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Time-Pressure micro dispensing

ANWENDUNGSBEISPIEL

Introduction

This application note shows how single droplets with a volume from 0.2 to 16 µl can be dispensed with a micro dispensing valve.

 

Used Material

  • 4-channel valve controller

  • B-drop mini (micro valve)

  • Pressure supply 0.5 bars

  • Pressure controller (SMC ITV)

  • Water container

  • Silicone Tubing

Time-Pressure dispensing

To dispense microliter droplets with a micro dispensing valve, a pressurized fluid container is necessary. The pressure inside the tank forces the fluid to the valve, similar to a water tap. An electric signal opens and closes the valve, what leads to dispensed droplets.

In the specific case of a micro dispensing valve, the opening times have to be only a few milliseconds in order to dispense very small droplets.

Time-Pressure dispensing

Short valve switching times

A micro valve is built with a strong magnetic coil and a very small and light moving plunger or membrane. Combined with a short plunger stroke, this leads to very short switching times that make micro dispensing possible. In the example of the B-drop mini, the switching only takes 0.3 milliseconds.

Controlling a micro valve

An electromagnetic microvalve is usually driven by a valve controller. The valve controller can adjust the valve opening time with a resolution in the microsecond range. It also allows a current reduction to prevent the valve coil from overheating. This mode of operation is called peak-and-hold waveform. In the special case of ReseaTech’s valve and controller, the electrical polarity can be switched to actively close the valve. The valve controller also has a trigger input to synchronize dispensing with a moving axis, e.g. of a printer or an automatic dispenser.

Influence of opening time, pressure and viscosity

The dispensed quantity of a micro valve is usually linearly dependent on the valve opening time. This is because the valve has a constant flow in the open state. However, the physical properties of the dosed fluid also influence the dispensed quantity. Thus, viscosity and pressure have a direct effect on the flow rate and thus on the dosed volume. For media whose viscosity is strongly dependent on temperature, a change in temperature also has an indirect effect on the dosed volume. In order to achieve exact time-pressure dispensing, the supply pressure must be controlled and the temperature must be constant. In addition, the medium must always be the same.