Vibration probe
A vibration probe (also known as a vibration level switch ) is a level limit switch for liquids and bulk solids. The basis is the observation that the movement of a vibrating tuning fork is dampened more when it comes into contact with solid or liquid than with air. In addition, the frequency changes .
Physical principle
Many different physical methods are suitable for monitoring the fill level of a container. The actual height rarely has to be measured continuously; it is often sufficient to reliably detect and signal when a limit level has been reached.
If a tuning fork swings freely in the air, it gradually becomes quieter and the decrease in amplitude can be measured. But if it vibrates under water, the amplitude will decrease more quickly because of the higher damping by the water. In addition, the frequency of the oscillation in water is lower than in air. Both physical effects - amplitude damping and frequency change - evaluate vibration probes.
Two metal rods, which sit symmetrically on a 1 mm thick metal membrane, are made to vibrate piezoelectrically at their resonance frequency. The ceramic disc then provides information - similar to a piezo microphone - how long this oscillation lasts.
Alarm function through frequency evaluation
The symmetrical structure of the vibration probes ensures that the probe is highly insensitive to contamination (buildup) and external vibration. The resonance frequency of the tuning fork (approx. 1,050 Hz) is reduced by approx. 15% by immersion in liquid. This triggers a switching process that stops pumps, for example. If the resonance frequency f a increases by +6.5%, for example due to fork corrosion , a fault message is generated.
The vibration probes can be used universally for viscous media, buildup, turbulence, external vibrations as well as air bubbles, gas formation and foam. They are ideal for use in the pharmaceutical industry, in hygienic areas such as food, and in the chemical and petrochemical industries.
Mechanical structure of a vibration probe for liquids
The oscillating bars of the symmetrical fork are made of stainless steel or a higher quality material. Coated versions are available for aggressive substances to which stainless steel is not resistant.
In the picture opposite you can clearly see a paddle-shaped end of the bars. On this paddle-shaped widened end, a notch marks the switching point in water when installed vertically from above.
The sensor of the vibration probe can be adjusted to the desired response level, the switching point, in the container using an extension tube. In cases in which the response level should not be known in advance, commissioning can be facilitated by using a so-called sliding sleeve.
Typical applications
Vibration probes for liquids can generally be installed in any position; installation from above (max detector / overfill protection) or from the side (min detector) is common. With highly viscous products, the fork is usually positioned so that the product can drip off. If installed on the side, the tuning fork should be tilted approx. 20 degrees with the paddles standing vertically. Paddles lying one on top of the other and installation from below are not advisable, especially with products that crystallize out, as this can actually provoke a build-up.
The list of media to be measured ranges from waste oil to food, e.g. B. milk, mineral water, beer and fruit juice to paints, varnishes, solvents and aggressive alkalis or acids, such as those found in chemistry.
literature
- Process automation - from field device to automation solution , Rüdiger Settelmeyer, 2007, ISBN 3865223052
- Level measurement technology. Basics and application examples , Ellen Amberger, 1999, ISBN 3478930146
- Process automation - measurement, control and automation solutions for production and logistics in the process industry , Endress + Hauser, 2007
- Level measurement technology - vibrating level switch for liquids , Endress + Hauser, 2004
Web links
- Example for bulk goods (PDF; 2.6 MB)