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Bespoke microdosing solutions from Bürkert

Bürkert recently assisted a customer with a time-pressure dosing system that required a bespoke dosing head that could deliver precise low dose volumes. Tony Brennan, Gas & Micro Field Segment Manager, describes the unique solution that Bürkert was able to provide, which could reactively adapt the dose according to temperature fluctuations, equalise the pressure in the liquid reservoir and CIP clean the flow path of the device in between every batch.

Bespoke time-pressure dosing system

The customer was initially using a pneumatic pump and the problem with that was that it was very expensive and needed a lot of maintenance. After so many cycles, you had to strip the whole thing down, which meant that the line was down while you were maintaining the pumps. They were keen to find an alternative way of doing things.

They looked at a number of different technologies but a time-pressure dosing system won out. It gave them quite a lot of flexibility in that you can change the dosing on the fly, so it’s not a fixed dose that you’re always dosing. It could vary the dose depending on the viscosity of the liquid going through. These were all positives as far as the customer was concerned.

As part of the project, we designed an integrated PCB board, which had an integrated pressure and temperature sensor on it. This allowed information on the temperature and pressure of the liquid being dosed to be sent back to the main system controller continuously. If the temperature changed during the dispense cycle, the liquid viscosity could change, which could introduce inaccuracies to the dosing volume. By monitoring the temperature continuously, it is possible to adjust the time the valves are energised on for on the fly to compensate for any changes in viscosity.

Other things to note on this system are that we had to have FDA conformity for components that were in contact with the liquid. The other novel thing about the unit is that it was controlled by a CANopen bus communication signal. It was optimised by having one wire to the unit and then daisy-chained to additional units with the single cable, through which power and data were transferred.

Precise, pressurised microdosing with CIP capabilities

The dispensing part of the application was quite stringent. The volume of the dosing liquid was relatively small. You couldn’t have much overdosing or underdosing without causing a problem. This solution consisted of 3-4 separate systems that we supplied for the machine builder to manufacture and supply to the end user.

In time-pressure dosing, you have to maintain the headspace pressure at the top of the liquid reservoir. This is usually achieved by using a 2/2 way proportional valve to inject a propellant gas. A second valve, usually a 2/2 way on/off valve, is used to exhaust the propellant gas from the liquid reservoir headspace if the setpoint pressure you are trying to achieve is exceeded.

The pressure in the headspace is continuously monitored by a pressure transmitter, which sends the process value to the system control unit in the dosing system. The system control unit compares the process value to the setpoint and calculates how far open or closed the proportional valve needs to be. A signal is then sent to the proportional valve to move it into the desired position.

This system means every time the dispense valves are open the pressure in the liquid dispense reservoir remains the same and therefore the rate of dispense is constant.

As that liquid level fell, we had to replace the pressure in the system, so that the head space was continuously pressurised at a constant pressure. We had a little unit that injected air, took a reading of the pressure and then would eject air if it was too much over the setpoint or inject more air if it was beneath the setpoint. It did that continuously. We had to develop a system that could do that and be integrated with the customer’s PLC.

In order to maintain the sterility of the whole system, we also had to develop a Clean In Place (CIP) system that would inject a caustic solution to kill anything in the flow path of the device after each batch. That involved valves and instrumentation to detect the change from water to caustic solutions, for example. These were the main sub-systems in this project.

Fast switching and repeatability from Bürkert WhisperValves

From a micro point of view, the really important part was the dosing head. Inside there you had the Type 6712 Bürkert WhisperValve, which has a very low dead volume and operates very quickly. It’s a very fast valve, which you need when you’re dosing at very low volumes. There are two things you need: for the valve to switch very quickly and for it to be repeatable. The Type 6712 has a dead volume as little as 5 μl, so it was the ideal valve technology for this application. It’s also very small, so you can fit several of them into a relatively small space envelope. That was also very important.

Inside the dosing head you had the Bürkert WhisperValve, which was on a custom manifold made from PEEK. Then we had a customised PCB board. When the customer set the system up they had to calibrate it to calculate how long they would need to keep the valves open to dose a certain amount of liquid. All of that information was stored on the EEPROM on the PCB board. It would store that as a lookup board in its system and when it was dosing. If it noticed the temperature falling or rising it could automatically adjust the time that the valves were held open or closed.

Time-pressure dosing to suit your application

This sort of time-pressure dosing system aligns itself towards pharmaceutical applications, biotechnology or the analytical instrument market. Wherever you’re dosing relatively low volumes of expensive liquids where you need a relatively high accuracy. Those are the sorts of applications where this system could be used. 

You’ll never find two Bürkert microdosing systems exactly alike. Every time we get a system enquiry, the top half of the dispensing manifold will remain the same but the point matrix over which you’re dispensing will always vary. The actual manifold for the dosing head will need to be bespoke to the application. We have our own machining tools to design and machine the manifolds used in these systems.

We’ve got various other different technologies we can use too. For instance, injection moulding is one. Or we can machine manifolds in plastics and in metal. As far as producing the manifold is concerned, that’s our bread and butter business.

For more information about bespoke time pressure dosing systems from Bürkert and our Type 6712 WhisperValve, or to discuss your application requirements in more detail, please contact our sales team today on +44 (0) 1285 648 720 or email sales.uk@burkert.com.

Type 6712

2/2-way WhisperValve with media separation

Type 6712
  • Highest chemical resistance combined with minimum internal volume
  • Compact design with 7 mm installation width
  • Orifice 0.8 mm
  • Switching noise < 36 dB
  • For dosing applications with excellent cleanability