When a Dispensed Volume Is Not Always the Same
The influence of pressure, viscosity and temperature on actual dispensing volumes
Pressure-time dispensing systems are an established standard in dispensing technology – for good reason. They are robust, cost-effective and technically sufficient for many applications. As long as material properties and ambient conditions remain stable, they deliver reproducible results.
However, dispensing is not a purely time-controlled operation; it is a physical process. And physical processes respond to change. As soon as processes operate within tight tolerances or material properties vary, a fundamental question arises: does the dispensed volume truly remain constant – or only under ideal conditions?
The Dispensed Volume as a Result of Physical Influencing Factors
In a pressure-time system, the discharged volume results from the interaction between applied pressure, opening time and the current flow behaviour of the medium. It is therefore directly dependent on the prevailing process conditions.
Viscosity is the key influencing factor. It is not a fixed constant but varies due to temperature deviations, batch-to-batch differences, ageing or mechanical shear stress. Even minor temperature changes can significantly affect the flow behaviour of highly viscous or filled media. In addition, system-related influences such as pressure losses or pressure fluctuations along the material path have an effect. In pressure-based systems, these variables directly impact the discharged volume. In applications with wider tolerances, this is often non-critical. In precision-relevant processes, however, even small deviations can lead to measurable changes in dispensing volume and consequently to quality variations.
Volumetric Dispensing: A Design-Based Approach
The decoupling of physical influencing factors is a key advantage of volumetric dispensing methods.
Volumetric dispensing follows a fundamentally different principle from the pressure-time method: the dispensing volume is not derived from current process conditions but is defined by a geometrically determined displacement. This displacement is not directly dependent on pressure or viscosity fluctuations. Changes in material behaviour may affect the mechanical load on the dispensing system, but not the delivered volume. The essential difference compared to pressure-based dispensing therefore lies in the method by which the dispensing volume is generated.
Technical Implementation: The Progressive Cavity Principle
An established technical implementation of volumetric dispensing is the progressive cavity principle. In this design, a rotor and stator form sequential cavities which continuously progress from the material inlet to the outlet due to the rotor’s eccentric motion. The medium is conveyed by continuous positive displacement. The dispensing volume results from the geometrically defined displacement per revolution and the rotational speed of the rotor. This generates an almost pulsation-free, continuous volumetric flow. The principle is particularly suitable for highly viscous, filled or shear-sensitive media where uniform and reproducible dispensing is required.
When Volumetric Dispensing Becomes Technically Advantageous
Whether a dispensing system operates with long-term process stability largely depends on the boundary conditions of the application. In processes with wider permissible tolerances and stable material properties, pressure-time systems remain an economically viable solution. However, when dispensing tolerances are reduced, material properties vary during the process, or high repeatability is required, the evaluation changes. In such cases, it is no longer only system robustness that is decisive, but the physical principle by which the dispensing volume is generated.
An additional technical aspect arises in automated applications: With variable path speed, the volumetric flow rate must be controlled proportionally in order to ensure a constant material volume per unit length. Without this synchronisation, acceleration and deceleration phases directly affect the applied volume per unit length.
With the application valves of the RotoStream series, you can implement exactly this necessary coupling: through speed-proportional control, the flow rate is synchronised in real time. This ensures that the dispensing volume per millimetre of path length remains absolutely constant – regardless of the complexity of the component geometry or the dynamics of your handling system.
“The transition from pressure-time systems to volumetric dispensing systems is particularly justified when processes are no longer defined solely by cost-efficiency, but by precision and reproducibility.”
Implications for Process Design
The stability of a dispensing process is not a matter of chance, but the result of the chosen dispensing principle. As long as material behaviour and ambient conditions remain constant, different dispensing principles deliver comparable results. As precision requirements increase, the question of whether the dispensing volume results from current process conditions or is structurally defined becomes a central criterion for process design. With the RotoStream series, we have translated these requirements into a practical system solution.
