Level Measurement How Wireless Vibrating Fork Frequency Monitoring Improves Process Safety

Author / Editor: Marianne Williams / Dr. Jörg Kempf

Vibrating fork technology has shown its value and reliability in a range of critical level monitoring applications. This article explains how frequency monitoring functionality provided by Wireless-Hart vibrating fork switches can support an increased range of applications that help improve safety and process performance.

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The Rosemount 2160 wireless level switch works well for monitoring and critical alarm duty applications where additional wiring would be costly and difficult.
The Rosemount 2160 wireless level switch works well for monitoring and critical alarm duty applications where additional wiring would be costly and difficult.
(Picture: Emerson Process Management)

The virtues of vibrating fork level switch technology are well known and extensively used by operators in the switch’s various primary roles, which include mission-critical overfill protection, fail-safe high and low level detection, and pump control and protection. But many industrial plant and process operators are now taking advantage of the additional frequency monitoring functionality of vibrating fork liquid level switches made possible by using Wireless-Hart communication.

The frequency monitoring functionality of vibrating fork switches is made possible by the unique design and sensitivity of the device, which is based on the principle of a tuning fork. A piezo-electric crystal oscillates the forks at their natural frequency and any changes to this frequency are continuously monitored. The frequency of the vibrating fork changes depending on the medium it is surrounded by. The denser the medium, the lower the frequency. Oil, for example, generates a much lower frequency than either water or air.


In its normal operating mode, for example when used to detect a low tank level, the liquid in the tank or pipe drains down past the fork, causing a change of frequency that is detected by the electronics and switches the output state from a wet to a dry condition. An alarm is sounded to alert the process operator that the level in the tank has dropped below the predetermined level.

In the same way, when used as a high level alarm, the liquid rises in the tank and makes contact with the fork, causing the output state to switch to a wet condition, alerting the operator to a potential overfill situation, or perhaps initiating an automatic response stopping the filling process.

Wireless vibrating fork switches have enabled devices to be installed quickly, cost-effectively and with minimum disruption to the plant or process. This has expanded the use and range of applications such as those found in remote parts of the plant where there is no existing network cabling. The additional functionality of vibrating fork devices is enhanced when paired with Wireless-Hart, which allows real-time data such as the output state detailed above, along with other parameters, to be transmitted over a secure wireless network.

Using the frequency monitoring functionality of vibrating fork devices means that you can accurately distinguish between fluids of differing properties, say, oil and water, or acid and water, at a given point in the production process, which provides measurable benefits. Being able to do so remotely with specific and accurate data, without exposing plant personnel to a potentially hazardous environment, also adds extra value in terms of safety and product integrity.

On the next page two examples serve to demonstrate and amplify the point.