Mass Flow vs Volumetric Flow
Understanding the difference between mass flow (standardized volumetric flow) and volumetric flow is essential when selecting the right instrument and flow measurement technology for your application. Both methods provide valuable information, but they serve different purposes depending on the conditions and requirements of the process.
Mass Flow
What is mass flow rate?
Mass flow represents the actual number of molecules moving through a system, regardless of temperature or pressure changes. This makes mass flow ideal for applications that require a consistent amount of molecules, despite varying conditions.
- Common units: Standard liters per minute (SLPM), standard cubic centimeters per minute (SCCM).
- Measurement basis: Number of gas molecules in motion, referenced to standard temperature and pressure conditions (e.g., 25°C and 14.696 PSIA).
- Considerations: Because mass flow calculations standardize conditions, they provide consistency when comparing flow rates across different settings.
Mass flow or also called “true mass flow” refers to the measurement of the mass of a fluid passing through a given point per unit time, typically expressed in units such as kilograms per second (kg/s). This is most commonly used in Coriolis technology, and is available on our M/MC-Series line under “engineering units”.
Where it is most applicable
Precise gas measurement and control become simpler with this approach, as it maintains a constant molecule count regardless of temperature or pressure changes. Because mass flow functions as a form of molar flow measurement, it delivers consistent results in various processes. Some example mass flow applications:
- Gas mixing applications – Ensures accurate gas ratios, as mass flow aligns with molar flow measurements, avoiding confusion with volumetric ratios.
- Custody Transfer – Provides precise measurement of gas mass for billing and compliance.
- Bioreactors – Regulates oxygen and CO₂ levels using Pulse modulated sparging for optimal oxygenation and CO₂ removal.
- Manufacturing optical fibers – Enables high-precision gas control for burner heating, thin-layer deposition, and argon flow to protect furnace elements.
Volumetric Flow
What is volumetric flow rate?
Volumetric flow rate, also known as actual flow rate, measures the volume of fluid passing through a system per unit time. In other words, it quantifies the amount of space that a gas or liquid occupies as it moves through a conduit over a specified period.
- Common units: Liters per minute (LPM), cubic centimeters per minute (CCM).
- Measurement basis: The amount of space a gas or liquid takes up as it moves past a point.
- Considerations: Since gas volume depends on temperature and pressure, volumetric flow measurements can change when these conditions vary.
Where it is most applicable
Cost-effectiveness and a focus on actual gas volume make this measurement type advantageous, particularly when density shifts have little effect on the process. It is also a suitable choice for straightforward flow-monitoring situations.
- Industrial hygiene and environmental air monitoring are two applications that utilize volumetric flow. The goal of each is to quantify the number of particles within a specific volume of air under ambient conditions.
- Basic flow monitoring – When gas density variations do not significantly impact the measurement.
Choosing between Mass Flow and Volumetric Flow
- Must comply with standardized measurements
- Density changes have minimal impact
Conclusion
Both mass flow and volumetric flow measurement methods play essential roles in different applications. Volumetric flow measures the physical space a fluid occupies as it moves through a system, while mass flow quantifies the number of molecules passing through per unit time.
The choice between these measurement types depends on the application’s focus: volumetric flow is suitable when the volume of fluid is the primary concern, whereas mass flow is preferable for applications requiring knowledge of the exact quantity of substance being transported, unaffected by changes in pressure or temperature. It is important to note that volumetric flow is directly influenced by the fluid’s density; changes in temperature and pressure can alter density, thereby affecting volumetric measurements.
Evaluating the priority between measuring volume and substance, along with considering potential variations in operating conditions, is essential in selecting the appropriate flow measurement method.
Alicat Mass Flow Instruments: Flexibility and Precision
Alicat’s laminar DP gas flow meters and controllers provide both mass and volumetric flow readings, allowing you to choose the most relevant measurement, compare both, or switch between control modes as needed. That is why Laminar DP Alicat instruments go beyond basic flow measurement—measuring all four key variables (mass flow, volumetric flow, temperature, and pressure) for unmatched precision.
For true mass flow measurement, Alicat also offers Coriolis devices for gas flow and Coriolis devices for liquid flow. The primary difference between mass flow and volumetric flow devices lies in their measurement techniques. Laminar Differential Pressure (DP) mass flow devices measure volumetric flow and apply temperature and pressure corrections to calculate mass flow, ensuring consistent readings across varying conditions. In contrast, Coriolis devices provide precise mass flow measurements regardless of the fluid’s properties, composition, and temperature.
Not sure which flow measurement suits your application best? Connect with our Application Engineers for personalized support!
