What is the difference between mass flow and volumetric flow?

What is the difference between mass flow and volumetric flow?

Understanding the difference between mass flow and volumetric flow is important when determining which flow instrument is right for your application. Here we introduce the concepts of mass flow and volumetric flow and discuss use cases for each measurement.

What is a volumetric flow rate?

Volumetric flow rate, also called the actual flow rate, is a measure of the volume of fluid that flows through the instrument per unit time. Common units of measure include LPM (liters per minute) and CCM (cubic centimeters per minute).

What is a mass flow rate?

In the gas mass flow industry, mass flow rates are an expression of volumetric flow rates corrected to standard conditions. Calculating mass flow requires a reference to standard temperature and pressure conditions, such as 25 °C and 14.696 PSIA or 0°C and 760 torr. Common units of measure include SLPM (standard liters per minute) and SCCM (standard cubic centimeters per minute).

Should I measure mass flow rate or volumetric flow rate?

It really depends on the goal of your application. When determining whether to use volumetric flow or mass flow, it is important to determine if you want to measure/control a set volume or the number of molecules of process fluid.

When to use mass flow

Many applications benefit from mass flow instruments, as they can precisely measure and control the flow of gas molecules into or out of a process. At any selected mass flow rate, the number of molecules flowing through the point of measurement is constant regardless of temperature or pressure conditions. Mass flow rates could also be considered molar flow rates.

Example mass flow applications

  • Gas mixing applications benefit from mass flow instrumentation, as gas mixtures are often defined in terms of volumetric ratios. This can be confusing because in reality these are molar ratios. As noted above, the flow industry defines mass flow as a form of molar flow measurement. Therefore, one should use mass flow rates for making up “volumetric” mixture ratios.
  • Custody transfer applications benefit from mass flow measurement, simply because it is necessary to know the mass of gas being used.
  • Mass flow is used to regulate oxygen and CO2 levels in bioreactors. Pulse modulated sparging utilizes mass flow controllers to produce uniformly sized oxygen bubbles into the bioreactor. The precise, low-flow control ensures the bioreactor is properly oxygenated and that toxic CO2 is effectively stripped.
  • When manufacturing optical fibers, it is necessary to have high-precision measurement and control. Mass flow devices are used in the burner control application to control gases that heat the preform and help control thin layer deposition. They are also used to facilitate the drawing process by feeding argon into a furnace. This prevents the furnace element from burning up.

For many more mass flow applications, check out the applications section of our knowledge base.

When to use volumetric flow

Volumetric flow instruments are generally less expensive than mass flow devices, and can be a great option if you need to measure or control a specific actual flow of gas without regard to the density of the process gas.

Example mass flow applications

Industrial hygiene and environmental air monitoring are two applications that utilize volumetric flow. The goal in each is to quantify the number of particles within a specific volume of air under the ambient conditions.

Alicat mass flow instruments

Alicat laminar DP gas flow meters and controllers provide readings for both mass flow and volumetric flow rates. This allows users to choose whichever measurement makes the most sense for their application and compare the two, or switch between closed loop control on mass flow rate and volumetric flow rate. Alicat also sells liquid flow meters. The primary difference between mass flow and volumetric flow devices is their use of temperature and pressure sensors.