Metering and dosing of ultra-low flow rates (g/hr) requires a different approach and understanding of the measuring principle compared to metering and dosing of higher flow rates. At these low levels physics start to play a significant role. In this article I will reveal some of the successes for measuring stable ultra-low flow rates.
Working with people to achieve stable and accurate low and ultra-low flow metering and dosing you learn that little things can make a big difference to the process output.
The first thing to do is understand the measuring principle that is being used, please see the video below for a short explanation on Coriolis measurement theory.
Once the principle has been understood then the challenges that we will face become easier to understand.
Due to the ultra-low flow nature of the application the influence of one area can affect the rest of the system, because of this we have broken our advice down into specific areas of discussion.
- For pressurised containers avoid using gas that dissolves in liquid, e.g. N2/Air
- If fluid is water, use Di-Water
- Always use a filter
- Ensure filter has a large internal volume to minimise pressure drop
- Outlet needs to be position up to ensure trapped AiR comes out during flush
- Make sure there are no leaks
- Make sure the entire system is filled with the same medium
- Ensure environment stability (temp, humidity, vibration)
- Use small volume tubes to avoid compliance
- Use a degasser to remove entrained AiR from the fluid
Image: example of pressurised container with degasser
Set-up for success
Firstly, you have to be able to pass the fluid through the tube. This is why you will always be asked to discuss the following points:
Knowing these three fluid parameters allows you to calculate (www.fluidat.com) the performance envelope that exists. This is something that is critical in instrumentation to ensure that you size a sensor correctly to achieve optimum performance following installation.
Once the above conditions have been considered and a sensor has been found to meet the requirements of both the fluid and the application you have only the conditions surrounding the sensor that can affect the quality of the readings generated.
All Coriolis instrument reading can be susceptible to external influence from vibration; this is why metering at low and ultra-low flow rates is such a challenging design process. If the environment around the application produces a vibration similar to that of the instrument it can cause false and inconsistent readings. Alongside external vibration issues it is possible to create ‘cross-talk’ where Coriolis instruments mounted next to each other incorrectly can cause interference.
Using mass blocks, considering your environment and allowing for physical functions of instruments in the design process will ensure you can achieve maximum performance.
It is important to discuss these limitations because they exist, however having seen Coriolis instruments on helicopters, at the bottom of the ocean and in submarines it just proves that due care and attention will mitigate low and ultra-low flow liquid metering challenges.