To detect higher pressures Bourdon tube-type pressure elements are used because their spring gradient is insufficient for detecting lower pressures or vacuums. Bourdon tubes are manufactured in C-type, helical, and spiral forms. 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. Helical and spiral elements are used when it is desired to reduce the stress in the tube and to increase the tip movement.
Some of the more desirable features of the Bourdon tube elements include their fast speed of response (usually 0.1 s full scale) and good sensitivity (0.01% of maximum pressure when unrestrained). Their limitations include a nonlinearity of about 0.5% full scale, which can be compensated mechanically or electronically. Most pressure gauges use a bourdon tube as their pressure-sensing for greater motion (and therefore greater gauge resolution).
Materials of Construction: Phosphor bronze, alloy steel, 316 and 403 stainless steel, beryllium copper, Monel, Ni-Span C
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