The free end motion of the C-Bourdon tube is insufficient to operate some of the motion balance devices, such as the transmitters. The spiral element is a series of C-Bourdon tubes joined end to end.
When pressure is applied, this flat spiral tends to uncoil and produces a greater movement of the free end, requiring no mechanical amplification. This increases the sensitivity and accuracy of the instrument because no lost motion or friction is introduced through the links and levers.
Standard spiral materials include bronze, steel, stainless steel, beryllium copper, Monel, and Ni-Span C. Spans as low as 10 PSI (69 kPa) are available for positive, negative, or compound ranges. Some start using helical elements at above 200 PSI (1.4 MPa) spans; others stay with the spiral element up to 4,000 PSI (27 MPa) and higher. Unflattened, heavy-wall, spiral tube springs that detect pressures up to 100,000 PSIG (690 MPa) are also available in direct indicators.
List of Prominent Suppliers: Afrisco, Ashcroft, Burkert, Dwyer, Noshok, Omega, PCI Instruments, Prisma, Schmierer, Trafag, Winters, WIKA
Instrumentation Basics 