Selecting suitable measuring devices

Liquid handling results depend first and foremost on reliable measurement results. If, for example, there is a large discrepancy between the delivered and the measured volume because of poor measurements, this will lead to misleading performance metrics. It is therefore crucial to use appropriate measuring devices. ISO 8655 and ISO 23783 describe different measurement methods and conditions, such as a the maximum systematic and random measurement errors that are allowed for various volume ranges. However, a basic requirement is to keep the ambient conditions as constant as possible and to exclude both external and internal influences.

Avoiding external influences

Even if the measuring devices are functioning correctly, unsatisfactory results may occur that are not caused by the dispensing or pipetting head. Some typical mistakes and countermeasures include:

• Air in the system
  • Possible causes: liquids that are not degassed, too many components, splitting of channels with increase in diameter, etc.
  • Countermeasures: rinse the system, avoid increasing the duct diameter from start to finish, reduce the number of components
• Particles in the system and leaking valves
  • Possible causes: contaminated products, insufficient filtration, incorrect process valves, damaged tubing, etc.
  • Countermeasures: clean products before installing them, rinse the system with higher pressure, use special fittings for liquids such as NLFA, use suitable air and liquid filters
• Poor repeatability
  • Possible causes: slow valve actuations, pressure drop between channels, pressure drop due to a reduced volume in the reservoir, tolerances of the components, etc.
  • Countermeasures: use a fast control unit (e.g. Festo VAEM-V), calibrate each channel/valve, use a liquid pressure sensor downstream of the reservoir
• Other ambient conditions
  • Possible causes: inconsistent temperature, vibrations, long tubing connections, etc.
  • Countermeasures: keep ambient conditions as constant as possible, minimise external influences, reduce length of tubing to a minimum

Eliminate internal influences

As already mentioned, external influences can drastically affect the results. Nevertheless, even small changes to a particular dispensing and pipetting head can improve results, especially for small target volumes. Festo uses a pressure-controlled dispensing and pipetting approach. In this case, it is absolutely crucial to use a valve with good repeatability and reproducibility without pause time effects. When it comes to dispensing applications, choosing the right needle is also very important.

Let's check this behaviour using an example with the Festo dispense head VTOE. The dosing results with different needle diameters reveal two main findings:

• High linearity: depending on the opening time and the needle, the dispensed volume will show a linear course.
• Different pulse times are necessary in order to achieve the same amount of dispensed volumes.

Looking at the corresponding channel CV values (also called intra-run or intra-assay CV) as a function of the dosing results of a needle, two further important findings are immediately noticeable:

• The CVs are very low; this means that the system has a very good performance.
• When the pulse time becomes smaller, the CVs become higher.

The reason for higher CVs with shorter pulse times is simple: there are many parameters and properties that can vary slightly when opening and closing the valves (e.g. time resolution of the control unit, switching behaviour of the valve, liquid flow, etc.). If the pulse time is low, the time portion of switching on and off is higher and therefore has a greater influence. This leads to a greater dispersion of the dosing results and poorer CVs.
To avoid this problem, the findings from the first diagram are significant. By using a needle with a lower flow rate (e.g. smaller inner diameter), the pulse time can be increased to achieve a certain amount of volume. This improves the CV values again while maintaining the same target volume.

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