Rotameters vs. Alicat flow meters in bioprocessing

Flow meters for bioprocessing: Rotameters vs. Alicats

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 two flow meter technologies: rotameters and Alicat 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).

Alicat flow meter operating principle

Alicat flow meters measure differential pressure across a flow body. They then convert this reading into a volumetric flow rate using the Hagen-Poiseuille equation. Finally, they calculate the mass flow rate using the volumetric flow rate and instantaneous fluid temperature and pressure measurements.

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 and Alicat mass flow meters.

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

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.

Priority Recommended device Reason
Precision 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, etc.).
Scalability Alicat flow meter Higher operating range allows a single device to monitor a much wider range of flows.
Ease-of-use Rotameter Simple installation, repeatability, and accuracy, with parts available off-the-shelf for any repairs or servicing.
Pricing Rotameter Lower-cost devices available.
FDA compliance Situation-dependent
  • 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.