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Which positive displacement pump (PD pump) is best for your dispensing needs, linear or rotary? Linear and rotary positive displacement pumps can provide exceptional accuracy and precision. However, each method has their advantages and disadvantages. Positive displacement pumping refers to a pump that retracts in a cavity to generate volume on the suction side and extends to close in the cavity on the discharge. Fluid flows into the suction side during retract and flows out of the discharge during contraction. This is a constant for each cycle.

Calibration:

In a rotary application, the volume of displacement is a factor of the pump angle relative to the motor axis. Once volume requirements are achieved, the pump module can be secured into its location. The displacement is a variable and in most cases must be checked to confirm calibration is accurate depending on validation requirements. In a linear application, calibration is not required. The pump module is set in a static location and volume is a factor of how far the piston retracts in the cavity only.  There is no mechanical set point to change volume, only software.

Cycle time:

Rotary pumps can produce faster dispenses throughout the cycle because they are not required to load fluid and valve between suction and discharge ports.  The loading of fluid and valving is simply part of their reciprocating motion due to operating on 2 different axis at the same time:  1. The motor axis.  2. The pump axis. Cycle time and the ability to produce an infinite volume (based on pump revolutions) are the rotary pumps strengths over the linear pumps. In high speed applications, the one condition is pulsation due to motor rotations.  The dispense profile mimics a sinusoidal waveform that is proportionate to the intake stroke versus the discharge stroke. An example of a great rotary application would be dispensing microliter range “dots” of fluid onto a substrate passing through a high speed automation system.

 

A linear pump must retract from the suction port to draw fluid, valve to the discharge port, and eject the fluid. In general, larger volumes will require more time, depending on pump size. The linear dispense creates a flat dispense profile with no pulsation throughout the entire chamber capacity of the pump module, making it extremely precise.  If more volume or a longer dispense is required, the pumps may be run in parallel.  An example of a great linear application would be a dispensing a constant line over a distance, such as a diagnostic reagent, or precisely filling a substrate with slow absorption rates. You can check out our technical animations of rotary and linear pumps here.