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.

MEMS calorimetric sensors measure mass flow rates with symmetrical thermopiles upstream and downstream of the heating element. Power is provided to the heating element to keep the temperature differential constant. When there is no flow, the sensors read the same value. When gas is flowing, the heated gas travels downstream, and the downstream sensor will be hotter than the upstream sensor. 

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.

MEMS thermal flow meter
Laminar DP flow meter
Operating range
Average 50:1
Up to 1,000:1
Up to 10,000:1
Communication & data logging
Manual (read and record)
Analog, serial communication, and/or Modbus RTU
Digital display, analog I/O, serial communication, and/or industrial protocols
Variable outputs
Volumetric flow rate
Mass flow rate and temperature
Mass flow rate, volumetric flow rate, pressure, and temperature
Limited customization
Available in manifold
All units custom designed to application needs
Simple installation
Easy incorporation with system equipment & data systems
Easy incorporation with system equipment & data systems
  • Gas meters not damaged by liquids
  • No power requirements
  • Parts commonly available
  • Small footprint
  • Calibration/repairs done at Alicat
  • Gas meters can handle process backsplash
  • Require power supply
  • Calibration/repairs done at Alicat

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.

Recommended device
Alicat flow meter
Tight tolerances across a wide operating range, with digital recording and communication for precise process control.
Integration & automation
Alicat flow meter
Analog and serial communication protocols, including industrial protocols (EtherNet/IP, DeviceNet, Modbus, PROFIBUS, PROFINET, etc.).
Alicat flow meter
Higher operating range allows a single device to monitor a much wider range of flows.
Simple installation, repeatability, and accuracy, with parts available off-the-shelf for any repairs or servicing.
Lower-cost devices available.
FDA compliance
  • Rotameters are advantageous because of their simplicity: they are mechanical devices which do not store or transmit data, and are not integrated with other data systems.
  • Alicat devices can better meet the PAT directive, but increased complexity brings increased regulatory oversight.