Purchase online at Metals4u or phone 01937 534318 for a speedy delivery. A compression coupling works by using clamping force to seal the fitting. The unit consists of a compression nut, a compression ring and a seat. The compression ring, often called a ferrule, is placed on the tubing with the nut behind it. The tubing is then inserted into the seat, and the nut is screwed onto the seat. As the nut is tightened, it compresses the ring around the tubing, creating a leak-free seal.