Bioprocessing: Rotameters vs. thermal flow meters vs. laminar DP flow meters

Any bioprocess needs flow meters to feed bioreactors, drive filtration and chromatography, and monitor critical process parameters. Here we provide a comparison of the operating principles and key features of three flow meter technologies: rotameters, MEMS thermal mass flow meters, and laminar DP mass flow meters. We then provide recommended use cases for each technology in bioprocessing applications.

Rotameter operating principle

Rotameters measure volumetric flow rate using the variable area principle. A rotameter consists of a float inside a tapered tube, and the upward force of a fluid passing through the tube dictates the position of the float. This creates a set operating range: the upward force of the fluid is a function of flow rate, and the float rises and falls linearly with the volumetric flow passing through the meter. It rises as the velocity head and buoyancy of the fluid passing through the meter increase (it is worth noting that buoyancy is primarily relevant for liquids, as it is negligible for gases). As the force of the fluid equalizes with the downward force of gravity, the float reaches a steady state and provides a stable flow measurement. The volumetric flow represents the volume of gas flowing through the system per unit time (e.g. liters per minute).

MEMS thermal meter operating principle

MEMS (micro-electromechanical systems) thermal flow meters use thermal heating elements, temperature sensors, and thermal properties of a gas to measure mass flow rates. There are two ways to utilize this setup: directly in the main flow path, or in a bypass tube. BASIS MEMS thermal mass flow meters measure flow directly in the flow path.

Laminar differential pressure (DP) flow meter operating principle

These mass flow meters measure differential pressure across a flow body. This reading is converted into a volumetric flow rate using the Hagen-Poiseuille equation. Finally, the mass flow rate is calculated using the volumetric flow rate and instantaneous fluid temperature and absolute pressure.

Whereas volumetric flow represents the volume of gas flowing through a system, mass flow represents the number of flowing gas particles. This is important because gases are compressible and gas volume changes with temperature and pressure conditions, while mass does not. Mass flow meters therefore allow flow rates to be measured and compared across a variety of process conditions. Mass flow is either displayed as a standardized volumetric flow (e.g. standard liters per minute) or a “true” mass flow in weight per time (e.g. grams per minute).

Feature comparison

The following table compares key features (technical and otherwise) of rotameters, laminar DP mass flow meters, and thermal flow meters.

Device recommendations for bioprocessing applications

Rotameters offer simplicity and low costs, while Alicats are likely advantageous to bioprocessing environments optimizing for process analytical technology (PAT) or Pharma 4.0. This table below contains device recommendations for your bioprocess flow meters based on your process development priorities.