The Helical Bourdon sensor produces an even greater motion of the free end than the spiral element, eliminating the need for mechanical amplification.
Other advantages of this design include the high over-range protection available. For example, a 0 to 1,000 PSIG (0 to 6.9 MPa) element may be safely exposed to 10,000 PSIG (69 MPa) pressure, and it is suitable for pressure measurement on continuously fluctuating services. The range of the helical coil is affected by the diameter, wall thickness, number of coils used, and construction materials. High-pressure elements might have as many as 20 coils, while low-span sensors can have only 2 or 3 coils.
Available materials include bronze, beryllium copper, Ni-Span C, and stainless steel. The various suppliers have different practices concerning the pressure levels at which they change from spiral to helical elements.
Helical coils can also be used as the element in differential pressure sensors if one of the pressures is acting on the outside surface and the other on the inside of the coil. Motion balance instruments with helical elements include direct indicators, recorders, controllers, switches, and transmitters with either pneumatic or electronic outputs. Their inaccuracy is normally in the area of ±1% of the span.
List of Prominent Suppliers: Afrisco, Ashcroft, Burkert, Dwyer, Noshok, Omega, PCI Instruments, Prisma, Schmierer, Trafag, Winters, WIKA
Instrumentation Basics 