Blog series: How to handle low liquid flows? Part 2

Part 2: Tips for flow meter selection

As you might remember from reading the first part of this blog series, for (ultra) low liquid flows, tiny disturbances in the process or ambient conditions can have a large impact on flow stability. This second blog aims to help users of low-flow fluid systems optimise the stability and performance of their system, by giving tips and recommendations for dimensioning, choice of material and best practice procedures, based on years of experience, gathered by Bronkhorst High-Tech.

The core of the low-flow fluid system is obviously a liquid flow meter or controller. Deciding which instrument type is best for the low-flow application largely depends on accuracy and stability requirements. However, ambient or media conditions which are difficult to control can also play an important role in choosing the best solution.

Coriolis-based liquid flow meter vs Thermal-based liquid flow meter

A Coriolis flow meter/controller…

• performs very well in situations where absolute accuracy and flow stability are essential; 
• has long-term stability and negligible thermal sensitivity;
• is a good choice when the medium density needs to be measured or monitored, in addition to the flow;
• can be used for mixtures of liquids with unknown properties (i.e. is media independent);
• is somewhat receptive to vibrations around the resonance frequency, which may necessitate the use of shock absorbing measures.

​On the other hand, a thermal flow meter/controller ...

• is a more economical choice if the processed liquids and (ambient) temperatures are stable and distributed evenly;
• performs well when reproducibility is more essential than accuracy;
• requires specification of the liquid density, viscosity, thermal conductivity and heat capacity;
• generally causes a relatively small pressure drop, which can help to keep the flow stable if the liquid contains a considerable amount of dissolved gas.

Tip 2: Provide a stable (inlet) pressure to the fluid system

A highly stable inlet pressure of a flow controller is a prerequisite for a stable low liquid flow rate. Two methods are popular to achieve this: use a pressure vessel where gas is used to pressurise the liquid or use a pump. Please refer to parts 3 and 4 of this series of blogs for more background details.

Tip 6: Purge the system before operation

This is especially important for skids. First connect all instruments and then purge before starting to control or measure. To this end, the Bronkhorst skids contain a purge setting.

In the next parts of the blog series, we will focus on low liquid flow supply using a pressure vessel or a pump and give more background details on how to manage external conditions. After all, choosing a good flow device is important, but insight into how this influences the application process is equally essential.