What is mass flow?

What is mass flow?

A common way to measure and control flow is with a mass flow instrument, but what is mass flow? The answer to this can be approached from a number of angles, which we cover in this article.

What do mass flow meters and controllers do?

Most mass flow devices output a mass flow rate. This mass flow rate is not a mass per unit time, but rather a volumetric flow rate that has been corrected to standard temperature and pressure conditions. Mass flow devices are used as process solutions for many flow applications, as they measure and control gas flows with high accuracy and precision.

What are the types of mass flow meter?

There are numerous types of mass flow instruments available on the market. Different mass flow technologies vary by method of flow measurement and also by performance and capabilities.

Most Alicat devices are laminar differential pressure-based mass flow instruments. These instruments calculate a highly accurate mass flow rate using known gas properties and real-time ambient temperature and pressure readings. Alicat also offers instruments that use Coriolis mass flow technology. These devices output highly accurate “true mass readings” instead of typical mass flow rates and they are able to measure and control fluids regardless of composition or ambient conditions.

What are the units of mass flow?

A mass flow rate is given as a standardized (or normalized) volumetric flow rate. Common mass flow units of measurement include standard cubic centimeters per minute (SCCM), standard liters per minute (SLPM), and standard cubic feet per hour (SCFH). Examples of common normalized mass flow units are NCCM, NLPM, and NCFH.

This nomenclature seems a bit misleading, as these units are volumes per time rather than units of mass. To calculate true mass from a mass flow rate, use the following equation:

True mass flow

True mass flow = (mass flow rate)(gas density at STP or NTP)

When should I use mass flow?

Mass flow devices are used for applications requiring high accuracy, precision measurement and control of the flow of gas molecules. There are so many applications that benefit from measuring and controlling mass flow, from bioreactor outgassing to fuel cell membrane testing to natural gas monitoring.

Mass flow rate is important precisely because of the ways it differs from volumetric flow: it is standardized, and will not fluctuate with changes to ambient temperature or pressure.

That said, volumetric flow is the right measurement for processes accounting for total gas volume, while mass flow is specific to instances where you are looking to control or measure the total amount of a specific gas. For more information on when to use volumetric flow or mass flow, read this article.

Does mass flow rate change with density?

Mass flow rate depends on the density of the gas that is being flowed. For laminar differential pressure devices, it is important to know the exact composition of the gas being flowed so that precise density corrections can be made.

Fortunately, Alicat differential pressure devices are pre-loaded with 98+ gases containing the relevant density information. A bonus of calculating and reporting at standard conditions is that it is fairly easy to apply correction factors to your flow data if you accidentally flow with the wrong gas selected.

True mass flow measurements also depend on density. Coriolis mass flow devices, however, do not use known gas properties to calculate flow rates. They are able to measure fluid densities themselves and accurately output true mass flow rates, even when flowing process fluids of unknown composition.