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Mass flow and volumetric flow, is there a difference?

Before deciding which flow instrument is right for your application it is vital you understand the difference between mass flow and volumetric flow. We will discuss both concepts and 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).

Which should I measure, mass flow rate or volumetric flow rate?

Your application’s objectives will determine whether you should use volumetric flow or mass flow. Your first question should be if you want to measure or control a set volume or the number of molecules of process fluid.

When to use mass flow

The benefit of mass flow instruments is that 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.

Mass flow application examples:

  • Gas mixing applicationsbenefit from mass flow instrumentation, becasue gas mixtures are often defined in terms of volumetric ratios which can be confusing because in reality these are molar ratios., As we previously mentioned, the flow industry defines mass flow as a form of molar flow measurement, therefore it should use mass flow rates for making up “volumetric” mixture ratios.
  • Custody transfer applications benefit from mass flow measurement, 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.

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 objective in each is to quantify the number of particles within a specific volume of air under ambient conditions.

Should you like to know more about Mass & Volumetric Flow, please contact our Sales Team. We will be happy to find you the right instrument to fit your application requirements. You can also see the Mass Flow Meter ranges we have available on our website.

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