At the heart of every Rheonics sensor is a resonator. Rheonics sensors are always in tune with the fluids they’re measuring!

The resonator vibrates in the fluid; the fluid influences the resonator’s vibrations. By measuring its effect on the the resonator, we can determine the fluid’s density and viscosity.

resonance of DV sensor

Rheonics resonators are influenced by fluids in two ways:

The denser the fluid, the lower the resonant frequency. A denser fluid increases the mass loading of the resonator.

mass loading effect on resonance

The more viscous the fluid, the broader and smaller the resonant peak of the sensor, Friction between the resonator and the fluid increases its damping.

viscous damping effect on resonance

The measurable properties of the resonator – its resonant frequency and damping – are both influenced by the properties of the fluid.

The Torsional Advantage

Many types of fluid sensors use lateral vibrations. Vibrating wire viscometers, for instance rely on the displacement of the wire perpendicular to its long axis. Flexural tuning fork resonators have two tines that vibrate as cantilever beams, with motion perpendicular to the plane of symmetry of the tuning fork.

In general, sensors that vibrate laterally are harder to isolate from the structures in which they are mounted. Mounting forces, the mass of the mounting structures, and even temperature can influence the response of the resonators in ways that are unpredictable, and therefore influence the repeatability of measurements.

Rheonics sensors vibrate in torsion. Their active elements twist about their own axes, rather than vibrating laterally. Torsional sensors are easier to isolate from the structures in which they are mounted. They are also less disturbed by ambient vibrations than are lateral resonators

Shape of the resonator - determines the measurements

The shape of the resonator determines the way in which it responds to the fluid in which it is immersed. Rheonics’ SRV series of sensors are cylindrical, vibrating parallel to their own surfaces. They are influenced primarily by shearing forces, and are therefore relatively insensitive to mass loading effects. They are useful for measuring viscosity but not density.

shape of the resonator

Rheonics’ DV series of sensors have flattened end masses. Parts of their surfaces vibrate parallel to themselves and therefore shear the fluid. These contribute to the damping of the resonator and determine its sensitivity to viscosity. Other parts of the surface vibrate perpendicular to themselves, and therefore displace the fluid. This results in mass loading of the sensor, and determines its sensitivity to density.

The Balanced Resonator Advantage

Resonant sensors fall into two further geometric categories – balanced and unbalanced.

A tuning fork is a typical balanced resonator. Its two tines vibrate in opposite directions, balancing out bending forces that are otherwise transmitted to the sensor’s mounting.

A single transversely vibrating beam ( a “half tuning fork” ) by comparison, exerts large reaction forces on its mounting, resulting in large energy loss compared to the balanced tuning fork geometry.

A vibrating wire, on the other hand, is an unbalanced resonator, and exerts substantial forces on its mounting structures.

In order to reduce the effects of mounting conditions on unbalanced resonators, their anchors must be relative large and massive compared to the size of the actual sensing element.

Insensitive to mounting conditions

Rheonics’ sensors use balanced resonators (patents pending). The DV series uses a torsional tuning fork configuration, in which the two tines twist in opposite directions. The SRV series uses a unique patented coaxial resonator, in which the two ends of the sensor twist in opposite directions, cancelling out reaction torques on their mounting.

DV balanced resonator eliminates anchor effects

Accurate Sensors need Accurate Electronics

Rheonics fluid sensing systems rely on patented technology that allows the use of one electronics platform – the evaluation unit – for all of our sensor products.

The core task of the evaluation unit is to drive and interrogate the resonant sensor in order to determine its resonant frequency and its damping. Once these two quantities have been determined, it is up to a sophisticated set of algorithms to convert these measurements into values for density and viscosity.

Our electronics platform is based on the phase shift method of evaluating the resonant frequency and damping of the resonant sensor, coupled with Rheonics’ patented gated phase-locked loop technology.

Rheonics Intellectual Property

A strong portfolio of patents (granted and pending) back the Rheonics (formerly Viscoteers) products & services.

  • 8752416 Resonant conductor measurement system and method
  • 8291750 Resonant measurement system and method
  • 7691570 Chemical analysis using dynamic viscometry
  • 5837885 Method and device for measuring the characteristics of an oscillating system
  • 4920787 Viscometer

Over a dozen patents pending covering cutting edge innovations in fluid sensing technology.


Rheonics products are based on three decades of research at ETH Zurich. The underlying technology is published in multiple journals, article and covered in numerous PhD theses. A small selection of published work is presented below:

ETH Zurich 2014 research bulletin article on Rheonics (previously Viscoteers)

Explore publications at ETH Zurich Institute for Mechanical Systems

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Rheonics, Inc.
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Sugar Land, TX, 77478

Tel: +1 832 998 0606