Select suitable measuring devices
First and foremost, liquid handling results depend on reliable measurement results. For example, if there is a large discrepancy between the volume delivered and the volume measured due to poor measurements, this will result in misleading performance metrics. Therefore, it is very important to use appropriate measuring devices. ISO 8655 and ISO 23783 describe various measurement methods and conditions, such as a maximum allowable systematic and random measurement error for different volume ranges. A basic requirement, however, is to keep the ambient conditions as constant as possible and to exclude both external and internal influences.
Avoid external influences
Even if the measuring devices are order, unsatisfactory results may occur which are not caused by the dispensing or pipetting head. Some typical mistakes and countermeasures are:
- Air in the system
- Possible causes: non-degassed fluids, too many components, splitting of channels with increase in diameter, etc.
- Countermeasures: flush system, avoid increasing duct diameters from start to finish, reduce number of components
- Particles in the system and leaking valves
- Possible causes: contaminated products, insufficient filtration, incorrect fittings, damaged tubing, etc.
- Countermeasures: clean products before installation, flush system with higher pressure, use special fittings for liquids such as NLFA, use suitable air and liquid filters
- Poor repeatability
- Possible causes: slow valve actuation, pressure drop between channels, pressure drop due to reduced volume in the reservoir, component tolerances, etc.
- Countermeasures: use of a fast control unit (e.g. Festo VAEM-V), calibration of each channel/valve, use of a liquid pressure sensor after the reservoir
- Other ambient conditions
- Possible causes: inconsistent temperature, vibration, long barbed tubing connectors, etc.
- Countermeasures: keep ambient conditions as constant as possible, minimize external influences, reduce length of tubing to a minimum
Eliminate internal influences
As mentioned above, external influences can drastically affect the results. Nevertheless, even small changes to a given 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 imperative to use a valve with good repeatability and reproducibility without pause time effects. When it comes to dispensing applications, selecting the right needle is also very important.
Let's check this behavior using an example with the Festo VTOE dispensing head. The dispensing results with different needle diameters show two main findings:
- High linearity: depending on the opening time and the needle, a linear course of the dispensed volume results.
- To achieve the same amount of dispensed volume with different needles, different pulse times are required.
Measuring specifications
- Compressed air supply 300 mbar
- Room temperature 23 °C
- Aqueous solution (water)
- 24 V without reduction of holding current
Looking at the corresponding channel CV values (also called intra-run or intra-assay CV) as a function of the dispensing results of a needle, two other important findings become directly visible:
- The CVs are very low – this means that the system has a very good performance.
- As the pulse time becomes shorter, the CVs increase.
Measuring specifications
- Compressed air supply 300 mbar
- Dispensing needle inner diameter 0.60 mm
- Room temperature 23 °C
- Aqueous solution (water)
- 24 V without reduction of holding current
The reason for higher CVs with smaller pulse times is simple: when opening and closing the valves, there are many parameters and properties that can vary slightly (e.g. time resolution of control unit, switching behavior of the valve, liquid flow, etc.). If the pulse time is short, the time portion of switching on and off is higher and thus has a greater influence. This leads to a greater scatter of dispensing results and poorer CVs.
To avoid this, the findings from the first diagram are important. 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. In this way, the CV values improve again with the same target volume.
Conclusion
In summary, it is obvious that a good liquid handling result depends on several parameters. First of all, suitable measuring equipment is required. In addition, it is very important to avoid external influences by designing suitable fluid channels, creating a clean working atmosphere and keeping the ambient conditions as constant as possible. Finally, by using the right components for each application, internal influences can be eliminated.
About the author
Christian Sampedro
Product Management Industry Segment LifeTech
Festo SE & Co. KG
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