Positive Displacement and Volumetric Precision Fluid Dispensing Systems

When specifying a fluid dispensing system, the volumetric accuracy of the system must be considered. This can be determined either in volumetric flow rate or in terms of actual dispense volume. One style of dispensing equipment that is inherently accurate and repeatable is a volumetric dispensing system that utilizes a positive displacement pump. This style of pump dispenser has some inherent benefits that make it a great choice for a liquid dispensing system.

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What is a positive displacement or volumetric dispensing?

A common way of characterizing a pump’s performance is to look at the volumetric output of a pump in response to changing discharge pressure. The closer to horizontal that the line is, the more consistent the pump output will be as ambient conditions vary. Positive displacement pumps provide this near constant volumetric output versus flow relationship and is why they can be useful for reliably dispensing precise amounts of fluid.

positive displacement is like a syringe

Unist has been designing fluid dispensing systems for over 60 years and has championed the use of positive displacement pump technology. The positive displacement pump can be thought of as a syringe style pump where when the pump piston moves, it moves through the pump chamber and positively displaces the fluid with the piston. The fluid must move somewhere as the piston displaces it. With the correct check valve arrangement, the fluid will flow in the desired direction.

The volume output by the pump is quantifiable by how large and how far the piston travels, or in the case of flow rate control, by how large and how fast the piston travels in the pump bore. It is not dependent on backpressure, fluid viscosity, or ambient temperature. The output is consistent, controllable, and quantifiable without the need for a fluid flow meter. Positive displacement pumps can be configured to provide continuous flow over a wide range of flow rates or discrete shots of fluid from microliters to milliliters and beyond.


What are all the elements of a positive displacement or volumetric precision dispensing system?

A typical positive displacement dispensing system will contain the following:

  • Fluid reservoir
  • Filter
  • Control system
  • Pump or a series of pumps
  • Tubing
  • Valves
  • Outlet tubing or nozzles as required.

The pump will draw fluid from the reservoir and direct it to any number of application sites. Pumps can be actuated pneumatically, electronically, or driven at a variable speed depending on what the application requires.

What are the benefits of positive displacement dispensing over other methods?

The main benefit of a positive displacement liquid dispenser pump is the repeatability of the dosing or flow rate. A volumetric pump is not affected by the ambient conditions as the output is not dependent on the viscosity of the fluid or the outlet pressure. Fluid dispensing systems that require accuracy can be created with less cost than a closed loop feedback control system using pressure controls, proportional valves, and expensive fluid flow meters. From micro dispensing systems to higher volumetric output systems, a liquid dispenser with a positive displacement pump will be accurate and repeatable. Positive displacement dispensing systems can be designed to work with a variety of viscosities from thin fluids such as isopropyl alcohol up to heavy gear oils and even greases.

What are the downsides or challenges compared to other methods?

As with any fluid dispensing system, fluid compatibility with the materials inside the pump and the seal materials is always a concern. Also, some positive displacement pumps might be less tolerant to debris or fluid contamination than other fluid dispensing solutions. A high quality fluid filter and maintenance schedule can help ensure long pump life. For very precise applications, users may wish to calibrate the pump output by measuring the volumetric output during system commissioning, however after this initial setup, the output will remain consistent.

Unist products for positive displacement and volumetric systems

Unist has a line of volumetric output pumps with nominal outputs between 1/30th and 1/10th of a milliliter per stroke. These can be configured to provide fluid in discrete shot amounts, or in a continuous spray delivery to any number of application points. Unist also has a continuous flow positive displacement piston pump capable of providing flow rates from 5 to 500 mL/hr into back pressures up to 200 psi. Please contact us today to discuss how a volumetric pump / liquid dispenser pump can improve your process.