Sensing Liquid Levels with Vibrating Fork Technology

In my Emerson Process Management RSS news feed was news of a new wireless vibrating fork liquid level switch from Emerson’s Rosemount measurement team. Not being familiar with this technology, I connected with product manager, Kevin Cullen. I asked him about this technology and how it fits in the world of liquid level sensing applications.

He first explained that there are a number of liquid level sensing technologies in addition to vibrating fork including ultrasonic, capacitance, and electro-mechanical. To understand the principle of operation, I turned to the Rosemount 2160 data sheet:

…designed using the principle of a tuning fork. A piezo-electric crystal oscillates the forks at their natural frequency. Changes to this frequency are continuously monitored.

The frequency of the vibrating fork sensor changes depending on the medium in which it is immersed. The denser the liquid, the lower the frequency. When used as a low level alarm, the liquid in the tank or pipe drains down past the fork, causing a change of natural frequency that is detected by the electronics and switches the output state to a Dry condition.

When the 2160 switch is used as a high level alarm, the liquid rises in the tank or pipe making contact with the fork, causing the output state to switch to a Wet condition.

The output state, along with other parameters, are regularly transmitted over a secure wireless connection to a Smart Wireless Gateway.

Kevin noted that these vibrating fork liquid level switches come in a range of models to address applications from very simple ones rated for unclassified areas to ones like the 2160 with advanced diagnostics rated for environments with temperatures from -94 degF (-70 degC) up to 500 degF (260 degC) and pressures to 1450 psig (100 bar g). This sensing technology is largely unaffected by liquid turbulence, foam, vibration, solids content, or coatings on the tuning fork. An adjustable delay for turbulent/splashing applications prevents false trips of the switch.

Kevin shared some of the advanced diagnostics available to spot problems with the device or surrounding process. These included external damage to the fork, internal damage to the sensor, coating buildup on the fork, and excessive fork corrosion. Alerts can be set to notify operations and maintenance of these conditions before they impact the process with a false trip condition.

The addition of WirelessHART communications is the part that Kevin said excites many process manufacturers. Most every plant, production field or platform, or mill has a need for level measurement in areas that are currently too costly to measure using wired switches, due to either inaccessibility or economics. One application example is using the 2160 for overfill protection in a tank that did not have convenient instrumentation wiring access.

Not knowing much about vibrating tuning fork technology before I started this post, I learned a lot from Kevin. I hope if you were similarly not well versed, that you did too.

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