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Rotameters vs. Alicats for bioprocessing

Rotameters vs. Alicats for bioprocessing

Flow meters are indispensable tools for bioprocessing, used to feed bioreactors 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.

Operating principle

Rotameters

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 meters

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 meterRotameter
Operating rangeUp to 10,000:1Average 50:1
Communication & data loggingDigital display, analog I/O, serial communication, and/or industrial protocolsManual (read and record)
Variable outputsMass flow rate, volumetric flow rate,
pressure, and temperature
Volumetric flow rate
CustomizationAll units custom designed to application needsLimited customization
IntegrationEasy incorporation with system equipment & data systemsSimple 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 based on your process development priorities.

PriorityRecommended deviceReason
PrecisionAlicat flow meterTight tolerances across a wide operating range, with digital recording and communication for precise process control.
Integration & automationAlicat flow meterAnalog and serial communication protocols, including industrial protocols (EtherNet/IP, DeviceNet, Modbus, Profibus, etc.).
ScalabilityAlicat flow meterHigher operating range allows a single device to monitor a much wider range of flows.
Ease-of-useRotameterSimple installation, repeatability, and accuracy, with parts available off-the-shelf for any repairs or servicing.
PricingRotameterLower-cost devices available.
FDA complianceSituation-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.

 

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