What is the difference between mass flow and volumetric flow?
Mass flow measures the number of molecules in a flowing gas. Volumetric flow measures the space that those molecules occupy. Because gases are compressible, volumetric flow rates can change substantially when pressure or temperature changes.
- Volumetric flow rate is a measure of the 3-dimensional space that the gas occupies as it flows through the instrument under the measured pressure and temperature conditions. Volumetric flow rate can also be called actual flow rate.
- Mass flow rate is a measure of the number of molecules that flow through the instrument, regardless of how much space those molecules occupy. Mass flow rate is often expressed as a standardized (or normalized) volumetric flow rate, which is the amount of space that those molecules would occupy if measured under standard temperature and pressure conditions (STP, or NTP).
Most Alicat gas flow instruments provide readings of both mass flow and volumetric flow, but determining which measure to use depends on the application objectives.
The ideal gas law
The Ideal Gas Law describes the relationship between mass, volume, pressure and temperature for static gases:
P = Static pressure
V = Volume
n = Molar mass
R = Gas constant
T = Absolute temperature
All other factors being constant, if the gas pressure (P) increases by a factor of 2, the volume of the gas (V) decreases by a factor of 2. This occurs because gases are compressible, and their molecules are pressed closer to each other as pressure increases. If the pressure decreases by half, volume doubles. The molar mass (n, the number of particles in the volume) stays the same, regardless of changes in static pressure.
Imagine a flexible container filled with 500 cm3 of air at atmospheric pressure (1 atm, about 14.696 psia) and standard ambient temperature (25 °C). If these conditions are defined as standard conditions (STP), then the mass can be expressed as a standardized volume (scm3).
- If the volume is compressed to 250 cm3, the air molecules move closer together, and the pressure increases to 2 atm. However, the number of air molecules (molar mass) remains the same, 500 scm3.
- If the volume is stretched to 1000 cm3, the original pressure of 1 atm is halved to 0.5 atm. Doubling the volume again to 2000 cm3 reduces the pressure to 0.25 atm. Again, the mass remains the same, 500 scm3.
In each instance above, no air has been removed or added from the container, so the mass of air inside the container never changes.
Flow rates and the ideal gas law
When air is put into motion as a flow of gas, the actual space that the air takes up per unit of time (volumetric flow rate) varies with pressure in the same manner as the static air. Doubling the line pressure halves the volumetric flow rate, and vice versa. However, the number of molecules of air that flow per unit of time (mass flow rate) does not change.
When to use mass flow rate vs volumetric flow rate
Most Alicat gas flow instruments provide readings for both mass flow and volumetric flow, so users can use the measure that makes the most sense for each application. Alicat’s gas flow controllers allow for closed loop control of either mass flow rate or volumetric flow rate.
- Choose mass flow when the focus of flow measurement or control is the gas itself. Mass flow measurement is best used when the number of molecules need to be monitored, regardless of pressure conditions. Gas mixing applications operate best when the controllers are measuring mass flow rates, which ensures that the concentrations of the constituent gases remain constant relative to each other. Custody transfer applications also benefit from mass flow measurement, as the mass of gas being used is more critical than the space that mass fills.
- Choose volumetric flow when the focus of flow measurement or control is what lies within the gas volume. Volumetric flow measurement is best used when monitoring the components of a gas stream under actual process conditions. Industrial hygiene and ambient air monitoring applications work best with volumetric flow measurement because the goal is to quantify the number of particles within the volume of air under the actual measured conditions.