选择合适的测量设备
首先，液体搬运结果取决于测量结果的可靠性。若测量不当，造成实际搬运量和测量量之间存在很大误差，那么会得出错误的结果数据。因此，选择合适的测量设备至关重要。ISO 8655 和 ISO 23783 对各种测量方法及测量条件做出了描述，包括不同体积范围内允许的最大系统及随机测量偏差。然而，需要满足这样的基本要求，周围环境条件要尽可能保持恒定，以便排除外部和内部影响。
排除外部影响
即使测量设备没有问题，实际结果也可能不尽人意，而这些结果并非由分液头或移液头引起。典型的错误及其应对措施如下：
- 系统内含有空气
- 原因可能是：液体未脱气，组件太多，管径膨胀导致管道裂开等等
- 应对措施有：清洗系统，从头至尾采取错误避免管径增大，减少组件数量
- 系统内有颗粒物，阀未密封
- 原因可能是：产品受污染，过滤不充分，配件选用错误，附件受损，等等
- 应对措施有：安装前对产品进行清洗，冲洗系统时增大压力，选用液体专用配件（如 NLFA），选用合适的气体及液体过滤器
- 重复性差
- 原因可能是：阀调节较慢，通道间压力下降，罐内容积减少导致压力下降，组件存在公差，等等
- 应对措施有：使用快速控制模块（如 Festo VAEM-V），对每个通道/阀门进行校准，在容器后侧使用液体压力传感器
- 其他环境条件
- 原因可能是：温度不恒定，存在振动，管道较长，等等
- 应对措施有：尽可能保持环境条件稳定，尽量减少外部影响，尽量减少管道长度
消除内部影响
如前所述，外部因素会极大影响搬运结果。尽管如此，即使对特定的分液头和移液头进行微小改动也能改善结果，当移液目标体积较小时尤其如此。Festo 采用压控式分液和移液方法。这种方式必须要使用重复性和再现性良好、且不受停顿时间影响的阀门。在实际分液的过程中，选用合适的滴定头也是非常重要的。
我们可以用 Festo 的滴定头 VTOE 来对此进行验证。通过观察使用不同直径滴定头的分液结果，我们可以得出这两条结论：
- 高线性度：容量一定时会产生线性过程，这一过程与开放时间和滴定头直径有关。
- 在针头直径不同的情况下，要想达到相同的分液量，就需要不同的脉冲时间。
Measurement specifications
- Compressed air supply 300 mbar
- Room temperature 23 °C
- Aqueous solution (water)
- 24 V without holding current reduction
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.
Measurement specifications
- Compressed air supply 300 mbar
- Dosing needle inner diameter 0.60 mm
- Room temperature 23 °C
- Aqueous solution (water)
- 24 V without holding current reduction
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.
Summary
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. And finally, internal influences can be eliminated by using the right components for each application.
