Common Pitfalls and How to Ensure Stability in Real-World Setups

This is Part 3 of our BP7 Performance Insights article series.
If you missed BP7 Performance Insights – Part 1: Stable Performance Needs Feedback or BP7 Performance Insights – Part 2: Detailed Performance Specification, we recommend checking those out first. They explain how the BP7 micropump performs under feedback control and how we validate that performance in controlled tests.

Start with: BP7 Performance Insights – Part 1: Stable Performance Needs Feedback

Continue with: BP7 Performance Insights – Part 2: Detailed Performance Specification

In this final part, we focus on what comes after testing:
From media viscosity to system layout, we explore the real-world conditions that can influence pump output and how to design around them for best-in-class stability.

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The Fluidic Dependency Triangle: Understanding the Key Influences on Fluidic Behavior in Our Micropump Systems

For simple transfer tasks or non-critical applications, open-loop control can be entirely sufficient, especially when media properties, tubing lengths, and flow paths remain stable. If all you need is to move fluid from A to B, open-loop setups can perform reliably even without feedback.

The fluidic performance of a system using our micropump is shaped by the dynamic interplay of three key factors. Our Dependency Triangle helps illustrate these influences and supports a holistic approach to system design:

Application Context

System architecture, fluid properties, and environmental conditions (such as backpressure, viscosity, or temperature) directly impact flow and pressure behavior.

Connected Equipment

Control electronics, tubing, valves, and power supply form the immediate equipment environment. Their configuration can enhance or dampen fluidic performance.

Pump Variability

Minor differences between pumps are an inherent part of precision manufacturing. These are well controlled and, when properly accounted for, enable reliable performance across applications.

Common Open-Loop Flow Behaviors

The following graphs and scenarios represent common patterns we’ve observed in microfluidic systems based on our experience. While these examples are qualitative in nature and not tied to specific measured values, they reflect the most likely behaviors encountered during open-loop operation of the BP7 micropump.

Open-loop performance can vary from system to system, depending on setup, media, and environmental conditions. These cases are meant to guide typical expectations and help identify potential pitfalls in real-world use.

Some of the challenges illustrated here primarily apply to liquid-based applications. Effects such as air bubble formation or priming-related inconsistencies are not typically relevant when operating the BP7 with gases, as gas-based setups are not affected by liquid-air interface dynamics.

Stable Flow With Syringe Priming (Dry Start Behavior)

Dry start means the pump is started with no liquid in the internal flow path. The chamber and channels are filled with air at startup.

In this case, the pump was actively primed using a syringe, which removes air from the system before startup by applying a vacuum from the outlet side.

As a result, the flowrate starts high and stays consistently close to the typical dynamic flow range (up to 14 ml/min with water).

The graph shows a stable curve from the beginning. No dips, no delays.
This method ensures reliable performance, especially in open-loop setups. While it greatly improves startup reliability, it does not necessarily guarantee maximum performance, as other factors like media properties or system resistance can still play a role.

Solution

→ Recommended for quick and repeatable priming in lab environments, as it delivers stable performance right from the start, making closed-loop control unnecessary in many situations.

Self-Priming with Air Bubbles in the Pump (Dry Start Behavior)

If air remains in the pump during self-priming, flowrate can stay consistently low.

The graph shows a flat curve, much lower than the typical dynamic range of up to 14 ml/min.

This is not a failure, but a common possible effect when the pump self-primes without assistance.
In this state, the pump is never fully filled with liquid, small air bubbles remain trapped inside. These bubbles compress during operation, which leads to energy loss:
Instead of moving fluid, part of the actuator’s motion goes into compressing air, reducing stroke efficiency and disrupting valve function.

As a result, the pump can’t reach its typical maximum flowrate, even though it runs continuously.

Solution

→ This can be avoided with active syringe priming, which removes residual air and ensures the pump chamber is completely filled before operation.

Unpredictable Air Release After Self-Priming (Dry Start Behavior)

This case starts with air in the pump, resulting in a low flowrate, again below the typical dynamic flow range of up to 14 ml/min.

After a delay, the trapped air bubble unintentionally detaches, often triggered by changes in pump parameters like frequency or amplitude.

The graph shows this clearly: flow stays low at first, then gradually rises and stabilizes at a higher, expected level.

Solution

→ The gradual flowrise from unpredictable air release during self-priming can be avoided by active priming and stabilized with closed-loop control, which detects the increase and actively reduces the pump output to stay on target.

Uncertain Performance with Wet Pump at Startup

In some cases, the pump is not completely dry, residual fluid remains inside from a previous run.
This can prevent proper self-priming, as trapped liquid and air create unpredictable flow conditions. A wet internal flow path can block self-priming just as much as trapped air.

The graph shows this symbolically: A dashed line with a question mark represents the uncertain outcome. The pump may not start at all or deliver erratic flow.

Solution

→ To restore proper function, either refill the pump reliably using syringe priming or dry it before reuse (by air or in a drying oven).

Gradual or Sudden Air Intrusion

Even after syringe priming and a clean start near the typical dynamic range (up to 14 ml/min), air can still enter the system over time.

Possible causes:

• Empty reservoir
• Outgassing from the liquid
• Leaky or loose connections
• Gas diffusion through tubing

Initially, this results in small air pockets that gradually reduce flowrate.
Later, a large air bubble can enter and cause a sudden drop in flow.

The curve shows a slow decline followed by a sharp drop: Exactly what happens when gradual air ingress turns into a disruptive bubble event.

Solution

→ Closed-loop control detects flowrate drops caused by gradual or sudden air intrusion and compensates in real time to maintain stable output.

Standard Distribution of Production Yield

Even with identical setup conditions, same fluid, same drive signal, same environment, the flowrate of individual BP7 pumps can differ.

The three graphs illustrate this effect based on schematic representations derived from real production data from pumps. While all pumps (pumps A, B, and C) ran under the same parameters, their flowrates vary slightly. This is a normal result of standard distribution within production: small differences in internal components or actuator response lead to measurable performance shifts.

For a deeper look at how we define these ranges during production, see BP7 Performance Insights – Part 2: Detailed Performance Specification

Here in Part 3, we highlight how those variations appear in real flow behavior and what that means for system integration and calibration.

Solution

→ In practice, these variations can be fully compensated using Closed-Loop Control, ensuring consistent output across systems, regardless of individual pump characteristics.

Low Flow Rates

At low flowrates, system sensitivity increases significantly. Every small disturbance, such as pressure fluctuations, media inconsistencies, or minor air inclusions, has a larger proportional effect on overall flow behavior.

This is because lower driving energy leads to reduced system inertia, making the pump more susceptible to internal or external variation. Stability decreases as the system operates closer to its physical limits.

The stroke volume of the BP7 pump is linearly dependent on the applied voltage:
At 250 Vpp, each membrane stroke produces a displacement of approximately 35 µm , while at 25 Vpp, the dispalcement is only 3.5 µm.

At low flow rates, only minimal actuator movement is required. However, this results in less force being applied to the medium compared with external influences, such as the system itself, backpressure, component selection, or environmental factors.
As actuation energy drops, so does the system’s ability to push against resistance and the effect of minor influences becomes more pronounced.

At a tragte flowrate of e.g. x = 100 µl/min, the flowrate gradually decreases over time in a fluctuating, unstable pattern, showing how small effects can accumulate when operating in a low-flow regime without compensation.

Solution

→ For low-flow applications, consider adding sensor feedback control with Closed-Loop Control, damping elements, or pressure monitoring to stabilize performance.

Pressure Head

When pumping from one reservoir to another, pressure head becomes a key performance factor. It refers to the hydrostatic pressure difference created by the fluid height in the source and target reservoirs.

This effect is especially noticeable in gravity-influenced open systems:

• If the target reservoir is already full, the pump must work against the full pressure head. As a result, little to no flow is achieved.
• If the source reservoir is completely filled at the start, the system supports the pump’s output, enabling a high initial flowrate.
• If both reservoirs are filled to an intermediate level, the flowrate reflects this and typically stabilizes around a medium value.

Below are three schematic illustrations showing typical reservoir conditions:

The graphs beneath these schematics visualizes the outcome:

Solution

→ For accurate dosing or long-term processes, pressure head effects must be factored in or balanced using closed-loop control or pressurized reservoirs.

Impact of Viscosity

Viscosity plays a key role in how the BP7 micropump behaves during operation. Fluids with different viscosities react differently to the same drive signal, affecting both flowrate and responsiveness. In the graph below, media ranging from high-viscosity oil (100 cP) to air (~0.02 cP) are shown under identical driving conditions (250 Vpp), illustrating how strongly viscosity influences performance.

More viscous media result in lower flowrates and shift the optimal operating frequency to lower values. For example, water (~1 cP) reaches peak performance near 100 Hz, while oil-based fluids perform best below 40 Hz. Gases, being less resistant, show optimal behavior at much higher frequencies.

These effects are caused by:

• Increased flow resistance with higher viscosity
• Damping of diaphragm motion, reducing effective stroke
• Shifts in resonance characteristics due to fluid mass and drag

Always consider the viscosity of your medium when tuning frequency, selecting components, or interpreting flow behavior. The performance you see is not just based on the pump, it’s also shaped by the fluid.

Solution

→ Closed-loop control can help stabilize performance by compensating for minor fluctuations, but it cannot fully overcome the physical limitations introduced by viscosity and won’t restore full flowrate if the fluid is simply too thick for the current system setup. In such cases, a multipump setup or a pressure-driven flow may be required

Conclusion

Open-loop setups can work well for simple transfer tasks. But when precision, consistency, or responsiveness matter, they often fall short. External influences like air ingress, pressure shifts, or media viscosity can cause performance to drift and without feedback, the system can’t correct itself.

Closed-loop control adds this missing stability. By adjusting the driving signal in real time, it compensates for typical system fluctuations and helps maintain a steady flowrate or pressure, even under changing conditions.

As shown in the flowrate stability graph from Part 1, closed-loop operation enables the BP7 to maintain highly stable performance, even when external conditions vary.

Such variations stem from system-level dynamics rather than the pump component, highlighting the limitations of open-loop control in complex environments.

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Bringing it full circle

From understanding how closed-loop control works to analyzing lab performance and real-world variables, we hope this series has offered useful insights into how to get the most out of the BP7 micropump in a wide range of applications.

If you’d like to revisit the foundations of stable flow control, start with
→ BP7 Performance Insights – Part 1: Stable Performance Needs Feedback

Or explore the test-based performance benchmarks in
→ BP7 Performance Insights – Part 2: Detailed Performance Specification

Whether you’re planning high-precision dosing or need robust long-term performance, we’re here to support your setup.