurate formation log.

To achieve this, the air is blown down the ring of the rod, creating an air-lift of the water and cuttings through the inner tube, ultimately forcing it into a sampling hose attached to the cyclone, and finally landing in a sampling bag to be taken for analysis.