Difference Between Resolution and Stability

When discussing microfluidic systems, there can sometimes be confusion with regards to the meanings of stability and resolution. Three parameters need to be examined to clarify the differences between these properties.

Flow Stability

This is the ability of the flow control device to restrict fluctuations or variations in the physical parameter controlled. The lower the value of the stability is, the smaller the variations are, and hence the more stable your flow is.

The Dynamic Resolution

The dynamic resolution is the smallest variation in a physical parameter which can be achieved in control mode by a microfluidic flow control system. For instance, if a control system based on pressure actuation is used, the dynamic resolution is the smallest difference between two pressure orders attainable using the control device.

The dynamic resolution is highly dependent on the regulation algorithm of the microfluidic instrument and its internal actuators and sensors. The flow can be tuned more sharply when the value of the dynamic resolution is smaller.

When using microfluidic controllers based on pressure actuation, dynamic resolution is expressed as a percentage of the full pressure range.

The Measurement Resolution

This is the smallest variation in a physical parameter which can be detected by a sensor. When using a flow control device based on pressure actuation, the measurement resolution is the lowest pressure variation sensed by the internal sensor.

Dynamic resolution and stability better than the measurement resolution cannot be achieved by microfluidic flow control devices based on feedback loops.

Figure 1. Comparison between flow controller 1 and 2

It can be seen in Figure 1 that the flow controller 1 has better stability and measurement resolution than flow controller 2. However, the dynamic resolution in the same.

Since the stability of flow controller 1 is high, the stability bands of flow controller 1 (before and after t=50 unit of time) do not cross each other. This controller does not cover the pressure band (from 200.5 to 201.5 pressure unit). Improving dynamic resolution will solve this problem.

The flow controller 2 has poorer stability and measurement resolution, and its stability bands cross each other. There is no gap in the pressure bands covered by this flow controller, but this is traded off with lower experimental performance.

Definition of Stability

Table 1. Table of definitions and equations

These three parameters need to be analyzed while choosing a flow control system:

• The stability band– This is the band defined by the minimum and maximum value determined for a specific set point. This band is normally given as a percentage of the pressure range for a pressure-driven flow control system.
• The standard deviation– This is a statistical tool for quantifying the variation of a parameter in comparison with the average value. The standard deviation can be used for defining confidence intervals assuming that the physical parameter to be controlled fluctuates around its mean value with a normal (Gaussian) distribution.
• The coefficient of variation– This is another way of using standard deviation. This is specified as the standard deviation divided by the mean value of the measured parameter. In contrast with the standard deviation, the coefficient of variation is a dimensionless quantity.

Microfluidics Flow Controllers Based on Mechanical Displacement

Microfluidic flow controllers based on moving components, such as a peristaltic pump or syringe pump, do not have pressure sensors or internal flow rate sensors. Their operating principle is founded on controlling a moving component and the relation between the mechanical movement and the flow-rate, but not on real time measurements.

This implies that the only information available is theoretical flow-rate values. However, these theoretical values may be affected by several external and internal parameters as seen in Table 2.

This may lead to a lack of stability,  poor repeatability, and decreasing performance, without the user even knowing it. No data is offered about whether the flow-rate orders have been reached or not.

To avoid this, all of the microfluidic flow control solutions available from Fluigent integrate high precision pressure or flow rate sensors. As well as their high stability (0.1%CV) and high dynamic resolution (0.03%), they are controlled by the powerful MAESFLO™ software which displays real time measured values with Flow Rate Control Module (FRCM).

Key Stability Factors

Table 2. Key factors affecting microfluidic stability