Liquid Flow Meter Measurement Principles
Advantages and Limitations
The market for flow meters is continuously growing. The specialized sensors are applied in more and more processes to increase reliability and efficiency. What do researchers need to look for when deciding which kind of flow meter with which measurement principle to use?
Accuracy & Repeatability: Key for Measurement Principles
Depending on the measurement principle, manufacturers offer two general types of sensors: sensors measuring the flow with direct contact to the media, and sensors that measure without any contact to the media. Direct contact with the media implies regular cleaning of the sensor, which is time-consuming and expensive. Even when disposable single-use sensors are implemented, the replacement is similarly time-consuming as well as expensive in the long run. Sustainable sensors without contact to the media are also called non-contact clamp-on sensors as they are clamped on the tubing and measure through the tubing wall.
Flow measurement is a complex topic for process engineers: They must decide which measurement method fits best to the process and at which point the flow measurement should be implemented. Additionally, the flow profile and the characteristic of the liquid are also important for the decision about the measurement device. Viscosity, temperature, pressure, presence and amount of particles, the sensitivity of cells and the purity of the liquid are only a few more questions, which should be discussed in the buying decision.
Considering flow as a critical process parameter (CPP) in bioprocessing, flow measurement becomes even more important in quality by design (QbD) strategies in the development and production of biopharmaceuticals. Particularly, reproducibility is a key factor to achieve a constant high product quality.
Thus, accuracy at low and high flow rates is one of the top requirements for flow meters in bioprocessing.
The operator’s side is also an important factor, which covers usability including process implementation, calibration and cleaning issues, as well as pricing and the sustainability of the flow meter. Considering all those points, a certain measurement principle can make the difference.
Physical Background
In general, flow measurement is the quantification of the movement of fluids. The flow can be indicated in several ways such as velocity, flow volume or mass flow rate. The flow volume is typically specified in millilitre or litre per period, whereas the mass flow volume is given in grams or kilograms per period.
In order to measure the flow, volume counters or flow meters are used.
Despite their similarity, the terms ‘counter’ and ‘flow meter’ do not refer to the same thing: ‘Counting’ describes the recording of a quantity without any reference to a period of time, whereas ‘measuring’ defines the recording of the quantity per a defined period of time. Thus, for example, volume counters indicate litres, whereas flow meters measure in millilitres per minute.
For counting the flow volume a number of devices, such as a turbine flow meter, paddle wheel flow meter and vortex flow meters are available.
For measuring the flow volume, volume and mass flow meters are used such as electromagnetic flow meters, Coriolis flow meter, and ultrasonic flow meters.
All measurement devices, counters and flow meters, are influenced by certain ambient conditions as well as the characteristic of the liquid. So far, there is no ready-to-use solution on the market. The measurement device must fit to the application and the environment.
Clamp-On Sensors
Single-Use vs. Reusable Flow Meters
As the demand for single-use material increases, the market for single-use flow meters is also rapidly growing. Today, for all the previous-mentioned measurement methods, single-use sensors are available, which are implemented in the process by using standard connectors with direct contact to the media. Depending on the complexity of the measurement method, certain types of single-use flow meters can be quite price-intensive considering they are used only once. Although single-use sensors are not designed to be cleaned, the high price of some sensors may tempt operators to perform one or more cleaning cycles.
Single-use implies fast implementation and usability, but the reality shows that single-use flow meters require much more effort than reusable clamp-on sensors in terms of process design, logistics, warehousing and sustainability.
Additionally, due to their reduced design, housing and lower price, single-use flow meters usually do not offer the same high accuracy and repeatability readings as reusable sensors.
In-line Measurement and Cleaning Issues
Except for the clamp-on ultrasound flow meters, all other sensors measure in-line and thus with direct contact to the liquid. Consequently, these flow sensors must be professionally cleaned on a regular basis, which is time- and cost-intensive. Depending on the design of the measurement channel, the cleaning procedure can be very complex. In general, it can be assumed that the simpler the setup of the measuring channel, the more efficiently the cleaning can be performed. However, there is always possible adhesion in the flow path inside of the flow meter. Generally speaking, this means that sensors without any moving parts in the measurement channel are more suitable for sensitive bioprocesses. By implementing single-use in-line sensors that are replaced after each cycle, contamination can be reduced to a minimum.
![[Translate to USA:] Non-contact flow meter made of stainless steel](/fileadmin/_processed_/6/b/csm_biotechnology-sonotec_2e93d5d861.jpg "Non-contact flow meters & bubble detectors for bioprocessing applications")
Stainless Steel Flow Meters
In order to avoid any surface contamination in GMP environments, flow meters with stainless steel housings are a viable option.
Stainless steel clamp-on ultrasound flow meters combine a number of advantages regarding cleaning and non-contact measurement in PAT-compliant bioprocessing.
