other element in oscillation or in natural frequency vibration and triggering a relay when the process material in the tank reaches the vibrating element and damps out the vibration. The reed, probe, and tuning fork variations are distinguished only by the frequencies of their oscillation (reed, 120 Hz; probe, 200 to 400 Hz; tuning fork, 85 Hz) and by their dimensions or physical shapes. Their shapes are similar, although the close spacing between the arms of the tuning fork could make it more susceptible to material buildup in sticky services than the others. This is not necessarily so on all types of coatings because, although any buildup changes the natural frequency of vibration, a limited amount of buildup will not collapse the oscillation. The applications of vibrating level switch designs are also similar and include powders of different plastics, toners for copiers, detergents, powdered sugar, flour, ground or instant coffee, chocolate, dried milk, tea leaves, cosmetic powders; granules of plastic pellets, rice, wheat, beans, carbon, sugar, and salt; plus slurries and liquids. VIBRATING LEVEL SWITCHES The design of the vibrating reed level switch is illustrated in Figure 3.21a. The unit consists of a driver, paddle, and pickup. The driver coil induces a 120 Hz vibration in the paddle that is damped out when the paddle is covered by the process material. The pickup end contains a permanent magnet and a coil that generates a millivolt output signal when the paddle is vibrating. When the paddle is covered, the signal decreases and as a consequencepoints. The device will detect liquid–liquid, liquid–vapor, and solid–vapor interfaces, because the switch is sensitive enough to detect relatively small changes in the density of the surrounding process material. The characteristics of this switch are described by Figures 3.21b and 3.21c. Figure 3.21b is a plot of millivolt signal strength versus paddle coverage. Curve 1 describes the switch behavior in flour, 2 in water, 3 in polyethylene pellets, and 4 in granular sugar.

It can be noted that, in all four cases, the switch will actuate before the paddle is fully covered by the process material. For example, in the case of water (2), switch actuation occurs when the water level is about 0.4 in. (10 mm) above the bottom of the paddle. The curves in Figure 3.21c refer to granular powders at various densities. Curve 1 is for 60 lbm/ft 3 (960 kg/m 3 ), 2 is for 50 lbm/ft 3 (800 kg/m 3 ), and 3 is for 40 lbm/ft 3 (640 kg/m 3 ). As would be expected, the lighter the powder, the more level has to build up before switch actuation occurs. In case of the 40 lbm/ft 3 powder (3), the switch will actuate at 1.0-in. level rise, which corresponds to a level buildup of 0.25 in. (6 mm) above the top of the paddle. The vibrating reed level switch can be used to detect both rising and falling levels, and it can be installed in tanks or pipelines. Its use for measuring density and viscosity are discussed in Chapters 6 and 8, respectively. If the process material has a tendency to adhere to the paddle, such buildup can be removed by periodically purging the line through a purge well but, in general, this unit is not recommended for applications where buildup is probable. The sensing wires between probe and receiver should be shielded and grounded at both ends. Supply voltage variations between 105 and 125 V will not interfere with the measurement. When used on wet powders, the vibrating paddle has a tendency to create a cavity in the granular solids. If this occurs, the vibration amplitude will be the same as if the paddle were in the vapor space. Therefore, this level switch should not be used in applications involving wet powders. Where the solids bins are purposely vibrated and the vibration frequency is close to 120 Hz, this method of level measurement is unreliable and therefore should not be considered. TUNING FORK The tuning fork type of level switch is oscillated at its resonant frequency of about 85 Hz by a piezoelectric crystal located near the head of the fork. Another crystal, also mounted in the head, detects the vibration or the lack thereof. TheWhen the probe is buried by the process material, the vibration decreases, and this decrease is used to trigger the switch. To make it immune to the effects of hopper vibration, the switch has a relatively high resonant frequency. Figure 3.21e illustrates that the sensing probe should be located so that the angle of repose will not cause false level indication when the solids level is low.

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