Creating an application specific solution for micro dosing


Tony Brennan, Gas & Micro Field Segment Manager for Bürkert Fluid Control Systems, looks at the technology involved in micro dosing and how the latest developments can be integrated into existing equipment

Creating a micro time-pressure-dosing system requires a range of components that need to be carefully linked together to deliver a reliable and effective solution

Many industries use micro dosing technology such as pharmaceuticals, medical applications including in-vitro diagnosis, environmental analysis and inkjet printing. However, despite a wide variety of industries using this technology, it's essential that the most appropriate design and materials choice is made depending on its use.

The two most common methods of micro dosing are time-pressure dosing and volumetric dosing. Each method has its strengths and will be more appropriate for some applications, therefore making the right choice is essential to a successful installation.

Control of time and pressure

Creating a micro time-pressure-dosing system requires a range of components that need to be carefully linked together to deliver a reliable and effective solution. The initial starting point is the pressure vessel where the dosing fluid is stored. This needs to be fitted with both level sensors and a highly accurate pressure transducer to ensure a constant supply of fluid at the correct rate.

The fluid is initially controlled by flow control valves which deliver it to the dosing system. In some cases, a conductivity probe can be fitted at this point to check that the correct fluid is being used. The dosing system itself will be based around a precision flow control valve that meets the specifications of the process.

Time-pressure dosing needs two controlled circuits to operate effectively. One circuit controls the dispensing of the fluid, and the other maintains the pressure of the propellant gas used to drive the liquid through the system. With a known system pressure, the volume being dispensed is controlled by the defined opening time of the valve. This operation is usually controlled by a Human Machine Interface (HMI) software programme (programmable logic controllers via a HMI).

This design has a number of benefits. Firstly, it's capable of delivering a high dosing frequency while controlling the dosing speed. Secondly, the dosing volume is variable, to the point that continuous dosing is also possible. Thirdly, this design is typically compact in dimensions and can be arranged to deliver several different fluids into one process.

Modern valve design has enabled micro precision whilst maintaining the highest hygiene standards. Onboard circuit boards enable calibration and process parameters to be retained within the valve assembly, while network connectivity is assured through fieldbus communications.

This type of system is more commonly seen in pharmaceutical, medical and biotech applications, as well as drinks vending machines, where speed and hygiene are prerequisite.

Volumetric dosing

For applications that still require accuracy and high dosing speed, micro dosing pumps provide the answer. These are excellent for continuous dosing applications and their more simplistic arrangement makes them very cost effective. They are often used in process filling, dosing adhesives or lubricants, or as environmental analysers.

Micro pumps are designed for a variety of applications. Often the dosing amount is controlled by a frequency generator and accuracy levels are typically ±2%. In some cases, all the wetted parts can be easily changed and the fluidic chamber can be designed as a disposable item.

Installed reliability and precision

The purpose of micro dosing systems is to deliver repetitive volumes of fluid quickly and accurately. Moreover, the accuracy must not decline over the lifetime of the dosing valve, so continued precision and long-term reliability are essential. To that end, Bürkert has developed a family of microfluidic valves that can operate at high pressures with a typical service life of 200 million cycles.

There are specialist products within this range, such as the Type 6712 Whisper valve, which offers fast acting control and an internal volume of just 5 µl. Products such as this can be combined with a host of compatible control equipment to create tailor-made systems.

From the initial application assessment, through design and construction, to installation and commissioning, working with a single supplier can reduce complexity and deliver the most efficient solution.

Its advantageous for these systems to fit seamlessly with existing process equipment, especially in terms of communications. As an expert in fluid control systems, Bürkert offers a complete turnkey solution for all micro dosing applications, including complete integration with the existing network topology.

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