When selecting a flow meter or flow controller for gases, it is important to understand the difference between the mass flow rate and volumetric flow rate. Unlike liquids, gases do not have a constant volume. The volume of a gas changes as the temperature and pressure change.

Referencing the Ideal Gas Law: PV=nRT

• When the temperature of a gas increases, the volume increases.
• When the temperature of a gas decreases, the volume decreases.
• When the pressure of a gas increases, the volume decreases.
• When the pressure of a gas decreases, the volume increases.

When you only know the volume or volumetric flow rate of a gas, you may not actually know how much gas you have. In this blog, we compare mass flow rate vs volumetric flow rate, how each are measured, and the advantages of each.

What is Mass Flow?

Hastings’ first thermal mass flow meter was introduced in the 1960s, and the phrase “mass flow” has been commonly used. However, the phrase “mass flow” is a bit of a misnomer. Thermal mass flow meters actually measure the flow rate of gas molecules or the molecular flow rate. Since the mass flow meter or controller measures the molecular flow rate, it is independent of temperature and pressure changes. It is not necessary to separately measure temperature and pressure, and temperature and pressure corrections are not required.

Teledyne Hastings Instruments mass flow meters and mass flow controllers use a thermal flow sensor to measure the molecular flow rate. The flow sensor is heated, and the flowing gas molecules transfer or “carry” the heat downstream. The rate of heat transfer is proportional to the mass / number of gas molecules flowing through the sensor which is used to determine the flow measurements.

Mass flow rates can be expressed in true mass flow units such as LB/hr, Kg/hr, and g/sec. However, they can also be expressed in standard volumetric flow rates such as SCCM, SLM, and SCFM. Standard volumetric flow units look like volumetric flow units, but they are mass flow units. Standard units are referenced to Standard Temperature and Pressure (STP). STP must always be defined. Teledyne Hastings Instruments typically uses STP: 0° C & 760 Torr; however, other STPs can be specified.

What Should Mass Flow Rate be Used For?

Mass flow rate measurement is ideal for any application where gas measurement is critical. As stated earlier, there is no need to simultaneously measure gas temperature and gas pressure which can introduce errors to your calculations. It’s ideal for applications requiring fast, reliable, repeatable, and accurate measurements.

Some common applications include instrument calibration, air sampling, non-destructive leak testing, gas blending (mixing), thermal spraying, thin film deposition, analytical instrumentation, and many more.

What is Volumetric Flow?

Volumetric flow rate is mathematically defined as the cross-sectional area of a tube multiplied by the velocity of the fluid. It simply measures volume per unit of time. Volumetric flow is ideal for liquids since their volume mostly cannot change. When using volumetric flow meters for gas measurement, the only way to determine how much gas is flowing is to calculate it. That requires either maintaining constant temperature and pressure or simultaneously measuring them. Examples of volumetric flow methods include rotameters, turbine meters, and critical orifices.

One of the most common volumetric flow meters is a rotameter or variable-area flow meter. It consists of a tapered tube and a float. The tapered tube is typically constructed of glass to allow the user to visually see the float. As the flow rate increases, the float travels up the tapered tube. The tube has graduated lines to indicate the volumetric flow rate.

Some common volumetric flow units are GPM, L/s, and CFM. Mass flow can be calculated from volumetric flow only if the density (ρ) is known. Q_mass = Q_vol*ρ.

What Should Volumetric Flow Rate be Used For?

Volumetric flow rates are fine in applications where the volume is constant such as incompressible liquids. They can also be found in gas applications where the temperature and pressure are fixed. In addition, volumetric flow meters typically have a lower initial cost than a mass flow meter. They are often found in low-cost gas measurement applications where molecular flow rate accuracy is not critical. When molecular flow rate accuracy is required, the user typically measures temperature and pressure separately and calculates it.

Summary

Both mass flow meters and volumetric flow meters are commonly used to measure gas and liquid flow measurements. For gas flow measurement, mass flow is more trusted as it measures the gas molecular flow rate. Mass flow meters and controllers function independently of gas temperature and gas pressure changes. This makes mass flow meters and controllers ideal for applications requiring fast, reliable, repeatable, and accurate measurements. Volumetric flow meters such as rotameters or critical orifice devices cannot routinely achieve these high accuracy levels on gases due to inherent inaccuracies and variations that occur as a result of pressure and temperature changes.

FAQS

• Q: Can Teledyne Hastings’ flow meters be used with liquids or any fluid?  
  A: No. Teledyne Hastings’ flow meters cannot be used with liquids. A fluid is defined as “a substance without a shape and can be either a gas or liquid”. Fluids used in our flow meters must be in the gas phase (gases only).

• Q: What is STP (Standard Temperature and Pressure)?
  A: In standardized volumetric flow units (SCCM, SLM, SCFM), the reference conditions, or “STP” temperature and pressure, define the amount of gas by determining the number of molecules using the Ideal Gas Law. In most cases, the selected reference conditions are 0°C & 760 Torr. Other reference conditions are also used, such as 20°C & 760 Torr, or 70°F & 760 Torr.

• Q: What does SCCM stand for?
  A: SCCM stands for Standard Cubic Centimeters per Minute. This is a unit of measure for volumetric flow. In standardized volumetric flow units, the reference conditions, or “STP" temperature and pressure, define the amount of gas by determining the number of gas molecules using the Ideal Gas Law.