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How Fluids Impact Variable Volume Pump Performance

Fluid properties can directly influence the pressure required to produce a specified flow rate, and failure to account for changes in fluid properties such as temperature or viscosity may result in pressures that exceed the safe operating limits of a pump. Effects of this can include motor stalling, system leakage, or even component damage. 

Lee variable volume pumps can be utilized with various flow mediums. Factors including fluid viscosity, temperature, chemical compatibility, and flow stability should be considered when designing a fluid system. Maintaining effective fluid function lets you reliably operate your variable volume pump to aspirate and dispense fluid in small, highly precise increments relevant to your specific application.

Fluid Viscosity

The fluid viscosity and overall system restrictions will determine the outlet pressure for a given dispense rate. High-viscosity fluids coupled with high inlet restrictions may cause the pump to stall or result in air being aspirated past the seal and into the fluid stream. Once air enters the fluid stream, compressibility can lead to problems with accuracy. Maximum fluid viscosity is system-dependent and affected by inlet and outlet restrictions and flow rate.

Fluid Temperature

Fluid temperature should also be considered when designing a variable volume pump into your system. Chemical compatibility at elevated or reduced temperature and temperature effects on pump components are two main considerations. Exceeding the pump’s operating temperature, either with a heated fluid or a hot environment, may cause permanent damage to the pump.

Chemical compatibility between the fluid and pump components must also be assessed at the operating temperature. Certain chemicals can become more reactive with elevated temperatures.

The viscosity of some fluids varies greatly with temperature, resulting in changes to the dispense pressure. If the fluid becomes too viscous, the dispense pressure can exceed safe operating limits and damage the pump or other system components.

Rated Pressure

The rated pressure of a variable volume pump is the maximum developed pressure wherein the pump will still meet operating specifications. The rated pressure is well below the maximum pressure that the pump can generate. In a pump, if both the inlet and outlet are closed or blocked, the pressure will increase until the motor stalls, the piston seal leaks, or a component in the fluidic circuit fails.

Lee variable volume pumps are positive displacement devices that generate pressure. The fluidic restriction on the pump outlet determines how much pressure can be generated. The generated pressure will be very low if the outlet restriction is low. Small passageways, valves, nozzles, and other fluidic devices will increase the system restriction and, in turn, increase the output pressure generated by the pump.

Chemical Compatibility

Lee variable volume pumps are offered in a variety of materials, and the selection of wetted materials should be based on application needs and fluid compatibility. In some cases, chemically aggressive fluids may not be compatible with one or more of the pump’s wetted materials. A barrier fluid can isolate the pump from incompatible fluids in these cases. In addition, some variable volume pumps can be configured with dual seal and flush ports, which prevent premature wear to sealing components due to incompatible fluids.

Whenever possible, chemical compatibility of the fluids in contact with the pump should be confirmed before selection. This includes the working fluid and any other fluids used for flushing, cleaning, and storage. The Lee Company can provide material samples for immersion testing by request.

Flow Stability

Flow stability is critical for certain Micro dispensing applications and can be influenced by the step rate, stepper controller, microstepping, and system design. 

A high step rate typically provides stable flow. The ideal pump for flow stability is one whose capacity is just above or equal to the required dispense volume. For example, if the dosing protocol calls for four dispenses of 250 μL, it may be best to choose a 1000 μL capacity pump instead of a 3000 μL capacity pump. This allows for more steps in each dispense and, therefore a higher dispense rate with less pressure pulsation. If the application is particularly sensitive to flow stability, choosing a 250 μL pump and completely aspirating and dispensing the fluid four times may be advantageous. In addition, micro stepping can create smoother flow due to smaller step increments.

The fluidic system design also affects flow stability. Pulsations in flow generated by the pump can be mitigated by using restrictors to create back pressure or using compliant dampening components such as tubing. Conversely, compliant flow controls expand and contract with pressure and can negatively impact accuracy and precision. Balancing the system to achieve optimal performance tailored to each application is important.

Fluid viscosity, temperature, chemical compatibility, and flow stability can impact effective fluid function in your variable volume pump. Consider these factors when designing your fluidic system to ensure continuous and reliable operation.

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