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What gas flow meter should I use?

Use cases for gas flow technologies

The broad selection of flow meter technologies on the market can make choosing the right one for your application an arduous task. Here we discuss the use cases of common gas flow meter technologies to simplify the process of determining a viable solution for your application needs.

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  1. Laminar differential pressure flow meters (Laminar DP)
  2. Variable area flow meters (rotameters)
  3. Thermal bypass flow meters
  4. Coriolis flow meters
  5. Ultrasonic flow meters
  6. Technology comparison chart

Laminar differential pressure flow meters (Laminar DP)

Laminar differential pressure-based gas flow meters require no warm-up time and are highly accurate due to the precise geometries in their construction. However, any clogging or condensation on the materials will change the geometry and therefore affect the system accuracy. These devices should therefore not be used to flow condensing or dirty gases. Laminar flow measurement also depends on accurate data for how a gas reacts with varying pressure and temperature, which means knowing the exact gas composition is required for accurate measurement.

Best for
  • Clean, dry gases
  • Operating temperature and pressure within sensor limits
Avoid for
  • Condensing or dirty gases
  • Unknown gas compositions
  • High viscosity gases

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Variable area flow meters (rotameters)

Rotameters are the lowest-cost gas flow meters on the market, and can be used with a wide variety of gases, including those that may not be clean enough to flow through laminar DP flow meters.

Rotameters come with three primary drawbacks. As they are dependent on gravity, rotameters must be installed vertically and cannot handle upward flows; gas that is visibly dirty, opaque, or coats the glass of the meter can make it difficult to measure the flow visually; and rotameters can only be calibrated to read volumetric flow rate or a mass flow rate at a specific pressure, limiting the pressure range of a device.

Best for
  • Low-cost
  • Near atmospheric pressure
Avoid for
  • Applications with varying pressure
  • Gases that are dirty, opaque, or coat glass

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Thermal bypass flow meters

Thermal flow meters are calibrated to specific gas properties, and to maintain accuracy are therefore used primarily for pure gases, as well as gases with non-changing, known compositions. The biggest advantage of thermal flow meters is that they can be inserted into large pipes to measure in-line flows of known gases. They can also withstand higher pressures, at the expense of creating a large pressure drop.

Thermal gas flow meters cannot be used for gases of unknown or variable compositions because mass flow calculations are based on the known thermal properties of the gas or gas mixture. Aggressive and coating gases may also damage the sensor or necessitate frequent maintenance, especially if contaminated with water.

Best for
  • Gases of known compositions
  • High pressure
  • Insertion into pipes
Avoid for
  • Unknown gas compositions
  • Aggressive gases

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Coriolis flow meters

Coriolis flow meters provide reliable, high accuracy mass flow measurements even for gases with changing densities. This makes them ideal for applications where gas composition is changing or has unknown physical properties. Because they can be constructed from various materials, Coriolis flow meters can be also used for sanitary applications and with corrosive, dirty, or aggressive gases.

While Coriolis flow meters tend to have a higher cost of purchase, they require very little maintenance and have lower lifetime costs of ownership than many other flow meters. Some Coriolis meters can even measure multi-phase fluids like slurries, although this is typically only available in higher flow models.  The main drawback of a Coriolis meter is sensitivity to external vibrations, which can be limited by attaching a large mass block.

Best for
  • Unknown compositions and/or physical properties
  • Sanitary applications
  • Corrosive or aggressive gases
Avoid for
  • Multi-phase fluids
  • Applications with vibrational noise

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Ultrasonic flow meters

Ultrasonic flow meters are accurate, repeatable, and function well at extreme pressures and temperatures. Their most unique advantage is that they can simply be clamped onto a pipe and offer non-invasive flow measurements. The meter therefore has no wetted parts and can be used when the pipes cannot be interfered with – although this does lower measurement accuracy. Furthermore since ultrasonic flow meters do not actually contact the flows, they can be used for sanitary, corrosive, or aggressive gases. And unlike most of the other meters discussed here, these meters can accurately flow gases with bubbles and eddies.

The primary limitation of these meters is that they can only flow gases that conduct ultrasonic waves. They are also particularly sensitive to process vibrations and they lose accuracy as the pipe diameter changes, such as when buildup accumulates on pipe walls.

Best for
  • Mixed phase fluids
  • Non-invasive flow measurement
Avoid for
  • Gases that do not conduct ultrasonic waves
  • Applications with vibrational noise

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Technology comparison chart

CoriolisLaminar DPThermalUltrasonicRotameter
AccuracyHighHighMediumMedium-HighLow
Clean, dry gas
Creates only a small pressure drop
High-viscosity gases
Unknown or changing fluid composition
Mixed-phase fluid compatible
Aggressive gas compatible
Opaque gases
Same unit works for liquid

Alicat offers several solutions for the metering and control of gas flow. Our selection of laminar DP and Coriolis solutions are compatible with a wide range of flow rates and process fluids. Contact one of our applications engineers to determine the best mass flow solution for your process needs.

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