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Mass Flow vs Volumetric Flow


Mass Flow vs Volumetric Flow:
Key Differences, Applications, and Measurement Guide


Understanding the difference between mass flow and volumetric flow is essential when choosing the right instrument and flow measurement technology for your application. While both approaches provide valuable insights, they serve different purposes depending on process conditions and measurement requirements.

Mass Flow

Volumetric Flow

What is mass flow rate?

Mass flow refers to the actual number of molecules moving through a system, regardless of changes in temperature or pressure. This makes it the preferred choice in applications that require a consistent quantity of molecules, even when environmental conditions vary.

What is volumetric flow rate?

Volumetric flow, sometimes referred to as actual flow rate, measures the volume of fluid moving through a system over a set period of time. Put simply, it quantifies the amount of space that a gas or liquid occupies as it passes a given point.

Common units of mass flow:

  • Standard litres per minute (SLPM)
  • Standard cubic centimetres per minute (SCCM)
  • Normalised litres per minute (NLPM)
  • Normalised cubic centimetres per second (NCCS)

Common units of flow rate:

  • Litres per minute (LPM)
  • Cubic centimetres per minute (CCM)

Measurement basis:

Mass flow is calculated according to the number of gas molecules in motion, referenced to standard temperature and pressure conditions (for example, 25 °C and 14.696 PSIA, or 0 °C and 100 kPa).

Measurement basis:

Volumetric flow is based on the space a fluid occupies in motion. However, since volume depends on both temperature and pressure, volumetric flow measurements can vary significantly under changing conditions.

Considerations:

Because mass flow standardises conditions, it ensures consistency when comparing flow rates across different systems and settings. “True mass flow” represents the actual mass of fluid passing through a point per unit of time, usually expressed in kilograms per second (kg/s). This type of measurement is most commonly achieved using Coriolis technology and is widely available across specialist mass flow meter ranges.

Considerations:

This measurement type is often more cost-effective and is particularly suitable where changes in fluid density have little impact on the process.

Where is mass flow most applicable?

Accurate gas measurement and control are made easier with mass flow, as it delivers a constant molecule count regardless of pressure or temperature fluctuations. Acting as a form of molar flow measurement, it provides highly consistent results. Typical mass flow applications include:

  • Gas mixing – Delivers precise gas ratios, ensuring accuracy as mass flow aligns directly with molar flow rather than variable volume measurements.
  • Custody transfer – Provides exact gas mass data for billing, regulatory compliance, and reporting.
  • Bioreactors – Controls oxygen and CO levels through pulse-modulated sparging for optimum gas exchange.
  • Optical fibre manufacturing – Enables precise gas control for burner heating, thin-layer deposition, and protective argon flow within furnaces.

Where is volumetric flow most applicable?

Volumetric flow is well-suited to straightforward flow monitoring where absolute gas density is not a critical factor. Common applications include:

  • Industrial hygiene and environmental monitoring – Measuring the number of particles within a specified volume of air under ambient conditions.
  • Basic flow monitoring – Tracking flow where gas density variations do not significantly affect results.

Mass Flow vs Volumetric Flow: How to Choose

When selecting between mass flow and volumetric flow measurement, the key question is whether your application requires the precise volume of space occupied by the fluid or the exact number of molecules being transported.

Alicat’s flow instruments are designed to handle both measurement methods, offering flexibility across a wide range of applications. They provide accurate volumetric flow readings even when temperature and pressure fluctuate, while also ensuring reliable mass flow measurements for consistent results.

  Mass Flow Rate Volumetric Flow Rate
Effect of Temperature and Pressure Stable under all conditions, unaffected by changes in temperature or pressure. Directly influenced by fluid density, which varies with temperature and pressure.
Ideal Applications
  • Consistency is required under changing conditions
  • Standardised measurement is essential
  • Processes requiring molar-level accuracy
  • Applications focused on the physical space occupied by fluid
  • Stable operating conditions with minimal density changes
  • General monitoring or environmental measurements
Measurement Units SLPM, SCCM, NLPM, NCCS LPM, CCM
Practical Selection Guide Use Mass Flow when accurate, standardised, and condition-independent data is critical. Use Volumetric Flow when monitoring the actual space occupied by a gas or liquid under stable conditions.

Summary: Mass Flow vs Volumetric Flow

Both mass flow and volumetric flow are essential in different contexts. Volumetric flow measures the space a fluid takes up, making it best for straightforward monitoring when density is stable. Mass flow, on the other hand, measures the number of molecules, ensuring precision and consistency under varying temperature and pressure.

The decision depends on your process requirements: choose volumetric flow when volume is the priority, and mass flow when substance consistency is key.

Mass Flow vs Volumetric Flow: Key Differences, Applications, and Measurement Guide

Alicat Mass Flow Instruments: Flexibility and Precision

Alicat’s laminar differential pressure (DP) gas flow meters and controllers deliver both mass flow and volumetric flow readings, giving you the freedom to select the most relevant measurement, compare both, or seamlessly switch between control modes. Unlike basic flow devices, Laminar DP Alicat instruments measure all four critical variables—mass flow, volumetric flow, temperature, and pressure—for exceptional accuracy and process control.

For applications requiring true mass flow measurement, Alicat also provides Coriolis flow devices for both gases and liquids. The key distinction between these technologies lies in their approach: Laminar DP devices measure volumetric flow and then apply temperature and pressure compensation to calculate mass flow, ensuring consistency under changing conditions. By contrast, Coriolis instruments directly measure mass flow, delivering unmatched precision regardless of the fluid’s properties, composition, or temperature.

Lab Unlimited is a leading distributor of Alicat flow instruments, trusted by professionals across industries. If you are unsure which flow measurement technology is best for your application, our dedicated sales team is here to help. Contact us today for tailored advice, product details, and support in selecting the right solution for your needs.


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