Mass Flow versus Volumetric Flow Rate
In this post, we compare mass flow rate to volumetric flow rate, revealing some distinct advantages for using thermal flowmeters which measure mass flow rather than volumetric flow.
What is volumetric flow rate?
Volume is a three-dimensional measure of the area a substance occupies.
Volumetric flow is the measure of a substance moving through a device over time. Common units of measure for volumetric flow rate are meters3 /second, milliliters/second or feet3/hour.
When heating gas, the molecules bounce off each other, the gas will expand and the same amount of gas will now have a different space (i.e., Assuming the volume is not fixed). If the volume is fixed, the pressure will increase when temperature is raised. Additionally, when increasing pressure the volume measurement will be reduced.
For this reason, when measuring gas using a volumetric flow meter, temperature and pressure is required, along with the gas density to convert volumetric flow to mass flow.
What is mass flow rate?
Mass is the measure of how much matter there is in a substance and is often expressed in weight (grams, kilograms and, tonnes, ounces or pounds).
Mass flow rate is the amount of a mass moving through an instrument over time, so the unit of measure is mass (or weight) per unit of time. It can be expressed as pounds /hour or kilograms/second or in some similar way. The advantageous thing about mass flow is that mass or weight does not vary depending on the temperature or pressure. However, weight does change with gravity, but as long as we’re dealing with applications on Earth, mass flow is not influenced since the gravity is the same.
Benefits of Mass Flow Measurement
There are some distinct advantages of using mass flow meters over volumetric meters. When using volumetric meters, there is the added expense of acquiring and installing additional equipment for temperature and pressure correction. There is also the added computation required to convert the volumetric flow to mass flow. Additionally, mass flow measurement is recognized as offering more reliable, added accuracy and better repeatability data than volumetric flow methods.
Examples of Mass Flow Meters
The following devices measure gas mass flow: Coriolis flow meters, thermal mass flow meters, ultrasonic meters and rotometers.
Examples of Volumetric Flow devices
The following devices measure volumetric flow: positive displacement meters, turbine flow meters, orifice plates, Venturis, vortex meters and Pitot tubes.
Related information can be found on Gas Meter Types for Flow Measurement.
This video describes density, mass flow vs volumetric flow rate.