When you next enjoy a packet of potato crisps, you almost certainly won’t notice a difference in flavour from one crisp to the next. This is thanks to the highly accurate dosing of relatively low volumes of strong yet concentrated flavouring, and the consistency in taste is an essential aspect of ensuring consumer quality. It’s the same issue in pharmaceutical products.
Achieving high-accuracy fluid control and metering for applications such as these can be traced back to Parisian mathematician and scientist, Gaspard-Gustave de Coriolis (1792-1843). He discovered an effect that relates to an inertial force acting on objects in motion within a rotating system, such as whirlpools.
In the field of liquid and gas flow, Coriolis control and metering devices involve a suspended, S-shaped measuring tube set in high-frequency magnetic vibration by an exciter coil.
When a liquid flows through the tube, the liquid’s inertia acts on the vibration of the tube, causing what is known as a phase shift. This vibrational change is directly proportional to the mass flow of the liquid, and is measured by two sensors, positioned at each end of the tube. The higher the flow rate, the greater the oscillation deflections of the measuring tube.
This process can also provide a density reading of the flowing liquid by measuring the vibration frequency, instead of the oscillation deflections. The lower the frequency, the higher the density. In fact, with a Coriolis-based device, it is possible to simultaneously measure mass flow, volumetric flow, density and temperature.
In recent years, technology has allowed the Coriolis principle to be used for increasingly lower flow rates, enabling dosing of the smallest volume of liquids. Bürkert’s Type 8756 mass flow meter/controller, based on the Coriolis effect, has a flow rate accuracy reading as low as ±0.2%. This is achieved with a repeat accuracy deviation of just ±0.1% for a flow rate up to 25 kg/h.
This reading is independent of the impact of the media, pressure, or temperature.
By comparison, a thermal mass flow meter, for example, takes a volumetric reading that is then corrected to mass. It is accurate to around ±1%, and also requires recalibration for every new fluid type introduced.
The Type 8756 can also be supplied with an integrated batch controller to provide fast measurement and control for fast dosing, which increases productivity.
Where there is an external pressure source, Bürkert combines the Type 8756 Coriolis flow sensor with a matched control valve suited to the process specification. This could range from a proportional valve for continuous pulsation free flow delivery, through to a Lorentz actuator valve for extremely fast dosing applications.
When an external pressure source isn’t available, Bürkert combines the Coriolis flow sensor with a gear pump. Pump speed is controlled by the Coriolis device itself to generate the precise pressure required for the desired flow rate. The fine teeth of the gear pump give continuous and extremely smooth flow delivery, according to the company. It is also possible to combine the Coriolis meter/controller with other pump technologies, like peristaltic pumps or motor speed controllers.
A video demonstration is available via www.is.gd/iyorur.