When bellows are used as the pressure-sensing element, it is desirable to add a spring for ranging and accurate characterization. Without the calibration spring, temperature effects and work hardening of the bellows would cause a loss of accuracy. The bellows are either brass or stainless steel, and the calibration spring is Ni-Span C, which ensures uniform motion throughout the range with practically no change due to ambient temperature variations.
The spring-loaded metallic bellows are compressed by the process pressure, which is forcing the lower end of the bellows upward against the opposing force of the spring. This vertical movement is transmitted through a suitable linkage or a torque tube assembly to the pointer. These units can also act as differential pressure detectors if the inside of the bellows is connected to a process pressure instead of being open to the atmosphere.
The spans of the single-bellows element vary with the diameter and construction materials of the bellows. By selecting the appropriate spring for the application, the sensor bellows can detect positive, vacuum, or compound ranges as a function of the location of the atmospheric pressure reference. The dual Bellows are available as differential pressure sensors. If either the low- or the high-pressure side is left open to the atmosphere, the unit will detect either gauge pressure or vacuum.
When relatively low pressures need to the measured, the process pressure is introduced into the inside of the bellows, while for higher pressures, the outside surface of the bellows is exposed to the process pressure. The force of the process pressure is applied to the lower end of the force bar. Due to the force balance nature of the unit, the force bar is constantly balanced; therefore, the sensing bellows do not move as long as the pressure detected is within the range of the instrument.
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Instrumentation Basics 