Measuring microliter droplets
Can droplets dispensed from an electromagnetic micro valve be measured in-line? The requirement for this, is a flow sensor that has a very short reaction time on flow changes. In the case of a micro valve, this reaction time has to be as low as 1 millisecond. The ReseaTech "Pulsed Flow Sensor" is capable of achieving that, as the following Application note will show:
To perform this Test, the Pulsed Flow Sensor (PFS) was used in combination with a micro valve (B-drop mini) to measure single droplet volumes from 0.5 to 13 microliters.
+ Pulsed Flow Sensor
+ PFS controller with User Interface
+ B-drop mini
+ Pressure supply 0.55 bars
+ Water container 50 ml
+ Silicone Tubing
The PFS was connected to a B-drop mini in a vertical setup. The tube between sensor and valve was kept as short as possible to make sure, the flow is not damped by the elastic tube.
The system was then pressurized to 0.5 bars (without pressure control*) and the measurement mode was set to “Droplet Mode”. This mode enables the sensor electronics to read the signal at the maximum frequency.
To control the switching cycle of the valve a peak-and-hold waveform was used.
*Note: To achieve very constant droplet volumes over a long dosing period, we usually install a pressure controller like for example the SMC-ITV0010.
The micro valve was operated with 5 different opening times from 2 to 50 milliseconds. Simultaneously the droplet volumes were measured with the Pulsed Flow Sensor. The measured diagrams show, that a constant flow of about 300 ul/s builds up after about 2 milliseconds. Having such a plateau phase, usually leads to very repeatable droplet sizes for the valve. And also leads to good droplet measurements.
With the very short opening times below 2 milliseconds, the valve already closes before a constant flow plateau can be reached. Therefore the repeatability is poorer. But a droplet of 0.5 microliters can still be detected.
Because the deviation of the measured droplets is a sum of the valve's deviation and the flow sensors deviations, we made a separate examination of the valves precision that will soon be available here.
The measured values:
|Opening time of the Valve|
|Drop no.||2 ms||5 ms||10 ms||20 ms||50 ms|
|Mean vol. µl||0.53325||1.2909||2.5878||5.40555||13.463|
|St. dev. %||1.23%||0.49%||0.38%||0.27%||0.28%|