Figure 8-8.–Dual bellows assembly.When in operation, the bellows will move inproportion to the difference in pressure appliedacross the bellows unit assembly. The linearmotion of the bellows is picked up by a drive armand transmitted as a rotary motion through atorque tube assembly (fig. 8-8). The indicatingmechanism multiplies rotation of the torque tubethrough a gear and pinion to the indicatingpointer.Bellows elements are used in various appli-cations where the pressure-sensitive device mustbe powerful enough to operate not only theindicating pointer but also some type of recordingdevice.PRESSURE SWITCHESOften when a measured pressure reaches acertain maximum or minimum value, it is desir-able to have an alarm sound a warning, a lightto give a signal, or an auxiliary control system toenergize or de-energize. A pressure switch is thedevice commonly used for this purpose.One of the simplest pressure switches is thesingle-pole, single-throw, quick-acting type shownin figure 8-9. This switch is contained in a metalFigure 8-9.—Typical pressure switch.8-5
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