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Microfluidics Stability and Resolution

In any discussion about stability, three parameters have to be distinguished:

  • Flow stability
  • Dynamic resolution
  • Measurement resolution

Microfluidic flow stability

This is the ability of the flow control device to limit the fluctuations, or variations of the physical parameter controlled. The lower the value of the stability, the more stable your flow.

To completely define stability, 3 criteria must be considered:

Stability band

Defined by the maximum and the minimum measured value for a requested set-point. For a pressure driven flow control system, this band is often given as a percentage of the full scale pressure range.

Standard-deviation

A statistic tool used to quantify the variation of a parameter compared with the average or mean value of a data set The standard deviation can be used to define confidence intervals.

Coefficient of variation (CV)

A relative way to express the standard-deviation. This parameter is given as one standard deviation divided by the mean value of the measured parameter. This is commonly expressed as a dimensionless percentage.


Dynamic resolution

It is the lowest variation of a physical parameter that can be reached in control mode by a microfluidic flow control system. For example, if you are using a control system based on pressure actuation, the dynamic resolution is the smallest amount of pressure that can be differentiated by the device. The dynamic resolution is highly linked to the regulation algorithm of the microfluidic instrument and its internal actuators and sensors. The lower the value of the dynamic resolution is, the finer one can control flow.


Measurement resolutiondynamic resolution schema

It is the lowest variation of a physical parameter that can be detected by a sensor. If you are using a flow control device based on pressure actuation, the measurement resolution is the
lowest pressure variation that can be detected by the internal sensor. Flow control systems based on a feedback loop cannot provide dynamic performance (stability, dynamic resolution) better than the measurement resolution of the sensor.

 

In this example, the flow controller 1 has a better measurement resolution and a better stability band. Both dynamic resolutions are the same. But due to its improved stability, the stability bands of the flow controller 1 (before and after t=50) do not cross each other. A pressure band (from 200.5 to 201.5) is not covered by this controller. Enhancing the dynamic resolution would solve this issue.

The flow controller 2 has poorer stability and measurement resolution: its stability bands cross each other. There is no pressure band uncovered by this flow controller but it provides the experiment with a lower level of performance. This can lead to difficulty in reproducing ones data.

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