Bourdon tube-type pressure elements are used to detect higher pressures (spans exceeding 30 PSI, or 200 kPa), because their spring gradient is insufficient for detecting lower pressures or vacuums. Bourdon tubes are manufactured in C, helical, and spiral forms (see Fig. 5.4a). The C-Bourdon element is made by winding a tubular element circularly to form a segment of a circle, the helical element is made by winding the tube several turns into a helix, and the spiral is formed by winding two or three turns in a spiral around the same axis. In each of these cases the circular cross section of the tube is flattened. When these elements are pressurized, their cross section tends to become more circular, which tends to straighten their shape (see Figure 5.4b). Helical and spiral elements are used when it is desired to reduce the stress in the tube and to increase the tip movement.